Detection thresholds of nonocclusive intestinal hypoperfusion by Doppler ultrasound, photoplethysmography, and fluorescein

Because clinical assessment of bowel viability is unreliable, other methods of determining intestinal perfusion have been recommended. Since none of these quantifies intestinal blood flow, we measured flow at the detection thresholds of Doppler ultrasound, photoplethysmography, and intravenously administered fluorescein, perfused the intestines at these threshold levels, and assessed histologic evidence of ischemic damage. The intestines of five anesthetized dogs were perfused for 4 hours via an in-line pulsatile extracorporeal circuit assembled between the iliac and superior mesenteric arteries at either relatively physiologic (approximately 20 ml/min/kg body weight) levels or reduced levels representing the flow detection thresholds of Doppler ultrasound or photoplethysmographic probes (approximately 4 ml/min/kg). Intravenously administered fluorescein was detected at even lower perfusion levels (approximately 2.1 ml/min/kg). Clear-cut ischemic changes were documented histologically in all subjects perfused at Doppler/PPG flow detection thresholds but in none of those perfused at normal levels. We conclude that threshold blood flow detection by any one of these methods, especially fluorescein, occurs at levels inadequate to guarantee tissue viability.