Pulmonary hemodynamics in neonatal lambs resuscitated with 21%, 50%, and 100% oxygen

Abstract

The effect of resuscitation with varying levels of O2 on pulmonary hemodynamics at birth is not well known. We hypothesized that the decrease in pulmonary vascular resistance (PVR) and subsequent response to pulmonary vasoconstrictors and vasodilators will differ following resuscitation with 21%, 50%, or 100% O2 for 30 min at birth in normal term lambs. Lambs at 141 d gestation were delivered by cesarean section and ventilated with 21% (21% Res; n=6), 50% (50% Res; n=6), or 100% 02 (100% Res; n=7) for 30 min followed by ventilation with 21% O2 in all three groups. A greater decrease in PVR was seen with 50% and 100% O2 ventilation than with 21% O2 (0.21 +/- 0.02, 0.21 +/- 0.02, and 0.34 +/- 0.05 mm Hg/mL/min/kg, respectively). Subsequent pulmonary vasoconstriction to hypoxia (10% O2) and the thromboxane,analog U46619 (0.5 and 1 mcirog/kg/min) was similar in all three groups. After inducing a stable elevation in PVR with U46619, impaired pulmonary vasodilation to inhaled NO (59 +/- 4, 65 +/- 4, and 74 +/- 5% of baseline PVR with 21, 50, and 100%Res, respectively) and acetylcholine infusion (67 +/- 8, 75 +/- 6, and 87 +/- 4% of baseline PVR with 21, 50, and 100%Res, respectively) and rebound pulmonary hypertension following their withdrawal were observed in the 100%Res group. We conclude that, while ventilation with 100% O2 at birth results in a greater initial decrease in PVR, subsequent pulmonary vasodilation to NO/acetylcholine is impaired.

Figure 1

Changes in PVR during the first 90 min of life. Lambs were ventilated/resuscitated with 21% (●, n = 6), 50% (▲, n = 6), and 100% (■, n = 7) O₂ for the first 30 min of life. Subsequently, lambs were ventilated with 21% O₂ for 60 min. The bar at the top of the figure indicates the fraction of inspired oxygen. *p < 0.05 compared with 21% Res lambs, †p < 0.05 compared with 50% Res lambs. Please note a change in the x axis scale after 30 min.

Figure 2

Changes in PVR during 10 min of hypoxic (10% O₂) ventilation and 20 min of recovery expressed as units (A) or percentage of baseline PVR before initiation of hypoxia (B). 21% Res (●, n = 6), 50% Res (▲, n = 6), and 100% Res (■, n = 7) O₂; †p < 0.05 compared with 50% Res lambs.

Figure 3

Decrease in PVR with inhaled NO at 20 ppm. Decrease in PVR during 10 min of NO inhalation with 21% O₂ ventilation and infusion of U46619 at 1 μg/kg/min. Rebound pulmonary hypertension was recorded 10 min after cessation of NO inhalation. 21% Res (●, n = 6), 50% Res (▲, n = 6), and 100% Res (■, n = 7) O₂, *p < 0.05 compared with corresponding baseline PVR.

Figure 4

Decrease in PVR following inhaled NO (20 ppm) and infusion of acetylcholine at 1 μg/kg/min expressed as a percentage of baseline PVR. 21% Res (open bars), 50% Res (solid bars), and 100% Res (hatched bars) O₂. *p < 0.05 compared with 21% Res group.

Figure 5

Changes in PVR during infusion of acetylcholine at 1 μg/kg/min. Rebound pulmonary hypertension was recorded 10 min after cessation of acetylcholine infusion. 21% Res (●, n = 6), 50% Res (▲, n = 6), and 100% Res (■, n = 7) O₂. *p < 0.05 compared with corresponding baseline PVR.