BMJ OnExamination Flashcards
Regarding pulse oximetry, are the following statements true or false?
True / False
Hypothermia produces a left shift in the oxygen dissociation curve
Is unreliable when used on the same side as the blood pressure cuff
The relationship between the partial pressure of oxygen in arterial blood (PaO2) and percentage oxygen saturation (%SaO2) is linear
Have an accuracy of +/-2% in the range of oxygen saturations of 70% to 100%
Is unaffected by the carboxyhaemoglobin level
True
True
False
True
False
Pulse oximetry is a non-invasive method of assessing arterial oxygen saturation and heart rate.
Any cause of poor peripheral perfusion causes unreliable readings including external compression by a blood pressure cuff.
Pulse oximeters have an accuracy +/-2% in the range of oxygen saturations of 70% to 100%, however, they are less accurate below 70%. Below an oxygen saturation of 70% the readings are extrapolated.
Other causes of error include abnormal pigments such as:
Methaemoglobin
Carboxyhaemoglobin.
Carbon monoxide poisoning overestimates and methaemoglobin underestimates the level of oxygen saturation.
The literature is a little confused on whether hyperbilirubinaemia affects pulse oximetry. Current consensus is that it has little or no effect on the accuracy of readings
When the partial pressure of oxygen is plotted against percentage saturation, a sigmoidal curve is produced. The middle range of the curve is therefore particularly important, as small changes in partial pressure will cause large changes in saturation.
Certain factors may result in ‘shifts’ in the curve, such as:
Reduced pH
Increased temperature
Partial pressure of carbon dioxide and
2,3-diphosphoglycerates.
cause a right shift in the curve. This means that haemoglobin gives up oxygen more easily to the tissues.
Which one of the following is a primary perioperative concern for patients taking chlorpromazine?
Reduction in anaesthetic agent requirements
Prolongation of neuromuscular blockade drugs
Avoidance of indirect-acting sympathomimetics
Interference with platelet function
Caution with desflurane use
Caution with desflurane use
Pharmacology
Patients taking chlorpromazine require special consideration with the use of desflurane during surgery due to the potential for sensitising the myocardium, which can lead to cardiac complications.
Chlorpromazine can sensitise the myocardium to the effects of desflurane, potentially leading to cardiac complications.
Avoidance of indirect-acting sympathomimetics is a concern for patients taking MAOIs, such as phenelzine or moclobemide, due to the risk of hypertensive crisis. It is not relevant for patients taking chlorpromazine.
Interference with platelet function is a concern for patients taking valproate, which can affect platelet aggregation. It is not a concern for patients taking chlorpromazine.
Prolongation of neuromuscular blockade drugs is a concern for patients taking lithium. Chlorpromazine does not significantly affect the duration of neuromuscular blockade.
Reduction in anaesthetic agent requirements is relevant for patients on lithium, as it can reduce the requirement for anaesthetic agents. It is not the primary concern for patients taking chlorpromazine.
A 23-year-old female is having a knee arthroscopy under general anaesthesia.
Anaesthesia is induced with fentanyl 1 mcg/kg and 2 mg/kg propofol. A supraglottic airway is inserted and anaesthesia maintained with an air oxygen mixture and 2.5% sevoflurane. The patient is allowed to breathe spontaneously through a Bain circuit. The fresh gas flow is 9 litres per minute. Over a period of 30 minutes the end-tidal CO2 rises from 4.5 kPa to 8.4 kPa. The capnograph has a baseline reading of 0 kPa.
Which one of the following is the most likely cause of the hypercarbia?
Malignant hyperthermia
Exhausted sodalime
Hypoventilation
Inadequate fresh-gas flow
Release of the tourniquet
Hypoventilation
Physics and clinical measurement
During anaesthesia a rise in end-tidal carbon dioxide is common in a spontaneously breathing patient. Having excluded rebreathing and increased equipment dead space, drug induced respiratory depression and hypoventilation is the most likely cause
Hypoventilation from the respiratory depressant effects of the opioid and sevoflurane are the commonest and most likely cause of a gradual rise in end-tidal CO2 (EtCO2) occurring during anaesthesia in a spontaneously breathing patient.
If the EtCO2 shows further progressive rise then other clinical signs of malignant hyperthermia should be sought.
Exhausted sodalime and inadequate fresh gas flow into the Bain circuit are causes of rebreathing and a rise in the baseline of the capnograph.
Deflation of the tourniquet would cause a sudden rise in EtCO2.
Which of the following does not affect resistance to flow in a blood vessel?
Haematocrit
Thickness of the vessel wall
Length of the vessel
Radius of the vessel
Pressure gradient
Thickness of the vessel wall
Physics and clinical measurement
Thickness of the vessel wall does not affect blood flow, but the stiffness of the vessel wall does affect blood flow.
