Operative creation of left to right cardiac shunts: pulmonary functional sequelae

Preoperative lung function in newborn infants with univentricular hearts compared with healthy controls

AIM

To measure the difference in lung function between newborns with univentricular hearts and healthy controls and study associations between lung function and pulmonary blood flow and pulmonary vasculature markings.

METHOD

Tidal flow-volume measurements and single occlusion tests were performed before surgery on 25 unsedated spontaneously breathing newborns with univentricular hearts recruited over a 3-year period. Seventy-five healthy control infants were measured. Pulmonary blood flow was graded according to the haemodynamic effect of the echocardiographically defined anatomy of the heart defect. Pulmonary vasculature was graded according to radiological markings.

RESULTS

The infants with univentricular hearts had a lower mean tidal volume of 4.7 ml (CI 2.3, 7.2, p < 0.001) (n = 24) and a lower mean compliance of the respiratory system of 12.7 ml/kPa (CI 4.6, 20.8, p = 0.004) (n = 14). Pulmonary blood flow grading was associated with respiratory rate (r = 0.53, p < 0.001), tidal volume (r =-0.48, p < 0.001), compliance (r =-0.55, p < 0.001) and resistance (r =-0.31, p = 0.043). Pulmonary vasculature grading was associated with compliance (r =-0.68, p = 0.006), resistance (r =-0.69, p = 0.007) and the time constant (r =-0.62, p = 0.042).

CONCLUSION

Newborns with univentricular hearts have reduced tidal volumes and reduced compliance of the respiratory system. The lung function abnormalities are associated with the degree of pulmonary blood flow and pulmonary vasculature markings.

Changes in respiratory mechanics among infants undergoing heart surgery

Children with excessive pulmonary blood flow (PBF) from congenital heart disease have abnormal respiratory mechanics. Exposure to hypothermic cardiopulmonary bypass (CPB) adversely affects lung function. We designed this study of 106 patients to determine the changes in respiratory mechanics in infants younger than 1 yr undergoing heart surgery. Dynamic respiratory compliance (Cdyn) and total respiratory resistance (Rrs) were measured before surgical incision, after sternal closure in the operating room, and after arrival in the intensive care unit. The following data were recorded: age, weight, preoperative pulmonary infiltrates, preoperative mechanical ventilation, evidence of increased PBF before surgery, duration of CPB, duration of aortic cross-clamp, duration of deep hypothermic circulatory arrest, use of steroids, and volume of ultrafiltrate removed. Repeated-measures analysis of variance with covariate analysis was used to determine the effect of each covariate on Cdyn and Rrs at the three time periods. Rrs improved after cardiac surgery correcting increases in PBF, and this was most pronounced in neonates. Among infants with normal or reduced PBF, cardiac surgery with CPB led to a reduction in Cdyn. We consider that the benefits of surgical correction of pulmonary overcirculation outweigh the negative effects of CPB on respiratory mechanics.

IMPLICATIONS

The benefits of surgical correction of pulmonary overcirculation outweigh the negative effects of cardiopulmonary bypass on respiratory mechanics in infants.

Pulmonary sequelae in infants treated with extracorporeal membrane oxygenation

The decision to place an infant on extracorporeal membrane oxygenation (ECMO) is based on predictions of expected morbidity and mortality. One unknown factor is the relationship between pre-ECMO pulmonary dysfunction and on barotrauma and post-ECMO pulmonary sequelae. To determine whether placement of infants on extracorporeal membrane oxygenation (ECMO) early is associated with less subsequent pulmonary dysfunction than placing infants on EMCO later, we evaluated pulmonary function in 25 neonates prior to ECMO, when the infants had come off EMCO, and at the time of nursery discharge. Pulmonary resistance (R) and compliance (CL) were determined by a pneumotachograph and esophageal manometry, and functional residual capacity (FRC) was determined by a helium dilution method. Maximal expiratory flow (VmaxFRC) was determined by thoracic compression at the time of discharge. Infants were assigned to an early ECMO group (< 36 hours of age, n = 12), or a late ECMO group (> 36 hours of age, n = 13). When first evaluated, the early group had a higher oxygenation index than the late group (mean value, 63 versus 48), but initial pulmonary function measurements were not different between the two groups. In the early group mean CL increase from 0.20 to 0.36 ml/cmH2O/kg, FRC increased from 7 to 20 ml/kg, and mean R decreased from 107 to 61 cmH2O/L/sec between the initial study and immediately after ECMO. In the late group, only FRC increased from a mean of 8 to 20 ml/kg. CL and FRC increased from post-ECMO to discharge in both groups (mean CL from 0.36 to 0.76 ml/cmH2O/kg in the early group, and from 0.30 to 0.79 in the late group). Mean FRC increased from 20 to 26 ml/kg in the early group, and from 20 to 25 ml/kg in the late group. VmaxFRC was lower in the late than the early group at discharge (mean, 1.14 versus 1.58 L/sec; P < 0.05). While both groups of infants had minimal pulmonary dysfunction at discharge, the infants placed on ECMO early had evidence of slightly less airway dysfunction despite a higher initial oxygenation index than the infants placed on ECMO late.

