These studies show that the left atrial booster pump action serves as a supercharger which can increase left ventricular stroke volume in the range of 25 percent; and in patients with aortic stenosis, stroke work in the range of 50 percent (4). These changes can occur in the face of increased resistance to left ventricular filling in clinical conditions such as aortic stenosis where there is diminished left ventricular compliance and in mitral stenosis where there is stenotic resistance to left ventricular filling from the atrium. In spite of this fact, assessment of left atrial function by measurement of cardiac output changes occurring after return from atrial fibrillation to normal sinus rhythm yields erratic and confusing results. The reason for this is that atrial function, per se, is not a primary determinant of steady state cardiac output. Sequential A-V pacing may temporarily increase stroke volume in an acute setting like myocardial infarction. Nevertheless, one cannot infer from such observations that the use of permanent transvenous A-V sequential pacing will augment steady state cardiac output over a period of time. This is an important point to remember when considering the use of sequential A-V pacing, since it requires insertion of more complicated pacing and sensing wires as well as a more complex circuitry. All of these features lead to an increased risk of pacemaker malfunction. This increased risk of malfunction is not justified unless there is good evidence that atrial contribution is important in a given patient.