This question refers to Poiseuille’s Law.
R = P/Q = 8nl/r4, where
P is the pressure gradient along the vessel
Q is the volume flow rate
r is the radius of the vessel
n is the viscosity (haematocrit) of the blood
l is the length of the blood vessel.
Thickness of the vessel wall does not affect blood flow, but the stiffness of the vessel wall does affect blood flow.
Which one of the following statements best describes a characteristic of high voltage transmission in the electrical supply system?
Efficient for long-distance transmission
Transforms DC to AC
Reduces the resistance of the conductors
Utilises chemical reactions for energy conversion
Safely used in domestic appliances
Efficient for long-distance transmission
High voltage transmission is a key strategy in the electrical supply system to reduce power losses over long distances. By increasing the voltage and decreasing the current, resistive losses are minimised, ensuring efficient and reliable delivery of electricity from power plants to consumers.
High voltage is used for long-distance transmission to minimise power losses due to resistance in the conductors. By increasing the voltage and reducing the current, the resistive losses (which are proportional to the square of the current) are minimised, making the transmission more efficient.
High voltage transmission does not reduce the resistance of the conductors. Instead, it reduces power loss by lowering the current for a given power level. The resistance of the conductors is a physical property that depends on the material and dimensions of the conductor.
High voltage is not used directly in domestic appliances due to safety concerns. Instead, high voltage is stepped down to safer levels using transformers before being supplied to homes and used in domestic appliances.
Transforming DC to AC is done using devices called inverters. High voltage transmission typically involves AC, not the conversion process itself. The main purpose of high voltage transmission is efficient energy transport over long distances, not transforming current types.
High voltage transmission does not involve chemical reactions. Chemical reactions are the basis of batteries and fuel cells. High voltage transmission relies on electromagnetic principles to transmit electrical energy over long distances.
Is it true or false that infrared analysers can be used to measure the following gases?
True / False
Oxygen
Carbon dioxide
Nitrous oxide
Nitrogen
Helium
False
True
True
False
False
Gases with molecules that contain at least two dissimilar atoms absorb radiation in the infrared region of the electromagnetic spectrum.
Therefore, carbon dioxide, nitrous oxide and all of the halogenated volatile anaesthetic agents can be measured using infrared absorption analysers.
Oxygen, nitrogen, helium and the inert (or noble) gases do not absorb infrared light and cannot be measured using this technology.
Oxygen is measured using the paramagnetic, galvanic or polarographic method.
Are the following statements true of digoxin? (High impact question)
True / False
Reduces intracellular calcium availability.
Toxicity is increased in the presence of hypokalaemia
Causes increased vagal activity.
Inhibits the cardiac sodium-potassium ATPase.
Is mainly excreted unchanged in the urine.
False
True
True
True
True
Digoxin has indirect effects via the vagus nerve as well as its direct effects.
Inhibition of the cardiac sodium-potassium ATPase increases the intracellular sodium concentrations and this leads to a displacement of calcium, increasing its availability. This is responsible for the positive inotropic effect.
Fifty per cent to 70% of digoxin is excreted unchanged in the urine and doses need to be altered in renal failure.
Toxicity is increased in hypokalaemia, hypomagnesaemia and hypernatraemia.
Are the following true or false of carbon dioxide monitoring?
True / False
Mainstream analysers cause more delay than sidestream analysers
Carbon dioxide absorbs infrared radiation at 4.28 µm
End-tidal CO2 is 0.6-0.7 kPa higher than arterial partial pressure
Is based on the principle of paramagnetism
The mainstream analyser can only be used to measure a single gas
False
True
False
False
False
Physics and clinical measurement
Capnography relies on the principle of infrared absorption. Carbon dioxide is made up of two dissimilar atoms and absorbs infrared radiation at a wavelength of 4.28 µm.
Capnography relies on the principle of infrared absorption. Carbon dioxide is made up of two dissimilar atoms and absorbs infrared radiation at a wavelength of 4.28 µm. End-tidal CO2 is normally 0.6-0.7 kPa lower than the arterial partial pressure.
Mainstream analyser is located within the patient’s breathing system and therefore increases dead space but allows rapid analysis. The IRMATM mainstream probes (manufactured by the company Masimo) are comercially available. They can measure CO2, N2O and five volatile anaesthetic agents (halothane, enflurane, isoflurane, sevoflurane and desflurane).
Sidestream analyser consists of a small diameter tubing that samples gases at 150-200 mL/min. It can cause sampling delay but multiple gases and anaesthetic vapours can be analysed from the same sample.
A patient is undergoing surgery under general anaesthesia at sea level. If the atmospheric pressure is 1 bar and the anaesthetic gas being used has a MAC of 1.5%, what is the partial pressure of the anaesthetic gas required to achieve anaesthesia?