Infant thoracic surgery: procedure-dependent pulmonary response

Respiratory insufficiency is a common complication of thoracic surgery in infants. To better define this dysfunction, pulmonary compliance (CL) and resistance (R) were measured for 17 infants who underwent common thoracic procedures: Blalock-Taussing shunting (n = 7) repair of congenital coarctation of the aorta (n = 10). Measurements were obtained preoperatively and 0, 1, and 3 days postoperatively. Preoperatively, CL was lower and R was similar for the two groups. Both groups had decreased CL and increased R on postoperative day 0; infants with coarctation had recovery to preoperative values by postoperative day 1 for CL, and day 3 for R. CL and R did not return to the preoperative values by postoperative day 3 in infants with a shunt procedure. The changes in R were greater than those in CL for both groups in the postoperative period. These data indicate that such thoracic procedures are associated with pulmonary morbidity that is airway-predominant, and that the degree of compromise and the time until recovery are, in part, procedure-specific.

Continued pulmonary recovery observed after discontinuing extracorporeal membrane oxygenation

Extracorporeal membrane oxygenation (ECMO) is a valuable therapy for the treatment of reversible lung disease in neonates. Associated with this treatment, however, are risks for complications that increase with the duration of therapy. We evaluated alveolar-arterial oxygen tension difference P(A-a)O2 pulmonary compliance (CL), and functional residual capacity (FRC) in 20 infants immediately after ECMO was discontinued, and again 24 hours thereafter. We measured CL by pneumotachography and esophageal manometry and FRC by helium dilution. Mean (+/- SEM) values for CL and FRC increased (CL from 0.28 +/- 0.02 to 0.35 +/- 0.03 mL/cmH2O)/kg and FRC from 18.6 +/- 1.4 to 22.2 +/- 1.1 mL/kg; P < 0.05), and P(A-a)O2 and the oxygenation index (OI) decreased (200 +/- 19 to 169 +/- 14 mm Hg and 6.9 +/- 0.44 to 5.4 +/- 0.5, respectively; P < 0.02), over the 24 hour period following ECMO. Nineteen of 20 infants experienced improvement in at least two of these parameters. Improvements were found to be greatest in the infant with the worst lung function immediately after discontinuing ECMO, and in the ten infants who had not received pancuronium bromide for inducing skeletal muscle paralysis, following decannulation from ECMO. These data indicate that improvement in lung function following ECMO will generally continue over the 24 hour period following the termination of cardiopulmonary bypass, and that borderline pulmonary status may not preclude discontinuation of bypass therapy.

High frequency jet ventilation: intraoperative application in infants

The potential advantages of the intraoperative use of high frequency jet ventilation (HFJV) when compared with conventional ventilation (CV) include the maintenance of adequate gas exchange and lung function with a relatively motionless surgical field. To determine the pulmonary response to HFJV ventilation in infants during cardiac surgery, we evaluated lung function in nine infants supported with CV and HFJV during a Blalock-Taussig shunt procedure. Infants were randomized to each mode of ventilation with inspiratory and expiratory pressures and FiO2 held constant. Heart rate, blood pressure, arterial blood gases, pulmonary mechanics (lung compliance and resistance), and functional residual capacity (FRC) were compared after 10 minutes of stabilization of each ventilation mode, with the infants in the thoracotomy position and the surgical field adequately exposed. Pulmonary mechanics were measured using esophageal manometry and pneumotachography, and FRC by helium dilution. There was no difference in vital signs, pulmonary mechanics, FRC, or PaO2 on HFJV ventilation when compared with CV. Arterial PaCO2 was lower with a lower mean airway pressure on HFJV when compared with CV. The surgical team subjectively observed a diminished need for lung manipulation and improved ease of access to the surgical field with HFJV. These results indicate that the use of HFJV during closed-heart cardiac surgical procedures in infants provides similar cardiopulmonary stability and some potentially important clinical benefits when compared with CV.