Lung ventilation/perfusion may reduce pulmonary injury during cardiopulmonary bypass

To the Editor:

During cardiac surgery with cardiopulmonary bypass (CPB), the surgeon is assured that all organs are perfused and/or protected. The heart—lung machine provides perfusion to all organs while the heart is either cardioplegically arrested or perfused.1 One assumes that the lungs are perfused, although pulmonary artery blood flow, the major source of blood supply to the lungs, ceases. Discontinuation of pulsatile flow and low mean pressures further accentuate decreased bronchial artery flow. The lung is the “target” organ, especially during prolonged CPB. Severe pulmonary dysfunction, manifested as poor gas exchange, pulmonary edema, and prolonged need for artificial ventilation, are consequences.

Imura and associates2 correlated low-frequency mechanical ventilation during CPB in pigs with suppression of ischemic derangements in tissue metabolism and histopathologic changes in the lungs. The technique appears simple and safe and could potentially be used in clinical practice.2

Why have cardiac surgeons not perfused/ventilated the lungs during CPB? Should they? The majority of surgeons were not trained in lung perfusion and most would say that, for short pump runs, the results are excellent. Perfusion of the lungs leads to blood in the operative field and decreases surgical precision. Lung perfusion does not necessarily have to be continuous. Intermittent lung perfusion (pneumoplegia) could be instituted at the same time that the surgeon administers intermittent cold cardioplegic arrest. This would eliminate the problem of visualization and blood in the operative field. A compromise, as suggested by Imura and associates,2 is simply to ventilate the lungs in the hope that some gas exchange would occur. Low-frequency ventilation2 is beneficial to the lungs of pigs under conditions of CPB. Can that be extrapolated to man?

An important issue to be addressed is whether lung ventilation/perfusion, during conditions of CPB, promotes gas exchange. We3 have performed all valve procedures with the heart beating, perfused simultaneously antegradely and retrogradely with warm blood, and with the lungs ventilated/perfused during the cardiac procedure, in an effort to decrease ischemia–reperfusion injury. End-tidal carbon dioxide (ETCO2), a measurement of pulmonary blood flow and cardiac output, was continuously monitored. ETCO2 provides a noninvasive estimate of cardiac output, through the ratio of change in ETCO2 partial pressure and CO2 elimination after a brief period of partial rebreathing.4 Expired CO2 is assessed by a noninvasive CO2 monitor that integrates the differential CO2 Fick partial rebreathing technique.5 Preliminary work shows that ETCO2 is an excellent indicator of lung perfusion, but its clinical value remains unclear. ETCO2 continued to be present once CPB was initiated and was unchanged throughout the operations. An example of one valve case is shown in Figure 1. Perfusion/ventilation of the lungs did not interfere with the operation.

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Figure 1 ETCO2 recorded in a patient during valve replacement. A, Before cardiopulmonary bypass (CPB) (ETCO2=22 mm Hg). B, During CPB (ETCO2=12 mm Hg). C, After CPB (ETCO2=33 mm Hg). PAW, Peak airway pressure; PEEP, peak expiratory airway pressure; RR, respiratory rate; MV, minute ventilation; TVexp, expiratory tidal volume; ETCO2, end-tidal carbon dioxide; ETO2, end-tidal oxygen; FICO2, inspired carbon dioxide fraction; FIO2, inspired oxygen fraction; Ppeak, peak pressure.

Recently, we received institutional review board approval for a prospective randomized clinical trial to evaluate lung perfusion/ventilation during CPB. Results will be forthcoming. On the basis of initial experience by Imura and associates2 in pigs and our own experience with humans, in long and complicated procedures, or in patients with pre-existing pulmonary diseases, lung perfusion/ventilation is recommended. Potentially, lung injury can be decreased in these patients.