Anaesthetics Flashcards
What is a critical illness?
Any patient at risk of serious deterioration/ has actual or impending organ failure
Give a differential of airway obstruction?
- reduced GCS
- foreign body
- aspiration
- oedema - anaphylaxis, burns etc
- bronchospasm
- pulmonary oedema etc
Wheeze is inspiratory
Stridor is expiratory
See-saw breathing e.g. paradoxical chest and abdominal movements are a sign of complete airway obstruction
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What is the tidal volume?
The volume of air that moves in and out per breath
usually about 500ml
What is the inspiratory reserve volume?
IRV is the amount of air above the tidal volume that a person can maximally inhale
(often around 3L)
What is the expiratory reserve volume?
The maximum amount, above the tidal volume that a patient can exhale
(normally about 1.2L)
What is the residual volume?
The amount of air left in the lungs after maximal exhalation
normal 1.2L
What is the functional residual capacity?
The FRC is the volume of air left in the lungs after normal expiration
(Normally 2.4 L)
What is the vital capacity?
The maximal amount of air that can be exhaled after a maximum inhale.
(Normally 4.7L)
What is an atomic dead space in the lungs?
All parts of the conducting airways e.g. trachea apart from alveoli which are not involved in gas exchange
About 1/3 of the tidal volume is lost to dead space
Physiological dead space - anatomical dead space + functional dead space (some alveoli do not have a good enough blood supply). How is it calculated?
Using the Bohr equation
e.g. physiological dead space may be increased in emphysema
How do you calculate the minute volume?
Tidal volume x respiratory rate
What is alveolar ventilation?
The portion of minute volume involved in gas exchange e.g. Minute volume - dead space
What is a shunt?
When perfusion exceeds ventilation e.g. there is a problem with the airways
What is dead space with regard Q/V?
When ventilation exceeds perfusion e.g. no oxygen can be gained as blood supply is inadequate
Examples include pulmonary oedema, pneumonia, ptx etc
CO2 is removed via alveolar ventilation (down a pressure gradient)
CO2 retention suggests low alveolar ventilation)
Some patients develop hypercapnia secondary to reduced hypoxia drive. What is the commonest cause?
Opiod analgesics which decrease respiratory drive
COPD is the other important cause
Partial pressure is the pressure that would be exerted by 1 gas in a mixture if it occupied an area on its own.
The partial pressure of 02 decreases from 21kPa in air to just 1 - 5 kPa in mitochondria
What are the 4 different types of hypoxia?
1) hypoxic hypoxia. —>pO2< 12 due to low ventilation, shunt or ventilation problems Vv
2) anaemic hypoxia —> anaemia or CO poisoning
3) Ischaemic hypoxia —> O2 OK but reduced blood flow e.g. clot or cardiac failure
4) Cytotoxic hypoxia —> tissues are unable to use O2 e.g. cyanosis
GIVE 3 clinical signs that suggest patients are ‘at risk’ of respiratory failure?
1) RR > 25
2) Sp02 < 94%
3) PaO2 <10 (normal is > 10.6)
Which 2 investigations should be performed in all patients with respiratory failure?
ABG
CXR
What is the difference between a fixed and variable performance oxygen device?
- fixed e.g. Venturi deliver fixed oxygen concentration regardless of respiratory effort. Should be used if you want to know the FiO2
- variable performance e.g. nasal cannulae - the O2 delivery depends on patient respiratory effort and size
Normal Fi02 = 0.21 as 21% of normal air is oxygen
This in increased to 0.28-0.36 by nasal cannula
Hudsons masks (the common ones) provide FiO2 of 0.4
Trauma masks e.g. non-rebreather masks provide an FiO2 of 60-80% but need a high flow rate (10-15L/min) to achieve this
Hudson masks use a flow of >4L/ min
Nasal cannula = 2-4L per minute
What are the target PaO2?
For a patient on air the PaO2 should be 10-13kPa
For a patient on oxyge, the PaO2 should be 10kPa less than the FiO2 concentration e.g. target Pao2 of 30 if FiO2 is 0.4
Venturi masks are the main fixed performance device
Remember the masks tell you what flow rate is required to get desired FiO2
Last reminder:
Shunt = inadequate ventilation e.g. wasted perfusion
Dead space = wasted ventilation as perfusion is inadequate
Hypoxia is usually due to s shunt
Shock is the failure to perfume vital organs
Shock is failure to perfume vital organs
What is the equation of oxygen delivery?
