Inferior Vena Cava Diameter Reductions Precede Changes in Traditional Vital Signs during Simulated Blood Loss

Abstract

Hemorrhage is a leading cause of trauma deaths. Many of these deaths could be prevented with early detection and appropriate treatment. Traditional vital signs such as heart rate (HR) and mean arterial pressure (MAP) can remain relatively normal despite the central hypovolemia that occurs during blood loss. Ultrasound measurements of the inferior vena cava diameter (IVCD) have been used clinically as gross indicators of central hypovolemia. However, it is not known if reductions in IVCD occur prior to changes in traditional vital signs during blood loss in humans. PURPOSE: To test the hypothesis that reductions in IVCD occur prior to changes in traditional vital signs during central hypovolemia. METHODS: Blood loss was stimulated using lower body negative pressure (LBNP) in fourteen healthy men (22±2 years). Pressure within the LBNP chamber was reduced by 10 mmHg every four minutes until the pressure reached -80 mmHg or subjects experienced pre-syncopal signs. Sagittal view images of maximum and minimum IVCD were obtained using B-mode ultrasonography between minutes two and four of each stage. The mean HR (ECG), MAP, pulse pressure (PP) and stroke volume (SV) (photoplethysmography) were obtained during the last minute of each stage. RESULTS: Maximum IVCD was lower than baseline (1.5±0.4 cm) at -20 mmHg (1.0±0.3 cm, P<0.01) and throughout LBNP (P<0.01). Minimum IVCD was lower than baseline (0.9±0.3 cm) at -20 mmHg (0.5±0.3 cm, P<0.01) and throughout LBNP (P<0.01). HR was only higher than baseline (70±16 bpm) at the final stage of LBNP (94±19 bpm, p<0.01). MAP was only lower than baseline (91±7 mmHg) at the final stage of LBNP (78±17 mmHg, p<0.0001). PP was lower than baseline (62±5 mmHg) starting at -40 mmHg (53±9 mmHg, P=0.03) and throughout LBNP (P<0.03). SV was lower than baseline (93±28 ml) starting at -50 mmHg (65±14 ml, P=0.01) and throughout LBNP (P<0.01). CONCLUSIONS: Reductions in maximum and minimum IVCD preceded changes in tradition vital signs during graded simulated blood loss. IVCD may be a useful tool to identify blood loss and guide treatment in patients prior to the development of vital sign abnormalities.