Monitoring anaesthesia Flashcards
3 components of balances anaesthesia
unconsciousness, analgesia and adequate mm relaxation
Goals of anaesthetic monitoring - 4
- ensure all components of balanced anaesthesia are provided
- maintain homeostasis
- detect any adverse effects of anaesthesia and correct immediately
Other than the patient, what else should the anaesthetist also monitor? 3
(Patient first)
Anaesthetic machine
Breathing system (+/- ventilator)
IV fluids
When do most anaesthetic deaths occur?
within 3 hours of the end of the procedure
What determines the level of monitoring?
Physical condition (animal) Surgical procedure planned (intense then use both invasive and non-invasive measures)
What is the anaesthetic record?
What should it include? 7
legal document
shows trends
reference for future anaesthetics
Should include:
Drugs (time, dose, route)
All intra-operative monitoring (blood tests, blood loss and urine output)
IV fluids (rate, type)
All procedures (intubation, invasive monitors)
Time of important events (induction, position, timing, extubation)
Unusual events/complications
Condition of animal at end of procedure.
What is the blood volume of dogs and cats?
Why is this important?
Dogs = 80ml/kg Cats = 60ml/kg
(useful to calculate and note the values for blood loss equal to 10%, 15% and 20% of blood volume to help in decision making as to whether to give crystalloids, colloids or blood)
What should you do if an anaesthetic machine doesn’t have low oxygen warning alarms fitted?
check cylinder (or pipeline) pressure and oxygen flowmeters constantly. If using NO2 on older machines without anti-hypoxic devices, it is also essential to check the O2:NO2 ratio regularly to ensure you aren’t delivering a hypoxic mixture to the patient.
What other machine checks should be made?
- Vaporiser settings and level of volatile anaesthetic in vaporiser
- Breathing system (+/- mechanical ventilator) for operation adn disconnedtion
- Fluid administration (rate, contents)
What 2 readings need to be measured by equipment during anaesthesia?
Arterial BP
End-tidal CO2
What can you listen for when monitoring anaesthesia? 5
Ventilation sounds Leaks (ETT or breathing system) Low oxygen pressure warning alarms Malfunctioning equipment Communications
What can you look for when monitoring anaesthesia? 3
Eye position
Chest movements
Colour of MMs
What can you feel for when monitoring anaesthesia?
Pulses - quality, rate, rhythm CRT Jaw tone Palpebral reflex Relaxation - limbs and reflexes
What can you smell for when monitoring anaesthesia?
Detecting volatile anaesthetic agent which would indicate leaks or disconnections
What is the anaesthetist’s first priority when anaesthesia is induced?
ABC assessment:
AIRWAY - placement of ETT, secured, cuff inflated
BREATHING - rapid, slow, regular/irregular
PULSE - strong, weak, thready, absent
Then once you are confident in the patient’s condition, make a quick assessment of the patient’s depth
How do you assess patient’s depth under anaesthesia? 4
- Eye position, nystagmus (species and drug dependent) lacrimation, pupil size and response to light
- Palpebral reflex - absent/present, brisk/sluggish, other CN reflexes.
- Muscular relaxation - jaw tone etc.
- EEG changes
What physiological parameters need to be monitored during anaesthesia?
HR
Pulse rate
RR, depth, quality
What can be used in small animals for physiological parameter monitoring?
oseophageal stethoscope - for heart and respiratory sounds.
What clinical observations should be monitored? 8
Depth of anaesthesia MMs Pulse Chest movements Pupil size Response to surgery Blood loss (Urine output)
What equipment observation should be monitored? 6
Respiratory gases Pulse oximeter Blood pressures ECG Thermometer Blood gases
What does an ECG inform you of?
electrical activity of the heart
What does an ECG not inform you of?
mechanical activity of heart
CO
BP
T/F: after a patient has been euthanased under GA, a normal ECG trace may remain for several minutes
True
Why is ECG monitoring recommended under GA?
cardiac arrhythmias occur in over 80% anaesthetised patients, most not clinically significant. most familiar departure from normal is sinus arrhythmia (so common it cannot be considered an abnormality, CO should be unaffected). abnormal ECGs have more significance if they indicate atrial contraction isn’t followed by ventricular contraction (2nd degree AV block) or ventricular activity is not preceded by atrial contraction (3rd degree AV block).