(High impact)
0.10 bar
0.75 bar
1.5 bar
1.00 bar
0.015 bar
0.015 bar
Physics and clinical measurement
The Minimum Alveolar Concentration (MAC) of an anaesthetic gas is a partial pressure value that represents the potency of the anaesthetic. The MAC is expressed as a percentage of the gas in the inhaled mixture at 1 atm.
The MAC value represents the percentage of the anaesthetic gas in the inhaled mixture at 1 atm to prevent movement in 50% of patients. Therefore, for a MAC of 1.5%, the partial pressure is 1.5% of 1 atm, which is 0.015 atm.
This value does not correspond to the calculation based on the given MAC value.
This is significantly higher than what the MAC value suggests.
This would imply that the gas constitutes the entire atmospheric pressure, which is not feasible for anaesthetic gases.
This exceeds the total atmospheric pressure and is not consistent with the concept of MAC.
Are the following true of intravenous induction agents?
True / False
The action of propofol is terminated by metabolism.
Ketamine causes loss of consciousness in less than 30 seconds.
Etomidate is an imidazole, which is sometimes used for sedation on ITU.
Barbiturates include thiopental and methohexitone.
Propofol is more ionised than thiopentone at plasma pH.
False
False
False
True
False
Etomidate is indeed an imidazole but it is not used on ITU for sedation due to its depressant effect on the steroid axis. Cortisol and aldosterone synthesis is depressed for up to 24 hours after a single dose. Increased mortality is seen with its use as a sedative infusion on the ITU.
Thiopental is the new British Approved Name for thiopentone and is a thio-barbiturate, and methohexitone is an oxy-barbiturate. Both are barbiturate intravenous induction agents. Methohexitone was used for day case procedures due to its rapid onset and offset and its use has persisted for electroconvulsive therapy (ECT) due to its epileptogenic effects on the electroencephalograph (in 20% of patients). It is no longer commercially available in the UK.
Ketamine takes at least 30 seconds to cause loss of consciousness following intravenous administration (not less than 30 seconds). The rapidity of induction is related to the proportion of the drug in its unionised form.
Hence it can be seen that propofol is more unionised at plasma pH than methohexitone, which is in turn more unionised (not ionised) than thiopentone (90%, 75% and 61% respectively).
The classical induction agent the action of which is terminated by re-distribution (not metabolism) is thiopentone, but the action of propofol is similarly terminated by re-distribution.
Propofol is metabolised much more rapidly than the barbiturates and this accounts for the reduced “hangover” seen with propofol.
Which of the following are correct gas pipeline pressures?
True/False
4 bar for oxygen
400 kPa for power tools
7 bar for Entonox
7 bar for nitrous oxide
400 kPa for nitric oxide
True
False
False
False
False
A piped gas supply is made up of networks of pipes and sockets which distribute medical gases from a central source to the point of demand.
In the United Kingdom only the following are supplied by such systems:
Oxygen
Nitrous oxide (not nitric oxide)
Medical air, and
Entonox.
All pipeline gases are supplied at 4 bar (or 400 kPa), but compressed air is also supplied at 7 bar for power tools.
Carbon dioxide and nitric oxide are usually only supplied in cylinders.
Which one of the following best describes a primary mechanism of filtration in heat and moisture exchangers (HMEs) used in anaesthesia?
(Please select 1 option)
Thermal inactivation of pathogens by heating elements within the HME. Electrostatic attraction capturing bacteria and viruses on filter fibres. Osmotic pressure gradients driving the filtration of gases and pathogens. Chemical absorption of pathogens into the HME material's core. Mechanical filtration through pores larger than the particles being filtered.
Electrostatic attraction capturing bacteria and viruses on filter fibres.
Physics and clinical measurement
A primary mechanism of filtration in heat and moisture exchangers (HMEs) is electrostatic attraction, which captures bacteria and viruses on the filter fibres, ensuring the prevention of cross-contamination.
Explanation
Electrostatic attraction capturing bacteria and viruses on filter fibres. This mechanism effectively traps pathogens on the fibres, preventing cross-contamination.
HMEs do not typically use chemical absorption to capture pathogens; their filtration relies on physical and electrostatic mechanisms.
Mechanical filtration in HMEs relies on pores smaller than the particles being filtered, not larger, to effectively trap pathogens.
HMEs do not use heating elements for thermal inactivation; they rely on passive mechanisms for filtration and moisture exchange.
Osmotic pressure gradients are not involved in the filtration mechanism of HMEs, which primarily use physical and electrostatic methods.
Starling forces can be applied to the pulmonary microcirculation in the same way as any other capillary bed.
Which one of the following pressures best approximates to interstitial oncotic pressure acting across a normal pulmonary capillary?
