Review Question - QID 217700

This patient who presents with substernal chest pain, lightheadedness, and ST-elevations most likely had an inferior myocardial infarction (II, III, and aVF) leading to decreased contractility and death. The cardiac function curve that best represents fatal myocardial depression is D.

The Frank-Starling relationship describes the interaction between sarcomere length and tension developed in the ventricles. Specifically, increased end-diastolic blood volume (EDV) stretches the cardiac muscle fibers and increases overlap between the myosin and actin in the sarcomeres. This allows for increased development of ventricular tension and increased stroke volume. The exception to this rule occurs at very high levels of EDV when the ventricular sarcomeres are stretched past their zone of useful overlap. The Starling curve can be shifted upwards or downwards based on cardiac contractility, which describes the intrinsic ability of myocardial cells to develop force. An example of increased contractility is during exercise due to catecholamines, and an example of decreased contractility is during heart failure states or myocardial infarction when cardiac tissue dies.

Russell et al. studied the effects of increasing left ventricular filling in patients after acute myocardial infarction. They found that patients with a flattened cardiac function curve had high mortality over 8 weeks.

Figure/Illustration A shows a series of cardiac function curves that are annotated on the illustration. In all cases, ventricular function will initially increase as the preload delivered to the heart increases but then decrease as the heart becomes overstretched.

Incorrect Answers:
Answers 1 and 2: A left shift in the cardiac function curve indicates states of increased cardiac contractility. An example would be the effect of catecholamines on the heart during exercise.

Answer 3: A depressed myocardial function curve that is sufficient for rest but is unable to accommodate to have sufficient output for walking or running is consistent with heart failure.

Answer 5: While no formal cardiac function testing was performed on this patient, given that he had a myocardial infarction it is certain that is cardiac output has reduced to some degree.

Bullet Summary:
Myocardial infarction leads to decreased cardiac function and cardiogenic shock, which can be represented as a downward shift in the cardiac function curve.