Compare and contrast the cardiovascular effects of an induction dose of propofol and ketamine.
The key words in this question were "compare and contrast", "cardiovascular effects" and "induction dose". Some candidates described aspects of both drugs other than the cardiovascular effects but gained no marks for this. Better answers used a combination of a table plus some explanation to contrast the cardiovascular effects of the two drugs concentrating on aspects such as heart rate, cardiac output, vascular resistance and blood
Many candidates were confused by the direct versus the indirect cardiovascular effects of
both drugs. Propofol probably has no direct negative inotropic effect. Ketamine has a direct myocardial depressant action but this effect is overridden by the centrally mediated sympathetic action of the drug. The effect of both drugs on the baroreceptor receptor response alone is a difficult area as there is significant interplay between the direct cardiovascular effects of the drugs and their effect on the baroreceptor reflex. Allowance was made for this in the marking. Propofol resets the baroceptor reflex producing a slower heart rate for a given level of blood pressure. Overall both drugs depress the baroreceptor reflex.
Comparison of the effects on cerebral, coronary, renal and hepatic blood flow earned extra marks.
|Net effects on cardiovascular performance characteristics|
|Blood pressure||Decreases||Increases or remains the same|
|Heart rate||Decreases||Markedly increases|
|Granular effects on variables which affect cardiac output|
|CVP||Decreases||Increases or remains the same|
|PVR||Decreases||Increases or remains the same|
|SVR||Decreases||Increases or remains the same|
|Cardiac reflexes||Suppressed||Remain intact|
|Regional circulatory effects|
|Cerebral blood flow||Decreases||Probably increases|
|Coronary blood flow||Slightly decreases||Remains unchanged|
|Renal blood flow||Decreases||Increases|
|Hepatic blood flow||Decreases||Increases|
|Pharmacodynamic mechanisms of these circulatory effects|
|Direct mechanisms||Propofol is a direct negative inotrope, but only in massive supratherapeutic concentrations. Under normal dosing conditions, it only decreases contractility by about 5-10% (Sprung et al, 2001).
Propofol has no direct vasodilator effects (Robinson et al, 1997)
|Ketamine is a direct negative inotrope. This effect is visible at therapeutic concentrations but is usually overwhelmed by the indirect sympathetic effects (Kongsayreepong et al, 1993).|
|Indirect mechanisms||Sympathetic suppression results in the haemodynamic effects||Sympathetic activation results in the haemodynamic effects|
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