Does blood cardioplegia solution cause deterioration in clinical pulmonary function following coronary artery bypass graft surgery?

Abstract

Pulmonary dysfunction following cardiopulmonary bypass surgery is a widely explored complication and a multitude of factors have been implicated, including but not limited to: operative trauma; the cardiopulmonary bypass circuit; cardioplegia; the type of donor grafts utilised; anaesthesia and fluid administered. There is a paucity of information regarding the effect of cardioplegia on the lungs. No studies have previously investigated whether allowing cold-blood cardioplegic solution to enter the lung parenchyma, during the period of cardioplegia delivery, has an effect on the clinical outcome of lung function following cardiopulmonary bypass surgery. For this reason an original study was done to determine the effect of preventing cardioplegia from entering the lungs, by evacuating overflow of cardioplegia not drained via the atriocaval cannula, by using a pulmonary artery vent. A total of 403 patients admitted to undergo full cardiopulmonary bypass were screened and 142 patients who fitted the criteria for inclusion and provided informed consent took part in this prospective double blind randomised clinical trial. The control group underwent routine cardiopulmonary bypass grafting. The study group had the intervention of a pulmonary artery vent sutured in position at the time the heart was cannulated for bypass. During cardioplegia delivery the cardioplegia was removed via the atriocaval cannula in the control group (A) and via the atriocaval cannula and the pulmonary artery vent in the study group (B). Aside from this difference, the two groups were managed identically intra- and post-operatively. Outcomes which were compared included eight time measures of arterial blood gases; electrolytes and shunt fraction; bedside lung spirometry measures over five time periods; radiographic measures of atelectasis and effusion over three time points; as well as physiotherapy and hospitalisation requirements. Numerous other potentially extraneous variables were measured and compared in order to monitor homogeneity of the study samples. The consistency of the results within each group throughout the study provides strong evidence that the measurements taken were accurate. The use of standardised equipment and vigilant adherence to the protocol ensured no extraneous deviation. The internal validity of this study was therefore good and accurate. The findings of the study however brought into question a previously accepted belief that the pulmonary artery vent prevents the overflow of cardioplegia, not drained from the right atrium, from entering the lungs. There was no literature or previous studies to confirm or dispute this accepted ‘observation’ by cardiac surgeons that the cardioplegia does enter the lung parenchyma. To therefore validate the findings of the study a further four original studies were designed and initiated. The objective of these studies was to establish the efficacy of the pulmonary artery vent and to determine whether cardioplegia indeed circulates through the lung parenchyma or merely accumulates and ‘pools’. Technetium (Tc-99m), a radio labelled isotope was added to the cold blood cardioplegia solution prior to delivery in order to determine this. In the four sub-studies it was confirmed that the pulmonary artery vent is 90-100% effective in retrieving any cardioplegic solution not drained by the atriocaval cannulae, thus confirming the effectiveness of the pulmonary artery vent in preventing cold blood cardioplegic solution from entering the lungs. The findings of the main study confirmed that respiratory impairment after uncomplicated cardiopulmonary bypass, even in low risk patients, is relatively common, as within each group there was a significant change in outcome measures over time. Inter-group comparisons however showed these changes were not significant, with both groups deteriorating by the same degree post-operatively, therefore establishing that these changes were independent of the intervention of the pulmonary artery vent. In the control group, the cold blood cardioplegia solution that did not drain from the atriocaval cannula entered the lungs and circulated the lung parenchyma during cardiopulmonary bypass. The study group made certain that none, or very little, of the cold blood cardioplegia solution entered the lungs. The main findings of this study are therefore that pulmonary function and gas exchange, although markedly reduced following cardiac surgery, are not affected by placement and suctioning via a pulmonary artery vent during the time of cardioplegia delivery intraoperatively. Furthermore, these studies strongly suggest that cold blood cardioplegia solution is innocuous to the lungs