Combined heart and lung autotransplantation and regulation of breathing

The effects of en bloc autotransplantation of the heart and both lungs on the regulation of breathing were studied in four mongrel dogs. Tidal volume, respiratory rate, airflow, airway pressure and electromyograms from the intercostal muscles and diaphragm were recorded before and after the transplantation. The dogs breathed air or a mixture of 5% CO2 and air. Airway closure at functional residual capacity (FRC) level and after insufflation of 200 ml air was used as a mechanical stimulus. The following observations were made. 1) Stimulation by 5% CO2 after the transplantation increased the ventilatory minute volume by increasing the tidal volume while the respiratory rate remained unaltered. Before transplantation, both tidal volume and respiratory rate increased in response to CO2 stimulation. 2) After transplantation, stretching of the airways did not cause apnea (Hearing-Breuer reflex abolished) as it did preoperatively. 3) CO2 inhalation increased the efficiency of the respiratory muscles as expressed by the ratio of mechanical work (tidal volume or pressure impulse) to electrical activities of the respiratory muscles. This was most obvious in the dogs with transplant when the airways were closed at FRC level. Elimination of the afferent impulses due to en bloc transplantation of the heart and both lungs therefore modified the efferent impulses to the respiratory muscles. This effect was seen after both chemical and mechanical stimuli.