(Please select 1 option)
17 mmHg 13 mmHg 6 mmHg 25 mmHg 0 mmHg
17 mmHg
Physiology and biochemistry
The interstitial oncotic pressure is relatively high in the pulmonary circulation due to the presence of lymph.
Explanation
The typical values of the Starling forces acting across normal pulmonary capillaries are as follows:
Interstitial oncotic pressure = 17 mmHg (estimated from measurements on pulmonary lymph)
Capillary hydrostatic pressure (Pc) is 13 mmHg (arteriolar end) to 6 mmHg (venous end) but variable because of the hydrostatic effects of gravity especially in the erect lung.
Interstitial hydrostatic pressure (Pi) - Variable but ranges from zero to slightly negative.
Capillary oncotic pressure = 25 mmHg.
A new opioid agent with a structure similar to fentanyl, has a pKa of 6 and is metabolised in the liver by the cytochrome P450 group of enzymes. Its lipid solubility is 10:1 compared with morphine
Which single one of the following is most likely to describe the kinetic behaviour of this drug?
(Please select 1 option)
The onset of action will be slower than for alfentanil The clearance will be greater than that of remifentanil The context sensitive half-time after a two-hour infusion will be shorter than that of fentanyl The terminal elimination half-life will be shorter than that of remifentanil The volume of distribution at steady state will be smaller than that of morphine
The onset of action will be slower than for alfentanil
Pharmacology
For weak acids and bases, the degree of ionization influences pharmacokinetics. The pKa of a drug relative to tissue pH influences concentration of the unionized proportion of the molecules. The greater proportion of unionised, non-polar and diffusible molecules, the faster the onset of action.
Explanation
The phenylpiperidine group of opioids are weak bases. Alfentanil has a pKa value of 6.5 and at physiological pH (7.4) approximately 90% is unionised which explains a rapid onset of action. With a pKa of 6, this new opioid will have less ionised molecules and therefore will have a slower onset of action compared with alfentanil. Remifentanil is unique because it is metabolised by non-specific esterases located in erythrocytes and other tissues. The propanoic acid-methyl ester linkage is hydrolysed. It therefore has a short terminal elimination half-life of 10-20 minutes and greater clearance because it does not depend on hepatic clearance from cytochrome enzymes.
For weak acids and bases, the degree of ionization influences pharmacokinetics. The lower pKa of this new drug (6) compared with fentanyl (8.5) means that the concentration of the unionized, diffusible form of this drug is much greater (>50%) than that of fentanyl (9%). This accounts for its more rapid onset-time and short half-life to equilibrium (t1/2 ke0).
For pharmacokinetic modelling purposes, like alfentanil this drug will have a smaller central compartment volume, a very much lower volume of distribution at steady state (Vss) and a lower clearance than fentanyl. As a result of these differences, fentanyl has a shorter context-sensitive half-time (CST) than this drug for short infusions (<2 hours). For drugs such as fentanyl, in which redistribution is the main mechanism responsible for the decline in plasma concentration after a brief infusion or bolus, the CST will initially be short. As the duration of infusion continues, redistribution becomes progressively less important and the CST increases. Therefore, fentanyl becomes a very long-acting drug if given at high infusion rates for many hours because it has a large peripheral compartment (V3) and redistribution is rapid (in contrast with propofol) and so plasma concentrations are well maintained despite rapid excretion.
Because of much greater lipid solubility, it is likely that this drug has a higher volume of distribution than morphine.
In the first year of life, are the following haematological features true?
True/False
The survival of erythrocytes is longer in infants than in adults
Erythropoietin production starts after 3 months Incorrect answer selected
Gamma chains are found in fetal haemoglobin (HbF)
The oxygen dissociation curve (ODC) for fetal haemoglobin lies to the right of adult haemoglobin
Sickle cell disease does not present in the neonatal period
False
False
True
False
True
Explanation
Fetal haemoglobin (HbF) differs from adult haemoglobin by gamma chains replacing the beta chains.
The oxygen dissociation curve for HbF is to the left of adult haemoglobin (not right), facilitating the transfer of oxygen from the maternal circulation to the fetus.
HbF is gradually replaced by adult haemoglobin during the first 6 months of life. Thus sickle cell disease does not present during the neonatal period and screening only needs to be performed after the age of 3 months (when the proportion of adult haemoglobin has started to increase).
Erythropoietin production starts in utero, and then production falls after birth with an increase seen at about 3 months resulting in a reticulocytosis.
Erythrocyte survival is shorter in neonates (not longer) than in adults.
Which of the following best describes the physical principles of temperature measurement by a thermocouple?
