Isoflurane, when administered to healthy patients in concentrations less than 1.0 MAC, will decrease all the following EXCEPT:

Isoflurane, when administered to healthy patients in concentrations less than 1.0 MAC, will decrease all the following EXCEPT:

• A. Cardiac output
• B. Myocardial contractility
• C. Stroke volume
• D. Systemic vascular resistance

Answer: (A)

The key idea is how volatile anesthetics affect the heart and circulation in healthy people at low concentrations. Isoflurane tends to dilate blood vessels, so systemic vascular resistance falls, which lowers arterial pressure. The body usually responds with a reflex increase in heart rate, which helps keep cardiac output from dropping. At the same time, there is some direct, mild depression of the heart’s ability to contract, and venous dilation can reduce venous return, lowering preload and potentially stroke volume. Put together, this means you commonly see a drop in afterload (and often preload) with a preserved or even slightly increased cardiac output due to tachycardia. So the one thing that does not typically decrease at less than 1 MAC is cardiac output, while myocardial contractility, stroke volume, and systemic vascular resistance are more likely to be reduced.

The key idea is how volatile anesthetics affect the heart and circulation in healthy people at low concentrations. Isoflurane tends to dilate blood vessels, so systemic vascular resistance falls, which lowers arterial pressure. The body usually responds with a reflex increase in heart rate, which helps keep cardiac output from dropping. At the same time, there is some direct, mild depression of the heart’s ability to contract, and venous dilation can reduce venous return, lowering preload and potentially stroke volume. Put together, this means you commonly see a drop in afterload (and often preload) with a preserved or even slightly increased cardiac output due to tachycardia. So the one thing that does not typically decrease at less than 1 MAC is cardiac output, while myocardial contractility, stroke volume, and systemic vascular resistance are more likely to be reduced.