12. Hypotension

Question 1

Intro

Answer 1

It is hard to write a generic answer to this critical incident,

as the answer you give in the exam will be tailored to the specific case presented to you.

For example, it may be appropriate to discuss cardiac tamponade as a cause of hypotension in a trauma patient but it would not be appropriate as a common cause of hypotension in a 16-year-old
undergoing an appendicectomy.

Approach this question using the physiological formula for calculation of mean arterial pressure (MAP):

Question 2

MAP =

Answer 2

= Cardiac Output × Systemic Vascular Resistance (SVR)

MAP = (Heart Rate × Stroke Volume) × SVR

Question 3

What are the causes of a low cardiac output?

Answer 3

> Bradycardia,

e.g. β-blocker, opioids, vagal response, hypoxia

> Arrhythmias,

e.g. electrolyte imbalances,
valvular or ischaemic heart disease

> Reduced circulating volume or 
venous return (preload),

e.g. hypovolaemia,
cardiac tamponade,
aorto-caval compression or
tension pneumothorax

> Impaired myocardial contractility,
e.g. ischaemia, hypoxia, acidosis

> Increased afterload,
e.g. aortic stenosis
(reduces left ventricular ejection fraction)
or pulmonary embolism

Question 4

What are the causes of a low SVR?

Answer 4

> Drugs,
e.g. intravenous anaesthetic induction agents, vasodilators, α-receptor blockers

> Spinal and epidural anaesthesia,
via sympathetic blockade

> Local mediators, e.g. potassium, nitric oxide

> Hypercapnia

> Pyrexia

> Sepsis

> Anaphylaxis

Question 5

How would you manage intra-operative hypotension?

Answer 5

Your management should be
tailored to the likely causes.

State that this is an anaesthetic emergency
and that you would call for senior anaesthetic
assistance.

> Administer 100% O2 to maintain tissue oxygenation.

> Recheck measurement and
ensure invasive monitoring equipment is
correctly positioned relative to patient.

> Check what the surgeons are doing,
e.g. exclude a sudden blood loss,
surgical caval compression and ensure normal abdominal insufflation pressures.

> Correct the physiology by maintaining cardiac output and SVR which, depending on the cause of hypotension, may require any of the following
interventions:

• Fluid challenge,
e.g. 10 mL/kg of crystalloid or colloid.
Assess response and repeat as necessary.
Transfuse blood if indicated.

• Vasoconstrictors, e.g. phenylephrine, metaraminol or noradrenaline.

• Inotropes,
e.g. ephedrine, dobutamine, dopexamine, milrinone.

• Treat arrhythmias,
e.g. electrolyte correction, anti-arrhythmics.

If hypotension becomes resistant to treatment, consider early use of cardiac output monitoring devices to guide further therapy. Remember that even
short periods of intra-operative hypotension may have consequences on end-organ function.

Decide on appropriate post-operative care in the critical care unit if indicated.