Ann Thorac Surg 2005;79:1094
© 2005 The Society of Thoracic Surgeons
Correspondence
Arterial Pressure and Pump Flow Rate During Long-Term Pulsatile and Nonpulsatile Cardiac Support: Reply
Stefan Klotz, MD,
Christof Schmid, MD
Department of Thoracic and Cardiovascular Surgery, University Hospital Münster, Albert-Schweitzer-Str 33, 48129 Münster, Germany
(E-mail: stefan.klotz{at}ukmuenster.de).
To the Editor:
We agree with Ündar and Myers that the energy equivalent pressure formula of Shepard and associates [1] is useful to quantify hemodynamic energy levels when comparing the different flow patterns of assist devices. However, this formula requires an arterial pressure wave that is possible to achieve only by using invasive methods. In our study [2], we used a protocol for evaluating and reevaluating transplant candidates, that is consistent with regulations of the European Transplant Organization (Eurotransplant, Leiden, the Netherlands). The protocol quantifies the contribution of the native left ventricle to total cardiac output and estimates left ventricular recovery, and the potential for weaning, as well as the risk of associated device dysfunctions [3]. Because the protocol avoids invasive arterial measurements, it was possible to perform all systemic pressure measurements in the group with a pulsatile left ventricular assist device (LVAD) noninvasively. In the nonpulsatile LVAD group, we used an invasive method in only a few instances.
Also, the calculation of energy equivalent pressure requires the area under the pump flow rate curve. This might be difficult in the case of nonpulsatile DeBakey ventricular assist devices because axial-flow devices have no pump rate.
In our study, we found that the patients' native hearts did eject significantly more in the nonpulsatile group compared with the pulsatile group. This was dependent on the time interval after LVAD implantation as shown in our Figure 5. Neither pump mode in the pulsatile devices (auto mode) nor number of revolutions per minute in the nonpulsatile devices (about 10,000 rpm) was changed during the study. A possible explanation for the higher pump output in the pulsatile group could be that pusher-plate devices generate suction to the left ventricle in contrast to the DeBakey axial-flow pump [4]. Therefore, recovery of the left ventricular function, which apparently took place in the nonpulsatile group, could have been obscured in the pulsatile group.
It is probably coincidence that the cardiac output after LVAD implantation in the nonpulsatile group (5.08 ± 1.04 L/min) is identical to the pump flow in the pulsatile group (5.09 ± 0.97 L/min). However, we apologize for the typographical error concerning right atrial pressure in the nonpulsatile group after LVAD implantation. The correct pressure is 4.4 ± 3.8 mm Hg, not 77.7 ± 6.2 mm Hg. Finally, all patients with a nonpulsatile device felt well with a mean arterial pressure of 78 mm Hg, which could be explained by the finding of almost identical systolic and diastolic pressures in these patients. Consequently, we did not try to increase the revolutions per minute. In addition, had we done so, there might have been negative effects on hemolysis. In our opinion, it is also important not to unload the left ventricle completely. The native heart should be included in the cardiac cycle, as this prevents blood clotting and might lead to better recovery.
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References
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- Shepard RB, Simpson DC, Sharp JF. Energy equivalent pressures Arch Surg 1966;93:730-740.[Abstract/Free Full Text]
- Klotz S, Deng MC, Stypmann J, et al. Left ventricular pressure and volume unloading during pulsatile versus nonpulsatile left ventricular assist device support Ann Thorac Surg 2004;77:143-150.[Abstract/Free Full Text]
- Deng MC, Wilhelm M, Weyand M, et al. Long-term left ventricular assist device support: a novel pump rate challenge exercise protocol to monitor native left ventricular function J Heart Lung Transplant 1997;16:629-635.[Medline]
- Farrar DJ. Physiology of ventricular interactions during ventricular assistanceIn: Goldstein DJ, Oz MC, editors. Cardiac assist devices. New York: Futura; 2000. pp. 15-26.
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