How do we monitor anaesthesia? -- BP Flashcards
- What is an arterial BP measurement?
- How can we measure BP?
- Measurement of the pressure exerted by blood on the walls of the blood vessels. It is an indirect indicator of blood flow.
- Indirect / non-invasive (NIBP) e.g. Doppler, Oscillometric.
- Direct / invasive (IBP) e.g. by placement of arterial catheter.
- Indirect / non-invasive (NIBP) e.g. Doppler, Oscillometric.
Comparison between direct and indirect BP measurement.
- Direct is more invasive than indirect due to catheter placement into artery (consider sterility and expense).
- Direct is more accurate than indirect.
- Direct is more reliable than indirect.
- Get beat-to-beat info w/ direct and but indirect detects trends.
- Indirect is easier than direct.
- Direct requires more experience and skill than indirect.
- Direct is faster than indirect.
- Indirect is easier than direct.
- Indirect is cheaper than direct.
- Advantages of Doppler.
- Limitation of Doppler?
- Disadvantages of Oscillometric.
- Advantages of Oscillometric.
- Inexpensive.
Efficient.
Detects pulse flow in low flow states.
Good in cats.
Quick results. - Systolic only.
- Less reliable and can be temperamental.
Interference/movement can interfere w/ readings.
Not so useful in SAs.
More expensive (initial setup). - Systolic, diastolic and MAP.
Doppler equipment list.
Doppler unit (microphone).
Sphygmomanometer (puffer).
Headphones.
Selection of cuffs.
Gel.
Spirit.
May have clippers.
- What is SAP?
- What is DAP?
- What is MAP?
- Systolic arterial pressure = Measure of the force the heart exerts on the walls of the arteries.
- Diastolic arterial pressure = The pressure in the arteries when the heart is between contractions.
- Mean arterial pressure = The intravascular pressure in the vessel during one complete cardiac cycle.
How does the Doppler work?
Doppler flow meters detect blood flow.
They emit an ultrasonic signal.
And produces an auditory signal generated by a frequency shift of underlying RBCs.
Inflatable cuff placed around the limb.
Cuff inflated.
At some point, the noise will stop (as flow stops).
The blood pressure is the measurement point when flow returns.
- How to select the correct cuff size for the patient.
- Consequence of using the wrong size cuff.
- The width of the cuff should be 40% of the limb circumference.
If the calculated width falls between 2 cuff sizes, go for the larger cuff. - Cuff too big = result will be artificially low.
Cuff too small = result will be artificially high.
Principle of the oscillometric BP measurement.
Automated.
Works on principle that the artery wall will oscillate when blood flows through it during cuff inflation and deflation.
The rapid increase in oscillation amplitude allows estimation of the systolic pressure.
Sudden reduction in oscillation allows estimation of the diastolic pressure.
* These are calculated at the machine level from empirically derived manufacturer specific algorithm.
Period of maximal oscillation used to estimate MAP.
- Cuff sizing w/ oscillometric.
- In what situations is oscillometric convenient?
- Accuracy?
- Same as w/ doppler.
- During anaesthesia.
- Accuracy sometimes questioned. Usually, systolic and mean are more accurate than diastolic. But does allow trends.
- How is MAP calculated?
- Why is it so important to monitor BP?
- Why is it so important to monitor BP during anaesthesia?
- CO x SVR.
- It is the driving force behind tissue oxygen perfusion.
- Anaesthesia agents impact BP.
- Drugs can cause vasodilation (ACP / VA).
- We are aiming to maintain adequate BP to maintain tissue oxygenation.
What to do when faced w/ hypotensive patient?
Ideal would be sys 90mmHg.
If low, identify underlying cause.
- try:
– reducing VA.
–> consider currently local blocks and topping up analgesia.
– If bradycardic, manage this.
– Consider fluids.
– Consider drug therapy (Anticholinergics / Beta 1 adrenergic agonists / vasopressors).
