CARDIOLOGY
Oxygen is chiefly transported to the tissue via haemoglobin and delivery is dependent on the cardiac
output. Cardiac output is the product of the stroke
volume per beat multiplied by the heart rate. The
stroke volume is generally decreased in patients suffering cardiac murmurs. The heart rate is increased to
preserve the cardiac output. Thus heart rate can be an
indicator of clinical stability.
Arterial blood pressure (figure 1) is the product of cardiac output multiplied by the systemic vascular resistance. During low cardiac output states, systemic
vasoconstriction is often present in order to preserve
an adequate arterial blood pressure (activation of the
sympathetic nervous system and the renin-angiotensin-aldosterone axis). Therefore, patients presenting
with pallor and tachycardia with an audible murmur
require pre-anaesthetic stabilisation.
Pre-anaesthetic stabilisation
A functional or innocent heart murmur in an otherwise healthy patient may not require any special preanaesthetic stabilisation.4 Patients which are already
on cardiovascular supporting drugs (pimobendin,
digoxin, angiotensin converting enzyme inhibitors,
diuretics etc.) but are otherwise stable and compensating may also not require stabilisation. Serum electrolytes (potassium, sodium and calcium) should be
measured in patients undergoing long term diuretic
or digoxin therapy, as they may present with low potassium or calcium concentration which would prolong the recovery. Sodium is important to maintain
the intravascular fluid volume (water shifts according
to the sodium gradient between the blood and tissue)
of circulating blood. If the sodium level is high it could
indicate that there is a decrease in the circulating volume and that fluid bolusing may be required. While a
low sodium level may indicate caution with fluid management as it would be easier to inadvertently fluid
overload these patients. Furthermore, routine haematology and serum creatinine and total serum proteins
(albumin of most interest) should be measured for
baseline readings in all patients. Collecting this data
Figure 2: Cat under general anaesthesia being monitored by
pulseoximeter
prior to general anaesthesia may assist in managing
patients suffering unexpected post-recovery complications.6, 11
• Patients presenting with cardiovascular instability
should be managed medically prior to general anaesthesia (Table 1 K