CHANGES IN RESPIRATORY MECHANICS IN CHILDREN UNDERGOING CARDIOPULMONARY BYPASS

Congenital heart malformations are often associated with altered pulmo nary hemodynamics. Lesions associated with increased pulmonary blood f low (PBF) or increased mean pulmonary artery pressure (MPAP) may in tu rn alter respiratory mechanics. Surgical correction of these cardiac d efects frequently involves the use of cardiopulmonary bypass (CPB), du ring which the lung may be partially or completely atelectatic for len gthy periods, further compromising lung mechanics. The aims of this st udy were to document the effect of PBF on respiratory mechanics in chi ldren and to determine whether the detrimental effects of CPB were out weighed by the potentially positive effects of the corrective surgery. Twenty-three children (2-120 mo) undergoing surgery were studied whil e anesthetized, paralyzed, and mechanically ventilated. Pulmonary to s ystemic blood flow ratio was used as an index of PBF. Seventeen childr en had lesions associated with increased PBF (group 1), while six had decreased or normal PBF (group 2). Respiratory mechanics were measured just before the commencement of CPB and within approximately 2 h afte r the cessation of CPB, with the chest closed. Dynamic elastance (ERS, dyn) and resistance (RRS) were calculated from flow, volume (V), and p ressure (Pao) measurements, using multiple linear regression with a vo lume-dependent single compartment model. Static elastance (ERS,st) was calculated from Pao and V measurements obtained when deflating the lu ng in steps from a maximal Pao of 30 cm H2O. ERS,dyn, ERS,st, and RRS increased significantly with increasing PBF to 220-330% predicted. The re was no correlation between MPAP and respiratory mechanics. After CP B, ERS,dyn and RRS fell to normal levels in group 1. The higher the in itial PBF, the greater the decrease in ERS,dyn and RRS after surgical correction of the left-to-right intracardiac shunt. Preoperative ERS,d yn, ERS,st, and RRS were lower in the children in group 2 with normal or decreased PBF compared with children in group 1 with increased PBF. After CPB, ERS,st increased significantly in this group, while ERS,dy n and RRS were unchanged. These results suggest that: (1) CPB may incr ease lung stiffness in children with decreased or normal PBF; and (2) respiratory resistance and elastance are increased in children with in creased PBF and that the beneficial effects of corrective surgery outw eigh any detrimental effect of CPB in the immediate postoperative peri od.