Cardiac output x arterial oxygen concentration
(remember that arterial oxygen concentration is a product of oxygen saturation x 1.34
What effect will systemic vascular resistance have on CO?
It will increase the afterload which decreases CO
What is the difference between BP and CO?
BP = pressure of blood in the systemic circulation
CO = flow of blood through the circulation
—> BP = CO x systemic vascular resistance
Cardiogenic shock = pump failure, typically due to reduction in SV. List some causes
- MI
- Acute valve incompetence
- VT
- Complete Heart Block
It is the contractility that is primarily affected
Obstructive shock is a reduced CO due to reduced SV as a result of compression. Give some examples?
PE
Cardiac tamponade
Tension ptx
It is the preload/ afterload that is primarily affected e.g. heart cannot fill or cannot empty
Distributive shock e.g. in anaphylaxis/ sepsis is caused by peripheral vasodilatation which reduces SVR —> fall in BP
Important that although CO increases, the tissue distribution is inappropriate so shock occurs (as well as reduced BP)
Where are arterial baroreceptors located?
1) Aortic arch (vagus)
2) Carotid sinus (glossopharyneal)
Remember that angiotensin 2 is potent vasoconstrictor.
It is produced in the lungs after conversion from angiotensin 2. Angiotensin 2 is produced by the action of renin on angiotensin 1
Angiotensin 11 also helps Na and water reabsorption
What are the 3 main consequences of shock?
1) Cellular hypoxia/ organ hypoperfusion
2) Effects of lactic acidosis
3) Inflammatory response
Normally glucose is metabolised to pyruvate the normal way, producing 36 molecules of ATP
In anaerobic metabolism, glucose is converted to lactate acid, producing only 2 molecules o ATP
(And eventually a metabolic acidosis)
What is the difference between severe sepsis and septic shock?
Severe sepsis = sepsis + evidence of end organ damage/ hypoperfusion
Septic shock = sepsis + refractory hypotension
List some signs of hypovolaemia
- tahcycardia
- hypotensive
- postural dizziness
- cool, slow cap refill
- oliguria
Signs of fluid overload
- SOB, crackles
- ankle oedema
- hypertension
- Increased JVP
- tachycardia (don’t forget)
Signs of sepsis:
- flushed, sweaty
- hypotensive
- tachycardia
- tachypnoea
- reduced LOC
- oliguria
- signs of underlying cause
- fever
Sepsis should ALWAYS be considered as a cause of hypovolaemia, especially if there is a poor response to fluid challenge
Sepsis should ALWAYS be considered as a cause of hypovolaemia, especially if there is a poor response to fluid challenge
What is the difference between crystalloid and colloid fluid?
Crystaloid = in solution can diffuse through a semi-permeable membrane e.g. saline
Colloid = are a suspension rather than a solution and will therefore not diffuse through a semi-permeable membrane
What is the defining abnormality of hypervolaemia?
A raised preload
In a bradycardic patient, what features on ECG suggest risk of asystole and therefore need for atropine?
1) Mobitz 11
2) Recent asystole
3) Complete heart block
4) Ventricular pause >3s
Patient with HR of 180 and signs of shock. After getting ECG etc what do you do?
Patient is unstable so up to 3 DC shock
If unsuccessful then give amiodarone 300mg over 10-20 minutes
Regular broad complex tachycardia = VT (amiodarone)
Regular narrow complex tachycardia = SVT (vagal and adenosine)
Course of action in an unconscious patient with a bedside glucose reading of <3.5mmol/L
Give 100ml of 10% glucose IV
(If any evidence of malnutrition or alcoholism, given 1mg/kg thiamine at the same time to prevent Werncike’s encephalopathy - confusion, opthalmoplegia and ataxia)
What is the triad of opiate toxicity?
Unconscious
Pin-point pupils
Respiratory depression
(consider naloxone - 0.4mg IV)
Always remember to use BEST response when considering GCS
The patient could have a focal weakness or similar
List 6 important causes of unconsciousness which require urgent investigation and management?
1) Sepsis/ shock
2) Drugs/ opiates
3) Raised ICP
4) Hypoxia
5) Meningitis/ encephalitis
6) Hypoglycaemia
What are the 3 areas of consideration when prescribing fluids?
1) Basal requirement
2) Existing deficit
3) Predicted losses
What is in saline?