When might AV blocks be seen under GA? 2
- bradycardic patients
- if sedated with an alpha-2-agonist
= so long as arterial BP is satisfactory,it may not be significant
What might VPCs be a warning of? 5
hypoxia hypercapnia acidosis underlying heart disease others - surgery to remove splenic tumours
Why should VPCs be monitored closely?
worsening can progress to ventricular fibrillation (VF)
What is the most common cause of delayed recovery from anaesthesia?
Hypothermia - can also cause other problems (bradycardia, increased susceptibility to infection, risks increasing the anaesthetic depth inadvertantly). Smaller the patient, the greater the risk (increased surface area to mass ratio)
What is malignant hyperthermia (MH)?
very rare, genetic condition
triggered by used of older anaesthetics (halothane, suxamethonium)
What is core temp. most effectively measured by?
thermistor probe in oesophagus (slightly more reliable) or rectum (slightly less reliable)
When does heat loss occur?
after pre-med
preparation (especially alcohol-based scrubs)
surgery (especially opening of body cavity)
List examples of warming devices
bubble wrap blankets (reflective) hot air blankets water blankets warmed IV fluids warmed fluids for flushing
Why be careful applying direct heat?
burning risk
increased vasodilation (further reduces core temperature)
hypotension
How can respiratory gases be monitored? 3
Inspired oxygen concentration
Capnography
Anaesthetic agent concentration.
What information does inspired oxygen concentration inform you of?
Method?
early warning that hypoxic gas mixtures are being delivered
METHOD = fuel cell fitted at the common gas outlet with a machine that measures % of oxygen OR can be measured at the end of the ETT via sidestream analysis (usually capnography) which allows measurement on a breath-by-breath basis (gives values for both inspired and end-tidal oxygen concentration, as a %).
What is the minimum concentration of oxygen that is usually provided during anaesthesia to small animals and large animals?
Smallies - minimum 30% inspired oxygen concentration
Large animals - minimum of 60% oxygen normally used.
This ensures that hypovetilation alone is unlikely to cause hypoxaemia.
Define capnography
measure of CO2 concentrations in respiratory gases - arguably the most useful monitoring equipment during anaesthesia - provides information on Resp, CVs and anaesthetic breathing systems
How can CO2 concentrations in respiratory gases be measured (capnography)?
- ) most commonly involves sidestream technology - gases are continuously suctioned from the breathing system (rate of about 150ml/min) into a sample cell within monitor (this effectively creates a leak in the breathing system).
- ) mainstream techology
* always remember to scavenge on a capnograph*
What are the normal components of a capnogram?
Phase 1 = inspiratory baseline
Phase 2 = Expiratory upstroke (mix of dead space and alveolar gas)
Phase 3 = alveolar plateau (not horizontal line but continues to rise gradually. End-tidal value for CO2 is taken at the peak of phase 3)
Phase 4 = inspiratory downstroke
Diffrentiate capnographs and capnometers.
Capnometers - give a numerical reading
Capnographs - give a graphical display of CO2 concentration with time. Most useful as shape of capnogram provides more info on respiratory system.
Define etCO2.
Normal values?
Intervention value?
end-tidal CO2. Reading is taken at the peak of phase 3 (the alveolar plateau). Indicates how well an animal is ventilating. Normal range is 35-45mmHg (4.5-6kPa). Values >60mmHg (>8kPa) may warrant IPPV.