(Please select 1 option)
The bimetallic strip has a coefficient of expansion that is proportional to temperature The bimetallic strip has a junction potential proportional to temperature The semiconductor at the measuring end has a junction potential that is proportional to temperature The bimetallic strip has a resistance that is proportional to temperature The semiconductor at the measuring end has a resistance proportional to temperature
The bimetallic strip has a junction potential proportional to temperature
Physics and clinical measurement
A thermocouple is constructed from a pair of dissimilar metals. The magnitude of the thermojunction electromotive force at the measuring end is proportional to applied temperature.
Explanation
A thermocouple is a device that is used to measure temperature “electronically”. It consists of a pair of dissimilar metal (bimetallic) wires or strips bonded together, typically copper and constantan (an alloy of 55% copper and 45% nickel). When these wires contact each other a small voltage is generated in the order of millivolts. The magnitude of the thermojunction electromotive force (emf) is proportional to applied temperature (the Seebeck effect). This physical principle can be applied to the measurement of temperature. The electromotive force at the measuring junction is proportional to temperature.
Two wires bonded together with different coefficients of expansion can be used as a switch for thermostatic control.
The resistance of the measuring junction of a thermocouple is irrelevant. The thermocouple does not employ semiconductor materials for temperature measurement.
Iron is an essential micronutrient in the human body.
Which one of the following options is the most abundant store of iron in the body?
(Please select 1 option)
Ferritin and haemosiderin Haem enzymes Haemoglobin Transferrin bound iron Myoglobin
Haemoglobin
Key Learning Points
Physiology and biochemistry
Haemoglobin is the most abundant store of iron in the body
Explanation
Iron is an essential micronutrient, as it is required for adequate erythropoietic function, oxidative metabolism and cellular immune responses. Although the absorption of dietary iron (1-2 mg/d) is regulated tightly, it is just balanced with losses.
There are 35-45 mg/kg iron in the adult body (about 4-5 g)
Iron is present in the following forms:
Haemoglobin 65%
Ferritin and haemosiderin 30%
Myoglobin 3.5%
Haem enzymes 0.5%
Transferrin bound proteins 0.1%
With regard to sodalime, which one of the following statements is correct?
(Please select 1 option)
The absorption of CO2 is an endothermic reaction The main component is calcium carbonate 4-8mesh is a metric measurement of granule size 100 grams of soda lime can absorb up to 26 litres of CO2 Passing sevoflurane through dry sodalime can generate clinically significant amounts of carbon monoxide (CO)
100 grams of soda lime can absorb up to 26 litres of CO2
Key Learning Points
Physics and clinical measurement
The main components of sodalime depend on the manufacturer.
Explanation
The main components of sodalime depend on the manufacturer. Typically it will be composed of:
Calcium hydroxide (93-97%)
Sodium hydroxide (0-3%)
Potassium hydroxide (0-1%)
Hardeners - Silicates (0-7.5%), Zeolite (0-5%)
Indicator (0.03%)
Water (13-17%)
Size of granules 4-8 mesh
First step:
Carbon dioxide and the water contained in the soda lime react to form carbonic acid.
CO2 + H2O = H2CO3
Intermediate step:
Carbonic acid reacts exothermically with sodium hydroxide to form sodium carbonate and water.
H2CO3 + 2 NaOH = Na2CO3 + 2 H2O + heat
Final step:
Sodium carbonate reacts with the slaked lime to form calcium carbonate and sodium hydroxide: Na2CO3 + Ca(OH)2 = CaCO3 + 2 NaOH
100 grams of soda lime can absorb up to 26 litres of CO2.
Desflurane and enflurane have been reported to produce substantial amounts of carbon monoxide (CO) when passed through dry sodalime. Isoflurane is said to produce less CO. Sevoflurane and halothane produce very little CO.
The granule size range of 4 to 8 mesh is an imperial measurement (inches) allows high absorptive capacity at low resistance to gas flow.
Typical features of aspirin overdose include:
True/False
Hyperventilation
Tinnitus
Hypothermia
Respiratory acidosis
Hyperkalaemia
True
True
False
False
False
Explanation
Severe aspirin toxicity occurs with serum concentrations >750 mg/L.
It may present with:
Tinnitus
Deafness
Pyrexia
Hypoglycaemia
Haematemesis
Hyperventilation, and
Hypokalaemia
Aspirin overdose typically produces a respiratory alkalosis due to a direct stimulatory effect on the respiratory centre. To compensate for this there is a loss of bicarbonate from the urine with loss of sodium, potassium and water.
Aspirin and NSAIDs can cause hyperkalaemia by inducing a hyporenin hypoaldosteronemic state caused by inhibition of prostaglandin synthesis but this is not typical, especially if the patient is euvolaemic and has normal renal function.
Do the following drugs inhibit enzymes?
