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The study investigates the effect of acute and incremental posthemorrhagic hypotension on pulmonary clearance of helium (CHe) introduced into the colon. Eighteen New Zealand White rabbits were cannulated and connected to a respirator at constant minute ventilation. A helium mass spectrometer was used to monitor airway gas. After 30 min stabilization, 10 ml/kg of helium were injected rectally while CHe and mean aortic blood pressure (BPm) were continuously monitored. Control animals (group 1, n = 5) achieved constant CHe (0.8-3.0 microliter/kg/min) by 20 min, with CHe and BPm continuing unchanged over a 90-min period. Group 2 animals (n = 5) underwent acute blood loss of 12 ml/kg with reinfusion after 30 min. Group 3 animals (n = 8) underwent incremental blood loss of 4 ml/kg up to a maximum of 28 ml/kg without reinfusion. Two animals in group 3 had electromagnetic flow probes placed around their distal abdominal aortae. At 12 ml/kg blood loss, group 2 and 3 animals experienced falls in BPm of 46 and 58% along with simultaneous falls in CHe of 33 and 53%, respectively. These changes were statistically significant (p less than 0.05). Reinfusion (group 2) caused initial parallel increases in CHe and BPm. However, CHe remained elevated as BPm returned to baseline, a finding consistent with colonic reperfusion hyperemia. At blood loss of more then 12 ml/kg (group 3), BPm and electromagnetic flow stabilized while CHe continued to decrease. Under these conditions CHe appeared to reflect shunting of intestinal blood flow away from the mesenteric bed.(ABSTRACT TRUNCATED AT 250 WORDS)

Acute Hemorrhagic Hypotension and its Effect on the Pulmonary Clearance of Helium Instilled into the Rabbit Colon