Figure 32-20 Unusual arterial pressure waveforms. A, Intra-aortic balloon counterpulsation with a 1:2 balloon-assist ratio produces a characteristic change in the arterial pressure waveform. Four cardiac cycles are shown, two with balloon assistance and two without. 0, Unassisted end-diastolic pressure; 1, unassisted systolic pressure; 2, dicrotic notch; 3, assisted or augmented diastolic pressure; 4, end-diastolic or presystolic dip; 5, assisted systolic pressure. Effective afterload reduction by the intra-aortic balloon is demonstrated by the presystolic dip pressure (4) lower than the unassisted end-diastolic pressure (0) and the assisted systolic pressure peak (5) lower than the unassisted systolic pressure peak (1). B, Arterial pressure waveform during cardiopulmonary bypass. Small phasic pressure variations (arrows) result from the mechanical action of the bypass roller pump. The bypass pump flow rate may be estimated by measuring these pulsations. Nineteen pulsations are recorded in a 3-second time interval. In this case, the pump configured with ⅜-inch tubing has an effective stroke volume of 27 mL. The pump flow rate may be calculated as follows: (19 pulsations/3 seconds) × (1 pump revolution/2 pulsations) × (27 mL/revolution) × (60 sec/min) = 5130 mL/min. This calculated pump flow rate should equal the flow rate displayed on the pump console (i.e., 5.2 L/min). (Redrawn from Mark JB: Atlas of Cardiovascular Monitoring. New York, Churchill Livingstone, 1998, Figs. 20-3 and 19-8.)