True/False
Piroxicam
Allopurinol
Trandolapril
Acetazolamide
Cimetidine
True
True
True
True
True
Explanation
Acetazolamide is a carbonic anhydrase inhibitor and a weak diuretic. It is used for prophylaxis against mountain sickness and in the management of glaucoma.
Allopurinol inhibits the enzyme xanthine oxidase which is responsible for urate synthesis and is used in the management of gout.
Piroxicam is a non-steroidal anti-inflammatory drug (NSAID) that inhibits cyclo-oxygenase.
Trandolapril is an angiotensin-converting enzyme inhibitor (ACE inhibitor).
Cimetidine is a histamine type-2 receptor antagonist but as a side effect it inhibits the mixed function oxidases (cytochrome P-450s) and so retards oxidative hepatic drug metabolism (phase 1 reactions).
Are the following statements true or false?
True/False
100 ml of arterial blood carries 47.5 ml CO2 in the form of bicarbonate
The chloride shift is the movement of chloride ions from red blood cells into the plasma
The Haldane effect describes the reduced buffering capacity of haemoglobin once deoxygenated
The Anrep effect relates to the heart rate response to hypoxia
The Bohr effect relates to increased carbon dioxide carriage by deoxygenated blood
False
False
False
False
False
In the red blood cell carbon dioxide is converted by carbonic anhydrase to carbonic acid, which in turn dissociates to hydrogen and bicarbonate ions.
The chloride shift (or Hamburger shift) is the movement of chloride ions into red blood cells (not plasma) as bicarbonate ions enter the plasma, which maintain electrical neutrality. The hydrogen ions are buffered mainly by haemoglobin.
The Haldane effect refers to haemoglobin’s increased buffering ability as it becomes deoxygenated.
The Bohr effect is the rightward shift of the oxyhaemoglobin dissociation curve associated with a rise in arterial PCO2.
In arterial blood approximately 50 ml of carbon dioxide is carried per 100 ml of blood: 45 ml as bicarbonate, 2.5 ml as carbonic acid and 2.5 ml as carbamino compounds. Venous blood carries 54 ml of carbon dioxide and 47.5 ml of this is as bicarbonate.
The Anrep effect describes the intrinsic regulatory mechanism of the heart in response to an increased afterload.
Cellular respiration involves an energy substrate and oxygen with the production of carbon dioxide and water. The respiratory quotient for a particular patient is measured as 0.7.
Which of the following is the most likely combination of energy substrates of this patient’s diet?
(Please select 1 option)
High carbohydrate, high fat and low protein Low carbohydrate, high fat and low protein Low carbohydrate, low fat and high protein High carbohydrate, high fat and high protein High carbohydrate, low fat and low protein
Low carbohydrate, high fat and low protein
Key Learning Points
Physiology and biochemistry
The RQ for a diet that is predominantly fat is 0.7. Lipids require more oxygen than carbohydrates for complete oxidation.
The Respiratory quotient (RQ) is the ratio of CO2 produced by the body to the volume of O2 consumed per unit time.
RQ = CO2 produced / O2 consumed.
Typically 200 mL/minute CO2 produced and 250 mL/minute O2 consumed. A mixed diet will typically produce and RQ of about 0.8.
The RQ will vary with the energy substrates in the diet.
Granulated sugar is a pure refined carbohydrate and is 99.999% carbohydrate with no significant lipid, proteins, minerals or vitamin content.
Glucose and other hexose sugars - RQ = 1
Fats - RQ = 0.7 (lipids require more oxygen than carbohydrates for complete oxidation)
Proteins - RQ is approximately 0.8-0.9
Ethyl alcohol - RQ = 0.67
Pure water at pH 7 has hydrogen ion concentration of:
(Please select 1 option)
40 nanomol/L 1000 nanomol/L 100 nanomol/L 70 nanomol/L 0 nanomol/L
100 nanomol/L
Key Learning Points
Physics and clinical measurement
pH is the negative log to the base 10 of hydrogen ion concentration. The relationship between hydrogen ion concentration and pH is exponential.
pH is the negative log to the base 10 of hydrogen ion concentration:
Logarithms
Ask the question what power produces the answer
pH = - log10 [H+], rearranging the equation:
[H+] = 10-pH or substituting pH, [H+] = 10-7
One nanomol = 1 x 10-9 or 0.000000001
10-7 = 1x 0.0000001 or 10 x 0.00000001 or 100 x 0.000000001
100 nanomol
Do the following respiratory changes occur in a normal pregnancy?
True/False
Respiratory alkalosis
Tidal volume increases
Functional residual capacity is increased
Lung compliance is reduced
Plasma bicarbonate is reduced
True
True
False
False
True
There is a reduction in total respiratory compliance due to a decrease in chest wall compliance, but lung compliance is not altered.
The functional residual capacity (FRC) is reduced by 20% (not increased), which is mainly due to elevation of the diaphragm.
