Question 8

a)    List four predisposing conditions and four precipitating factors which may lead to the occurrence of dynamic left ventricular outflow tract obstruction in critically ill patients.
(40% marks)

b)    What specific cardiovascular clinical signs on physical examination may be present in a patient with left ventricular dynamic outflow obstruction?    (20%marks)

c)    What are the principles of medical treatment in a patient with shock secondary to dynamic outflow obstruction?    (40% marks)

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College answer

a)    Predisposing conditions
Hypertrophic cardiomyopathy
Left ventricular hypertrophy (e.g. Hx of hypertension or aortic stenosis)
Post AVR or TAVI for aortic stenosis
Post MVR

Precipitating factors
Vasodilatation e.g. anaesthesia, sepsis, nitrates, liver failure
Inotropic agents
b)    Ejection systolic murmur lower left sternal edge which may vary in intensity over time as the gradient changes
An associated MR murmur is common
Signs of low CO syndrome i.e. hypotension, oliguria, lactic acidosis, end organ hypoperfusion
LVF signs

c)    Fluid loading
–increase preload
Vasocontrictors (preferably without b effect i.e. phenylephrine / vasopressin) increase afterload without increasing heart rate
Negative inotrope / chronotrope e.g. b blockade
–control heart rate
–manage arrhythmias
Treat underlying conditions

Examiners Comments:

This was generally answered well. Candidates who did poorly didn't know how to manage patients with outflow tract obstruction and recommended dangerous therapies including inotropic therapy and vasodilator agents.


Given that this was "generally answered well",  one might assume that a detailed breakdown of the subject is probably superfluous and that only cosmetic changes could be made to enhance the already excellent college answer. That is reasonably accurate. Unfortunately, the same cannot be said for the question. Specifically, the distinction between "predisposing conditions" and "precipitating factors"  is difficult to parse. Is a mitral valve replacement a "condition" or a "factor"?  If it's a predisposing condition, then why isn't liver failure a predisposing condition? Confusion prevails. In order to make sense of this, "predisposing conditions" here are interpreted as "structural factors" and precipitating factors are interpreted as "functional factors".

Predisposing conditions

  • HOCM
  • Concentric LV hypertrophy
  • Eccentric septal hypertrophy
  • Systolic anterior motion of the mitral valve (SAM)
  • Structural causes of anterior mitral valve leaflet or apparatus position
    • Redundant anterior leaflet
    • Redundant posterior leaflet
    • Papillary muscle displacement 
    • Prosthetic valve placement
  • Infiltrative cardiac disease
    • Cardiac amyloid
    • Fabry disease
    • Danon disease
    • Friedrich ataxia
    • Cardiac oxalosis
    • Mucopolysaccharidoses
    • Sarcoidosis

Precipitating factors

  • Insufficient diastolic filling pressure
    • Hypovolemia
    • Atrial fibrillation
  • Insufficient diastolic filling time
    • Tachycardia
    • SVT or VT
  • Insufficient afterload
    • Vasodilation due to sepsis
    • Vasodilator drugs, eg. nitrates or general anaesthetics
  • Excessive contractility
    • Inotrope drugs
    • Stress
    • Pain
    • Anxiety
    • Exercise

Clinical features include:

  • Haemodynamic instability associated with stress
  • Ejection systolic murmur, louder with Valsalva (whereas AS becomes softer)
  • MR murmur
  • Brockenbrough–Braunwald-Morrow phenomenon: "a paradoxical decrease in the arterial pulse pressure and an associated increase in the LV systolic pressure in the beat following a PVC" (Trevino & Buergler, 2014). 
  • Brisk arterial pulse with rapid systolic rise and rapid drop-off
  • Characteristic "spike and dome" appearance of the aortic pressure trace

Management consists of targeting the following parameters:

  • Preload: keep it high-normal. 
  • Rate: keep it slow. 
  • Rhythm: keep it sinus. 
  • Contractility: bring it down. Negative inotropes are often called for, eg. beta-blockers
  • Afterload: keep it high. Use vasopressors with minimal beta effect (eg. vasopressin or phenylephrine)


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