Applied Physiology for Anaesthesia

Question 1

How does anaesthesia and surgery affect the CVS?

Answer 1

1. The cardiovasucular depressant effects of many anaesthetic agents
2. The stress response to surgery which has neural and hormonal components
3. Potential blood and fluid loss during surgery

Question 2

What are the indices of CVS function?

Answer 2

a) Cardiac Output (CO=SV x HR)
b) Stroke Volume (preload, contractibility and afterload)
c) Cardiac Index (CO divided by body surface area)
d) Blood Pressure (MAP=CO x SVR)
e) Ventricular filling

Question 3

What is CO?

Answer 3

Volume of blood leaving the heart each minute i.e it is a measure of flow.

Question 4

How much CO in an adult at rest?

Answer 4

5 l/min

Question 5

What does an increase in venous return (pre-load) have?

Answer 5

It increases the ventricular end-diastolic volume which stretches the cardiac muscle fibres and increases the tension that develops. This increases the contractility (like Frank-starling principle dictates).

Question 6

What determines afterload?

Answer 6

Systemic vascular resistance (SVR)

Question 7

What determines SVR?

Answer 7

SVR is determined by vessel diameter and blood viscosity. Anaemia will increase SVR

Question 8

What is the Frank-starling principle?

Answer 8

Increasing the ventricular end-diastolic volume (preload) will increase myocardial stretch and thus increase the contractility with an increased SV as a result.

Question 9

What is the average cardiac index in an adult?

Answer 9

3.2 l/min/m

Question 10

What affects ventricular filling?

Answer 10

The atrial systole contributes

Question 11

What controls the HR?

Answer 11

The SA node is pacemaker.

Question 12

What affects CO?

Answer 12

heart rate, preload, myocardial contractility, and afterlaod

Question 13

Which factors decrease preload?

Answer 13

hypovolaemia, haemorrhage, general anaesthesia, neuraxial anaesthesia, intermittent positive pressure ventilation, autonomic neuropathy. (dec preload: decr vol, anaesthesia, PPV, neuropathy)

Question 14

Which factors decrease afterload

Answer 14

anaemia

Question 15

Which factors increase afterload?

Answer 15

polycythaemia

Question 16

Which factors increase contractility?

Answer 16

incr preload

Question 17

Which factors decrease contractility?

Answer 17

decr preload

Question 18

Which IV agents reduce CO?

Answer 18

Propofol, etomidate, thiopentone

Question 19

Which IV agents invrease CO?

Answer 19

Ketamine

Question 20

What is the most CVS stable IV agent?

Answer 20

Etomidate.

Question 21

Do all inhalation agents decrease CO?

Answer 21

Yes. to a greater or lesser degree. This is due to a combination of direct myocardial depression; inhibition of central and peripheral nervous system function; vasodilation, and/or altered baroreceptor activity. Often these agents are assoc/ compensatory tachycardia to maintain CO.

Question 22

Which inhalation agents are the most CVS stable?

Answer 22

iso, sevo, and des

Question 23

Is halothane CVS stable?

Answer 23

No, it is a potent myocardial depressant and dysrhthmogenic.

Question 24

Which muscle relaxant increases the HR and therefore increases CO?

Answer 24

Pancuronium

Question 25

Which muscle relaxant can decrease the HR?

Answer 25

Vecuronium

Question 26

Describe Histamine releasing drugs on CO.

Answer 26

Some opiates and muscle relaxants stimulate histamine release, and this causes vasodilation, which can decrease afterload and have varying effect on CO.

Question 27

What is the basis for vasodilators (i.e afterload reduction) therapy in cardiac failure?

Answer 27

To decrease the SVR (systemic vascular resistance) which may result in an increase of CO despite a drop in BP. However if the BP drops significantly then diastolic myocardial perfusion may be compromiseed, resulting in ischaemia, cardiac failure and a decreased CO.