Progesterone stimulates the respiratory centre producing primarily an increase in tidal volume with a relatively constant respiratory rate, this leads to a respiratory alkalosis which is compensated for by a reduction in plasma bicarbonate levels.
A 30-year-old woman presents with vertical diplopia and difficulty reading and walking downstairs. On examination, her left eye appears slightly elevated and adducted. She reports that tilting her head to the right improves her vision.
Which one of the following cranial nerve palsies is most likely?
(Please select 1 option)
Abducens nerve palsy Optic nerve palsy Trochlear nerve palsy Trigeminal nerve palsy Oculomotor nerve palsy
Trochlear nerve palsy
Key Learning Points
Anatomy
Trochlear nerve palsy leads to vertical diplopia and patients often adopt a head tilt to compensate. Recognising the clinical presentation helps in diagnosing and managing cranial nerve palsies effectively.
Trochlear nerve palsy: This presents with vertical diplopia and compensatory head tilt (to the side opposite the lesion) to align the vision, as the superior oblique muscle is affected, causing the eye to be elevated and adducted.
Abducens nerve palsy: This would cause horizontal diplopia and difficulty abducting the eye but would not typically cause vertical diplopia or issues with head tilt.
Oculomotor nerve palsy: This would cause the eye to be down and out with ptosis, rather than the symptoms described.
Optic nerve palsy: This would result in visual loss, not diplopia or issues with head tilt.
Trigeminal nerve palsy: This affects facial sensation and mastication muscles, not eye movement.
Which one of the following is the primary role of a capacitor in an AC circuit?
(Please select 1 option)
To measure the frequency of the AC signal To convert electrical energy into heat To provide a steady output voltage To block direct current while allowing alternating current to pass To act as a power source
To block direct current while allowing alternating current to pass
Key Learning Points
Physics and clinical measurement
In AC circuits, capacitors are used to block direct current while allowing alternating current to pass through. This characteristic makes them essential in applications like coupling and decoupling in signal processing and filtering unwanted noise from signals.
Explanation
Capacitors block direct current (DC) because they cannot maintain a constant flow of charge, but they allow alternating current (AC) to pass through because they can continuously charge and discharge in response to the changing voltage.
Power sources provide the necessary energy to power a circuit, such as batteries or power supplies. Capacitors store and release electrical energy but do not generate it.
Capacitors can smooth voltage fluctuations but do not regulate voltage on their own.
Measuring the frequency of an AC signal is typically done with frequency counters or oscilloscopes. Capacitors do not have the capability to measure frequency.
Capacitors store and release electrical energy but do not convert it into heat as their primary function.
Are the following true regarding labetalol?
True/False
Has alpha blocking action
Decreases bile secretion
Has a half life of two hours
Causes bronchodilation
Is 70% protein bound
Causes bronchodilation
Explanation
Labetalol is a combined beta and alpha adrenergic receptor antagonist with a ratio of activity between 2:1 and 5:1 respectively.
It is selective for alpha-1 receptors but is non-selective for beta receptors.
It is used to treat severe hypertension and pre-eclampsia and in hypotensive anaesthesia.
Labetalol has a half life of approximately four hours (not two) and is approximately 50% protein bound.
It has been shown to cross the placental barrier, but not the blood brain barrier. It is metabolised in the liver and excreted in the urine and faeces.
Oral administration of the drug undergoes extensive first-pass metabolism.
Severe hepatocellular damage has been reported after both short and long term use and the reduction of bile secretion may rarely lead to jaundice.
Which one of the following is the primary role of a capacitor in an AC circuit?
(Please select 1 option)
To block direct current while allowing alternating current to pass To provide a steady output voltage To convert electrical energy into heat To measure the frequency of the AC signal To act as a power source
To block direct current while allowing alternating current to pass
Key Learning Points
Physics and clinical measurement
In AC circuits, capacitors are used to block direct current while allowing alternating current to pass through. This characteristic makes them essential in applications like coupling and decoupling in signal processing and filtering unwanted noise from signals.
Explanation
Capacitors block direct current (DC) because they cannot maintain a constant flow of charge, but they allow alternating current (AC) to pass through because they can continuously charge and discharge in response to the changing voltage.
Power sources provide the necessary energy to power a circuit, such as batteries or power supplies. Capacitors store and release electrical energy but do not generate it.
Capacitors can smooth voltage fluctuations but do not regulate voltage on their own.
Measuring the frequency of an AC signal is typically done with frequency counters or oscilloscopes. Capacitors do not have the capability to measure frequency.
Capacitors store and release electrical energy but do not convert it into heat as their primary function.
In a clinical trial of a new intravenous induction agent, the plasma concentrations of this drug are taken at regular time intervals following a bolus.