How does the etCO2 relate to arterial pressure (PaCO2)
In animals with healthy lungs, the two are very closely related usually (PaCO2 is usually about 5mmHg higher) and thus may be used as a surrogate for vales obtained by blood gas analysis. In animals with diseased lungs, etCO2 measurements should be compared with arterial blood gas results if possible, especially at the beginning of anaesthesia.
Define hypercapnia
increased CO2 in the blood
Causes - hypERcapnia - 4
- Hypoventilation (anaesthetic-related resp.depression)
- Significant increases in inspired CO2
- Increased CO (if anaesthesia too light)
- Hyperthermia (increased metabolic rate)
Causes - hypOcapnia - 4
- Artefact (patients with low tidal volumes, high rate of gas sampling by the capnograph may result in dilution of expired gas with fresh gas from the breathing system)
- Hyperventilation (anaesthesia too light)
- Decreased CO (anaesthetic overdose, shock)
- Hypothermia (decreased metabolic rate)
Causes - absent etCO2 - 4
- apnoea
- cardiac arrest (no CO2 containing blood delivered to lungs)
- disconnection of breathing system
- airway obstruction
What is the usual cause of increases in inspired CO2 concentration during anaesthesia?
Usually due to rebreathing. (e.g. insufficient fresh gas flow with a non-rebreathing system, exhausted CO2 absorbent or a malfunctioning one-way valve in a circle system)
Define etCO2.
Normal values?
Intervention value?
end-tidal CO2. Reading is taken at the peak of phase 3 (the alveolar plateau). Indicates how well an animal is ventilating. Normal range is 35-45mmHg (4.5-6kPa). Values >60mmHg (>8kPa) may warrant IPPV.
How does the etCO2 relate to arterial pressure (PaCO2)
In animals with healthy lungs, the two are very closely related usually (PaCO2 is usually about 5mmHg higher) and thus may be used as a surrogate for vales obtained by blood gas analysis. In animals with diseased lungs, etCO2 measurements should be compared with arterial blood gas results if possible, especially at the beginning of anaesthesia.
What information do pulse oximeters provide? 3
Arterial blood Hb saturdatin (SpO2, espressed as %)
Adequacy of tissue perfusion
Pulse rate
Causes - hypERcapnia - 4
- Hypoventilation (anaesthetic-related resp.depression)
- Significant increases in inspired CO2
- Increased CO (if anaesthesia too light)
- Hyperthermia (increased metabolic rate)
Causes - hypOcapnia - 4
- Artefact (patients with low tidal volumes, high rate of gas sampling by the capnograph may result in dilution of expired gas with fresh gas from the breathing system)
- Hyperventilation (anaesthesia too light)
- Decreased CO (anaesthetic overdose, shock)
- Hypothermia (decreased metabolic rate)
Causes - absent etCO2 - 4
- apnoea
- cardiac arrest (no CO2 containing blood delivered to lungs)
- disconnection of breathing system
- airway obstruction
Define hypoxaemia in terms of PaO2
PaO2 < 60 mmHg is hypoxaemia.
What can be measured when looking at anaesthetic agent concentration? When are they most useful?
inspired and expired volatile anaesthetic concentrations. machines doing this becoming more affordable.
Used with knowledge of MAC, helps assess depth of anaesthesia.
USES: especially when low gas flows are used in a circle system (when actual inspired concentrations may bear little relationship to vaporiser setting).
How can you assess oxygenation? 3
Clinically -cyanosis
Arterial blood gas analysis
Pulse oximetry
What information do pulse oximeters provide?
Arterial blood Hb saturdatin (SpO2, espressed as %)
Adequacy of tissue perfusion
Pulse rate
What should the SpO2 be in healthy, anaesthetised animals?
95-100%
What does an SpO2 of 90% correspond to in terms of PaO2?
PaO2 of approximately 60mmHg (i.e. a hypoxic leve)
Equation for oxgyen delivery (DO2)
= arterial oxygen content (CaO2) * CO
What should you do if SpO2 is low on 100% oxygen? 4
- check O2 supply
- check probe position
- check BP (hypoperfusion)
- have you used something causing vasoconstriction (aslpha-2-agonist)?
