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Accurate and relatively simple monitoring is essential in managing patients with multiple injuries, and becomes particularly important when there is substantial occult blood loss. Tachycardia, said to occur following a 15% blood loss, is generally regarded as the first reliable sign of hemorrhage. However, heart rate is a nonspecific parameter which is affected by factors other than changing intravascular volume. The purpose of this study was to evaluate available means of monitoring volume status and to identify the parameter which is the earliest and most reliable indication of blood loss.Sixteen mongrel dogs were anesthetized and bled by increments of 3% of their total blood volume until the onset of sustained hypotension or a 25% blood loss. All dogs were monitored with a Swan-Ganz catheter and an arterial line. Vital signs, full hemodynamic parameters, and arterial and mixed venous blood gases were measured after each 3% blood loss. Statistical analysis of the data demonstrated that only Cardiac Index and Mixed Venous Oxygen Saturation showed linearity as function of measure blood loss. Linear regression analysis generated r values that ranged from 0.85–0.99 with a mean of 0.95 for Mixed Venous Oxygen Saturation; r values for Cardiac Index ranged from 0.39–0.98 with a mean of 0.85. Furthermore, all dogs had increased tissue oxygen extraction after 3–6% blood loss. Because Central Venous Blood Oxygen Saturation mirrors Mixed Venous Oxygen Saturation and is easily and rapidly measured, we extended our study by repeating all of the previously measured parameters, with the addition of CVP blood gases in an unanesthetized animal model. Seven mongrel dogs were implanted with Swan-Ganz, CVP, and carotid lines, allowed 24 hours to recover, and then bled in a conscious state according to our previous protocol. The results obtained were in agreement with the results as noted above. Statistical analysis revealed that r values for CVP Blood Oxygen Saturation ranged from 0.79–0.98 with a mean of 0.89.We conclude that Mixed Venous Oxygen Saturation is a sensitive and reliable parameter with which to gauge acute blood loss under controlled experimental conditions. Serial CVP Blood Oxygen Saturation measurements may be a useful way to identify patients with occult or ongoing blood loss following trauma. The availability of 40-cm fiberoptic intravascular oxygen saturation catheters makes continuous monitoring feasible.