The following data are acquired:
Time following injection (hours) Plasma concentration (mcg/mL)
2 400
6 100
10 25
14 6.25
From these measurements which one of the following options approximates best to the plasma half life (T½) of the drug?
(Please select 1 option)
4 hours 1 hour 10 hours 2 hours 8 hours
2 hours
Key Learning Points
Pharmacology
The plasma half life (T½) of a drug is the time it takes for the initial plasma concentration to fall by 50%.
Explanation
The plasma half life (T½) of a drug is the time it takes for the initial plasma concentration to fall by 50%.
Once injected into the central compartment (intravascular space) the concentration of the intravenous induction agent decreases exponentially as it is eliminated and redistributed.
Extrapolating the values from the plasma concentration vs. time data:
Plasma concentration at time zero = 800 mcg/mL
Plasma concentration at 2 hours = 400 mcg/mL
Plasma concentration at 4 hours = 200 mcg/mL
Plasma concentration at 6 hours = 100 mcg/mL
Plasma concentration at 8 hours = 50 mcg/mL
Concerning the femoral nerve and its branches, which statement is correct?
(Please select 1 option)
Lies within the femoral sheath A branch supplies the skin of part of the foot Is formed from the dorsal divisions of the L1-L4 anterior rami of the spinal cord Lies medial to the femoral artery in the femoral triangle Has a branch which supplies the skin of the scrotum
A branch supplies the skin of part of the foot
Key Learning Points
Anatomy
The femoral nerve is formed from the dorsal divisions of the L2-L4 ventral rami of the spinal cord and runs down the medial side of the leg, supplying the medial side of the calf and the medial side of the dorsum of the foot, before terminating in the region of the ball of the big toe.
Explanation
The saphenous nerve, a branch of the femoral nerve, runs down the medial side of the leg and supplies the medial side of the calf as far as the medial malleolus. It terminates in the region of the ball of the big toe and may supply the medial side of the dorsum of the foot. The scrotum derives its sensory supply from the pudendal, posterior femoral cutaneous, genitofemoral and ilioinguinal nerves. In the femoral triangle the femoral nerve is lateral to the artery which in turn is lateral to the femoral vein. It lies outside the femoral sheath which “houses” the femoral artery and vein, lymph nodes, femoral canal and loose connective tissue. The femoral nerve is s formed from the dorsal divisions of the L2-L4 ventral rami of the spinal cord.
Which of the one of the following statements about the adult human trachea best applies?
It is a midline structure that bifurcates at T4
The right main bronchus angles off the trachea at 45°
It is a midline structure that starts at C4
The left main bronchus angles off the trachea at 25°
It is approximately 25 cm long incorrect option
It is a midline structure that bifurcates at T4
Key learning points
Anatomy
The human trachea is approximately 15 cm long and bifurcates at the level of T4, with the right main bronchus separating at a 25° angle and the left main bronchus separating at a 45° angle.
Explanation
The human trachea starts at C6 at the inferior edge of the cricoid cartilage and extends to the level of T4 where the trachea bifurcates. It is approximately 15 cm long in an adult. The right main bronchus separates from the trachea at a 25° angle, whereas the left main bronchus separates at an angle of 45°.
Is it true or false that the approximate pH of intravenous fluids is as follows?
True / False
3% hypertonic saline is 7.4
Hartmann’s solution is 6.5
0.9% sodium chloride is 5.5
5% glucose is 6.8
Gelofusine is 7.4
False
True
True
False
True
Explanation
The pH values of intravenous fluids may vary slightly between the different manufacturers, but approximate pH values (+/- 0.3) are as follows:
Sodium chloride 0.9% is 5.5
Hartmann’s is 6.5
Glucose 5% is 4.15
Gelofusine is 7.4
3% hypertonic saline is 5.0
Are the following statements true or false about volatile agents?
True/False
Desflurane has a lower boiling point than halothane
Sevoflurane has a higher minimal alveolar concentration (MAC) than desflurane
Isoflurane and enflurane have the same minimal alveolar concentration (MAC)
Halothane and isoflurane have a similar saturated vapour pressure (SVP)
Sevoflurane has a higher molecular weight than desflurane
True
False
False
True
True
Isoflurane and enflurane are structural isomers and have the same molecular weights, but different minimal alveolar concentration (MAC) values.
Sevoflurane has a molecular weight of 200, desflurane has a molecular weight of 168. The MAC values of sevoflurane and desflurane are 1.8 and 6 respectively.
At 23.5°C, desflurane has the lowest boiling point of all the volatile agents in common use, and requires a specialised vapouriser.
Halothane has a boiling point of 50°C.
Saturated vapour pressure (SVP) is an indicator of volatility and is measured in kPa. The SVP is similar for both halothane and isoflurane at approximately 32kPa (240 mmHg).