+ if true there are 2 options:
- ) increased RHS -> LHS shunting (anatomical or intrapulmonary perfusion of unventilated alveoli)
- ) impaired diffusion (v. rare)
Why is pulse oximetry not a good measure of ventilation?
animals with healthy lungs breathing 100% oxygen can never hypoventilate enough to reduce their PaO2 to hypoxic levels (unless they stop breathing). Pulse oximetry gives no information about CO2 elimination.
What happens in diseased lungs to SpO2?
serious reductions in SpO2 even when breathing 100% O2 because of problems matching ventilation with the blood flow through the lungs and the development of intrapulmonary shunting of blood.
What happens at the end of anaesthsia in terms of ventilation?
Animal is disconnected from the circuit –> hypoventilation (even in healthy animals) –> can result in dangerous reductions in PaO2 and hence SPO2. This is when pulse oximetry is most useful in healthy animals. Always keep pulse oximeter probe attached to tongue for as long as possible, until you are happy that SpO2 is stable on room area.
What is important to remember about pulse oximeters? 4
- Gradually falling SpO2 often indicates compression of pulsatile vascular bed (move probe).
- Peripheral vasoconstriction and hypoperfusion affect oximeter performance
- Readings are averaged over 10-20 seconds so machines can’t detect acute desaturation
- Measure O2 saturation but no direct information on O2 content of blood (depends on Hb concentration) nor O2 delivery to tissues (depends on CO).
Equation for oxygen content (CaO2)
= (1.36[Hb]SpO2)+(0.003*PaO2)
Therefore anaemic animals may have low arterial oxygen content, even though SpO2 is normal.
What is the most accurate reading from an oscillometer?
mean ABP (this is advantageous since it is the vale that is the true driving force for tissue perfusion)
What type of ABP does the Doppler technique with the cuff measure? What about cats?
systolic ABP - although in cats the value given is probably closer to the mean. Some authorities recommended adding 14mmHg to the value to obtain the true systolic pressure in these species.
When do oscillometers struggle to give readings?
excessive patient movement
presence of poor perfusion
irregular pulse (arrhythmias)
Define ABP. What is it used for?
Arterial Blood Pressure
(used to assess the adequacy of tissue perfusion because it is easier to measure than CO which would be measured in an ideal world).
Define NIBP
Non-invasive blood pressure measurement
How can NIBP be performed? 2
- ) Cuff arund limb/tail base- –> inflated above systolic ABP then gradually deflated until BF returns –> this is detected with a Doppler Ultrasound probe distal to the cuff OR oscillometry. Accuracy determined by cuff-size
- ) Oscillometers give values for systolic (SAP), mean (MAP) and diastolic ABP but tend to underestimate the true pressures.
How do you select the right cuff size to measure NIBP?
cuff width should be 40% of the circumference of the limb.
Small width cuffs –> over-reads BP
Large width cuffs –> under-read BP
Large determiner of NIBP accuracy.
Measured pressure alters with the site of measurement and its relationship to the level of the heart (hydrostatic pressure)
What is the most accurate reading from an oscillometer?
mean ABP
What type of ABP does the Doppler technique with the cuff measure? What about cats?
systolic ABP - although in cats the value given is probably closer to the mean. Some authorities recommended adding 14mmHg to the value to obtain the true systolic pressure in these species.
When do oscillometers struggle to give readings?
excessive patient movement
presence of poor perfusion
irregular pulse (arrhythmias)
At what mean ABP does renal perfusion start to reduce? When is this particularly important? 2
When MAP is < 60mmHg (ischaemia and necrosis). Maintaininf pressure >60mmHg is even more important in animals with pre-existing renal disease or those that have had NSAIDs pre-operatively.
