Effects of pulmonary restriction on hypercapnic responses of heart-lung transplant recipients

Previous studies of hypercapnic ventilatory responses (HCVR) in human heart-lung transplant recipients (HLTX) have yielded conflicting results. We compared the HCVR of restricted transplant recipients (HLTX-R) to recipients with normal pulmonary function (HLTX-N), and normal controls (C). HLTX-R exhibited limited tidal volume responses, whereas their frequency responses were essentially identical to those of other subjects. Accordingly, HCVR of HLTX-R (1.45 ± 0.59 l · min^-1 · Torr CO^-1/₂) were significantly depressed compared with both HLTX-N and C (2.90 ± 0.55 vs. 3.05 ± 1.23, respectively) (P < 0.02). Despite undoubtedly greater ventilatory impedances, airway (mouth) occlusion pressure responses (Pm_0.1) during hypercarbia of HLTX-R (0.46 ± 0.28 cmH₂O) were similar to those of C (0.43 ± 0.20) and paradoxically blunted compared with HLTX-N (0.83 ± 0.36) (P < 0.02). We conclude that pulmonary reflexes are superfluous for maintenance of HCVR in HLTX with normal respiratory mechanics, whereas the presence of moderate restriction results in profound depression of CO₂ responses among these subjects.