Accuracy of noninvasive oxyhemoglobin saturation, end-tidal carbon dioxide concentration, and blood pressure monitoring during experimentally induced hypoxemia, hypotension, or hypertension in anesthetized dogs.

OBJECTIVE

To determine reliability of noninvasive methods of arterial oxyhemoglobin saturation (SpO2), end-tidal CO2 concentration (PEtCO2), and blood pressure (BP) determination during periods of hypoxemia and systemic arterial BP perturbations.

ANIMALS

7 healthy, conditioned dogs weighing 19 to 22 kg.

PROCEDURE

3 pulse oximeters, 2 capnometers, and 2 oscillometric BP monitors were used to measure oxygen-carrying capacity of the blood, heart rate, ventilatory status and arterial BP changes during hypoxemia, and altered arterial BP. Pulse oximeter-derived SpO2 and PEtCO2 were determined during rapidly induced plateaus of hypoxia (decreased fractional in-spired oxygen concentration [FiO2]) and altered systemic arterial BP. A lead-II ECG was used to monitor heart rate.

RESULTS

Pulse oximetry provided an accurate assessment of fractional oxyhemoglobin saturation (SaO2) at SpO2 > 70%. As SaO2 decreased from 70%, the magnitude of the SpO2 error increased (20% error at SpO2 < 30%). The PEtCO2, was accurate at PaCO2, ranging from 30 to 55 +/- 5 mm of Hg under all experimental conditions. When PaCO2 was > 55 mm of Hg, both capnometers produced values that were as much as 20 mm of Hg less than the corresponding PaCO2. Mean BP was least dependent on pulse wave quality, consistently underestimating mean arterial BP by approximately 10 mm of Hg.

CONCLUSIONS AND CLINICAL RELEVANCE

The pulse oximeters tested provided an accurate estimation of SaO2 at SpO2 > 70%. A PEtCO2 value > 55 mm of Hg may represent hypercapnia that is more profound than indicated. Systolic BP determinations were most accurate during hypotensive states and least accurate during hypertension. Diastolic BP measurements were generally more accurate during hypertension than normotension. Accuracy is not appreciably affected by hypotension resulting from vasodilation or blood loss. The tendency to underestimate systemic arterial BP should not interfere with trend detection during unstable clinical conditions.