When is IABP indicated?
when rapid changes in haemodynamic state are indicated and continuous measurement of BP is essential. More accurate but may be associated with more complications
Distinguish SaO2 and SpO2
SaO2 = arterial oxygen saturation measured by blood gas analysers SpO2 = arterial oxygen saturation measured with a pulse oximeter
What should you do if blood pressure is low? 4
- ) reduce depth (anaesthesia)
- ) fluid bolus (5-10ml/kg)
- ) positive inotropes (dopamine, dobutamine)
- ) avoid NSAIDs
Where is the level of the heart in dorsal and lateral recumbency?
DORSAL: point of shoulder
LATERAL: manubrium sterni
What are normal ABP values (systolic, diastolic and mean)?
Systolic= 100-160mmHg Diastolic = 60-100 mmHg Mean = 80-120 mmHg
Causes - hypotension
- ** reduced CO as a result of hypovolaemia
- reduced CO
- reduced vascular resistance
At what mean ABP does renal perfusion start to reduce? When is this particularly important? 2
When MAP is < 60mmHg (ischaemia and necrosis). Maintaininf pressure >60mmHg is even more important in animals with pre-existing renal disease or those that have had NSAIDs pre-operatively.
Name 2 cartridge-based blood-gas analysers.
What do they measure ? 2
What do they calculate? 3
iSTAT and IRMA.
Measure pH and partial pressures (O2 and CO2) in a blood sample.
Calculated derived variable such as plasma bicarbonate concentration, base excess and saturation of Hb with oxygen.
What PaO2 and SpO2 values should cause concern?
NORMAL: PaO2 >80mmHg (>10.6 kPa), SpO2 >95%
SERIOUS HYPOXAEMIA: <75%
What should you do if blood pressure is low? 4
- ) reduce depth (anaesthesia)
- ) fluid bolus (5-10ml/kg)
- ) positive inotropes (dopamine, dobutamine)
- ) avoid NSAIDs
What is the gold standard for evaluating gas exchange?
Arterial blood gas analysis (also provides information about the acid-base status of the patient)
What are the normal ranges for arterial PCO2 (PaCO2)?
Normal range = 35-45mmHg (4.5-6kPa)
Value < 35mmHg indicates hypERventilation
Value >45mmHg indicates hypOventilation
Value >60mmHg is the upper limit, warrants artificial ventilation (IPPV).
Value < 25mmHg, <3.3kPa) indicates severe hypoventilation and can be associated with severe cerebral vasoconstriction, which may reduce cerebral perfusion and oxygen delivery.
What shoyld the PaO2 be in an anaesthetised animal be?
PaO2 (mmHg) should be 5 times the FiO2 (=21%, normal PaO2 in a healthy animal breathing air) OR alternatively the ratio of PaO2/FiO2 should be >300.
T/F: PaO2 measures the pressure of oxygen dissolved in blood and is independent of Hb concentration
True
How is Hb saturation linked with PaO2?
oxyhaemoglobin dissociation curve.
What PaO2 and SpO2 values should cause concern?
NORMAL: PaO2 >80mmHg (>10.6 kPa), SpO2 >95%
SERIOUS HYPOXAEMIA: <75%
Why might a capnograph have an elevated plateau (increased etCO2)? 2
Hypoventilation
Increased metabolic rate
Why might a capnograph have an elevated baseline AND plateau (increased etCO2 AND inspired CO2)?
rebreathing
Why might a capnograph have a sudden etCO2 decrease to zero? 4
- airway obstruction
- circuit disconnection
- obstruction in sampling tube
- ventilator failure
Why might a capnograph have a plateau that quickly decreases to low or zero values of etCO2? 2
- leaks in breathing system or around ETT cuff
- Reduced CO (= reduced delivery of CO2 to lungs)
Why might a capnograph have a low etCO2 without a good plateau and slow rate of rise? 2
- expiration not complete before next inspiration (e.g. as seen with rapid RR, partial airway obstruction)
- contamination of sample with fresh gas
