Introduction: A less-invasive method for monitoring cardiac output (CO) has been developed. This new method, called PRAM (pressure recording analytical method), can derive beat-to-beat values of CO from the arterial pressure waveform analysis . The concept behind the measurement and monitoring of CO based on the analysis of arterial blood pressure waveform is not new and similar approach has been studied by several authors. However, PRAM does not require any external calibration and can be implemented in various conditions of flow . Our purpose were (1) to assess the accuracy of PRAM in cardiac surgery using the bolus thermodilution (TD) and the roller pump device (RP) as the reference gold standard methods; and (2) to identify patients developing low cardiac output during weaning from extra-corporeal circulation (ECC).
Method: In 20 patients undergoing cardiac surgery, CO values obtained by PRAM from the analysis of radial artery were compared with TD (a series of five thermal indicator injections) before and after ECC, and with the RP readings during ECC before and after aortic clamping. During spontaneous circulation, PRAM results were based on the simultaneous evaluation of the pulsatile and continuous components of cardiac output flow as expressed by the arterial waveform . During ECC, in the absence of cardiac activity, a sinusoidal waveform was produced by the pump, representing only the continuous component of flow. Thus, PRAM flow measurements during ECC were based on the determination of the stroke volume (obtained by each sinusoidal wave), multiplied by the number of sinusoidal waves (i.e. the number of the roller pump revolutions per minute). No clinical decision was taken based on PRAM results.
Results: The estimates of blood flow measured by PRAM closely agreed with TD (r2 = 0.71; P < 0.0001; at Bland-Altman analysis mean = 0.06, +2SD = 0.74, −2SD = 0.66) and simultaneous RP readings (r2 = 0.70; P-value < 0.0001; at Bland-Altman analysis mean = 0.09, +2SD = 0.71, −2SD = 0.53). During weaning from ECC, two patterns of haemodynamic adaptation were documented by PRAM following resumption of cardiac contraction. (1) Most patients had a rapid and stable recovery of CO (a reduction <10% as compared to ECC flow) at the decreasing of RP-infused volumes. (2) Four patients showed a marked fall in CO >10% up to 40% and required longer ECC assistance, weaning time and inotropic support.
Conclusions: PRAM seems to provide accurate and continuous quantification of flow during each phase of cardiac surgery and may allow early recognition of low CO during weaning from ECC.
1 Romano SM, Pistolesi M. Assessment of cardiac output from systemic arterial pressure in humans. Crit Care Med
2002; 30: 1834-1841.