154 of Na and Cl
NaCl goes into the extracellular fluid e.g. interstitial and plasma BUT beware in hypernatraemia
What is in Hartmann’s?
Na 131
Cl 110
K 5
Just like NaCl it goes into the ECF
Why is 5% dextrose rubbish for resuscitation?
It is essentially just water so equilibrates between ICF and ECF equally
What is in gelofusine?
0.9% NaCl —> 154 Na, 154 Cl
+ Big molecules which exert an oncotic pressure
What is pain?
‘An unpleasant sensory of emotional experience associated with actual or potential tissue damage
Remember pain activates the sympathetic nervous system, prevents deep breathing/ cough reflex and reduces mobility —> DVT, pressure sores etc
Remember pain activates the sympathetic nervous system, prevents deep breathing/ cough reflex and reduces mobility —> DVT, pressure sores etc
Describe the Tayside 0-3 pain scale
0 - no pain at rest or movement
1 - no pain at rest, mild on movement
2 - intermittent at rest, moderate on movement
3 - continuous at rest, severe on movement
Remember we normally step up from paracetemol —> paracetemol + opioid —> opioid + non-opioid
In post-op pain we start at the top and step down
What is the standard way to treat acute pain?
1) Dilute morphine to a 1mg in 1ml solution
2) Titrate in 1-2mg increments until patient is comfortable
In opioid induced poisoning naloxone is given as a bolus of 400 micrograms
In opioid induced sedation e.g. excess post-op analgesia it is given in 40 microgram increments with continual reassessment
What are the side effects of an epidural?
1) post-dural puncture headache
2) hypotension
3) infection
4) epidural haematoma
5) inadequate relief
What is the standard PCA prescription?
1mg morphine with 5 minute lockout
How do you convert from IV to oral morphine?
Double it e.g. 60mg IV = 120mg oral
What is isoflurane?
A general anaesthetic
Ranitidine is a gastric acid inhibitor
ACTRAPID is an anti-hyperglycaemic
Salbutamol is a beta 2 agonist which can cause hypokalaemia
Salbutamol is a beta 2 agonist which can cause hypokalaemia
Remember antibiotics generally increase activity of warfarin (bleeding more likely)
OCP, carbamazepine and barbituates generally reduce activity
Atenolol is a cardioselective beta 1 receptor antagonist
Reduce heart rate and contractility
Can mask signs of hypoglycaemia
How does morphine work?
It binds to the mu opiod receptor and stimulates dopamine release
Morphine has extensive 1st pass metabolism —> t1/2 = 3 hours
Benzos work by binding to and enhancing the activity of GABA receptors
Remember GABA is the main inhibitory neurotransmitter
Glutamate is main excitatory transmitter
What does a central line measure?
It is placed in the subclavian vein and then threaded to the SVC
It measures central venous pressure in the RA —> preload
What is an intra-arterial line used for?
1) continuous BP measurement
2) continuos ABG
3) can be used for drawing blood samples
How does paracetamol cause problems?
1) Normally it is absorbed, conjugated and peed out
2) In overdose, the conjugation is overwhelmed and paracetamol is metabolised via an alternative pathway —> NAPQI
(N-acetyl - p - benzoquinine imine)
3) This is initially OK as it is inactivated by glutathione
4) BUT glutathione is used up quickly —> free NAPQI which causes necrosis of the liver/ kidneys
5) Toxicity is increased with other activators of the P450 system e.g. alcohol
Doses for the treatment of anaphylaxis?
1) Adrenaline = 500 micrograms
2) Fluid challenge 500ml
3) Hydrocortisone 200mg
4) Cholrphenamide 10mg
PR interval
AV conduction time
Normally 0.12-0.2 (3-5 small squares)
what should be given to a post patient with a RR of 6 who is drowsy and difficult to rouse due to opioids?
Naloxone - give 40 microgram increments, titrated to effect
when is warfarin stopped prior to elective surgery?
5 days
which drug do you give in asystole?
1mg adrenaline IV
what is the typical blood gas in a patient who has had a sudden cardiac arrest?
respiratory and metabolic acidosis due to apnoea and ischaemia
Where is the epidural space?
outside the dura mater (epidural is the same as extra-dural)
What is in the epidural space?
Adipose tissue
Spinal nerve roots
venous plexus
Why does an epidural cause reduction in CO and BP?
it blocks the sympathetic system –> vasodilatation
