45. IHD Flashcards
Tell me about the principles of anaesthetising a patient with ischaemic
heart disease (IHD).
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.
Supply
Supply of oxygen = Blood flow × Oxygen content
Blood flow is determined by:
- Heart rate
determines coronary blood flow because diastole shortens with increasing heart rate. - Aortic pressure (particularly diastolic)
- 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.
- 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. - Metabolic factors. Coronary blood flow closely parallels myocardial
metabolic activity.
Oxygen content is determined by:
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)
Demand
Determinants of myocardial oxygen demand are:
Preload, which determines LVEDP Remember Laplace’s law
Afterload
Heart rate
Contractility
clinical practice this means avoiding
- Hypoxia!
- Hypotension –
especially diastolic - 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. - Tachycardia – shortened diastole
- Stimulation of the autonomic nervous system
Monitoring options include:
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.
What are the ‘risk periods’ during anaesthesia for patients with IHD?
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.
Do you know any ways of modifying these responses?
Pre-medication with anxiolytics, oxygen and usual cardiovascular medication
Beta-blockers
More recent
evidence, however, has given grounds for less enthusias
- Caution with induction agents to avoid significant hypotension. An arterial
line prior to induction ensures continuous monitoring of the blood pressure - Opioid-based anaesthetic technique
- Lignocaine spray to the cords (prior to intubation and extubation)
- Use of neuroaxial blocks (prevents stress response but beware hypotension
- Post-operative oxygen
Beta blockers
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
How do you assess cardiac risk?
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
Scoring systems
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.