Hypoxic pulmonary vasoconstriction during and after infusion of group B Streptococcus in neonatal piglets. Vascular pressure-flow analysis

Spatial and temporal heterogeneity of regional pulmonary blood flow in piglets

Regional pulmonary blood flow (PBF) in adult animals varies over space and time, following a fractal pattern. We hypothesized that PBF would follow a fractal pattern in young animals. Five, two-week old piglets were sedated and mechanically ventilated. After stabilization, fluorescent microspheres were injected via the right atrium at baseline and then again at 5, 20, 20.5, 40 and 60 min. The lungs were subsequently excised, dried, inflated, and cored into 0.12-cm3 pieces (mean n=561+/-106 per animal) with the spatial coordinates recorded for each piece. Regional PBF was spatially and temporally heterogeneous with a spatial coefficient of variation of 43.3+/-7.9% and a temporal coefficient of variation of 14.3+/-0.4%. PBF followed a fractal pattern with a fractal dimension of 1.20+/-0.06 at 20 min, remaining stable throughout the experiment. PBF decreased with distance from the hilum but did not follow a lobar pattern. Temporal heterogeneity did not significantly increase with time but low flow regions demonstrated the greatest temporal variability throughout the study. Hence, PBF in young piglets was characterized both spatial and temporal heterogeneity.

Nitric oxide synthase inhibition restores hypoxic pulmonary vasoconstriction in sepsis

Hypoxic pulmonary vasoconstriction (HPV) is inhibited by inhaled nitric oxide (NO) in healthy animals and is blunted in endotoxemia. We investigated whether the loss of HPV during sepsis could be reversed by NO synthase (NOS) inhibition. Hypoxic challenges were induced in intubated, awake sheep breathing 100% nitrogen to the left lung and 100% oxygen to the right lung. HPV was assessed as the decrease in left pulmonary blood flow during hypoxia, measured with an ultrasonic flow probe around the left pulmonary artery. Group I (n = 5) received carrier solutions and Groups II (n = 6) and III (n = 8) received an infusion containing Pseudomonas aeruginosa. After 24 h, Group III also received an infusion of 6.6 mg.kg.h-1 N omega-monomethyl-L-arginine (L-NMMA). After 24 h of sepsis, HPV decreased from 60 +/- 9% in Group II and 56 +/- 4% in Group III to 27 +/- 2% and 26 +/- 4%, respectively. Group I showed no change in HPV. During infusion of L-NMMA, HPV increased to 38 +/- 4%. Pulmonary shunt during hypoxia increased in Group III to 161 +/- 10% of its baseline value, and decreased to 121 +/- 11% during infusion of L-NMMA. We conclude that L-NMMA improves but does not restore HPV, indicating that other vasodilatory mediators besides NO also influence HPV in sepsis.