45. IHD

Question 1

Tell me about the principles of anaesthetising a patient with ischaemic
heart disease (IHD).

Answer 1

This is a question about myocardial oxygen supply and demand. You
must relate the physiology to clinical practice.

The over-riding principle is to avoid cardiac ischaemia by ensuring that supply
of oxygen to the myocardium always meets demand.

Question 2

Supply

Answer 2

Supply of oxygen = Blood flow × Oxygen content

Question 3

Blood flow is determined by:

Answer 3

1. Heart rate
   determines coronary blood flow because diastole shortens with increasing heart rate.
2. Aortic pressure (particularly diastolic)
3. Extravascular compression of the coronary arteries (by a contracting
   ventricle). .
   Blood flow in the coronary arteries is therefore greater in diastole.

Compression during systole is greatest at the endocardium and this
explains why this area is particularly susceptible to ischaemia. Blood flow in
the left coronary artery may be reversed during systole.

4. Neurohumoral factors. These are not of great importance. Autonomic nerve
   stimulation causes coronary vasoconstriction but this is offset by the
   vasodilatation that accompanies an increase in the myocardial metabolism.
5. Metabolic factors. Coronary blood flow closely parallels myocardial
   metabolic activity.

Question 4

Oxygen content is determined by:

Answer 4

Haemoglobin concentration (but this alters viscosity and therefore blood flow!).

SaO2
PaO2
Oxygen content = (1.34 ml ofO2 × Hb g/dl × %SaO2) + (0.003 × PaO2)

Question 5

Demand

Answer 5

Determinants of myocardial oxygen demand are:

Preload, which determines LVEDP Remember Laplace’s law
Afterload

Heart rate
Contractility

Question 6

clinical practice this means avoiding

Answer 6

1. Hypoxia!
2. Hypotension –
   especially diastolic
3. Hypertension –
   because it causes increased myocardial wall tension and thus
   further extravascular compression.
   It should be noted that pressure work
   increases myocardial O2 consumption much more than volume work
   (increasing cardiac output) so hypertension must be avoided.
4. Tachycardia – shortened diastole
5. Stimulation of the autonomic nervous system

Question 7

Monitoring options include:

Answer 7

CMV5 Leads II and V5 together detect 95% of ischaemic events.
PAFC
TOE
Arterial line To detect hypotension quickly

A patient with ischaemic heart disease should be monitored for signs of
ischaemia so that treatment can be instituted.

Question 8

What are the ‘risk periods’ during anaesthesia for patients with IHD?

Answer 8

The ‘risk periods’ are those times during the anaesthetic when the patient is
more likely to develop hypotension, hypertension and tachycardia.

Anxiety pre-operatively causes hypertension and tachycardia.

Induction can cause hypotension.

Laryngoscopy and intubation can cause hypertension and tachycardia as can:

Surgical incision

Extubation

Pain post-operatively

Hypoxia is frequently responsible for ischaemic cardiac events

post-operatively.

Question 9

Do you know any ways of modifying these responses?

Answer 9

Pre-medication with anxiolytics, oxygen and usual cardiovascular medication

Beta-blockers
More recent
evidence, however, has given grounds for less enthusias

3. Caution with induction agents to avoid significant hypotension. An arterial
   line prior to induction ensures continuous monitoring of the blood pressure
4. Opioid-based anaesthetic technique
5. Lignocaine spray to the cords (prior to intubation and extubation)
6. Use of neuroaxial blocks (prevents stress response but beware hypotension
7. Post-operative oxygen

Question 10

Beta blockers

Answer 10

Class I
Beta-blockers should be continued in patients undergoing surgery who
are receiving beta-blockers to treat angina, symptomatic arrhythmias,
hypertension, or other ACC/AHA Class I guideline indications.
Beta-blockers should be given to patients undergoing vascular surgery
who are at high cardiac risk owing to the finding of ischaemia on
pre-operative testing

The large POISE study (2008) has added some caution in that,
although the treatment arm (metoprolol) had less cardiac events, the
overall mortality was significantly higher as a result of more strokes,
probably due to hypotension

Question 11

How do you assess cardiac risk?

Answer 11

History and examination to identify risk factors.

Plan appropriate intervention to reduce risk and improve outcome.

Investigations: the history may dictate the need for some assessment of
functional capacity

ECG
Arrhythmias predict peri-operative cardiac events

Exercise ECG Negative test correlates with low-risk

Dobutamine stress echo

Ambulatory ECG
Can detect silent ischaemia

Echocardiogram Questionable value as a predictor of
post-operative cardiac events

Coronary angiography Expensive but useful
Decrease in mortality rate afforded by
CABG negated by risks of procedure itself

Question 12

Scoring systems

Answer 12

The Revised Cardiac Risk Index is now widely used

Goldman was one of the first (1977) nine factors. Max score = 53

Detsky (1986) – additional factors to Goldman: MI at any time, unstable
angina within 6 months, CCS angina Class III and IV, pulmonary oedema.
Max score = 120.