Systolic Arterial Pressure in Dogs Pharmacologically Induced to Vasoconstriction and Vasodilation

Accuracy of Measurement by Photoplethysmography

Abstract

Background: Systolic arterial pressure (SAP) monitoring is an essential parameter in veterinary anesthesiology, enabling early detection of hemodynamic instability. Arterial catheterization is considered the gold standard; however, non-invasive methods such as Doppler (DOP) and photoplethysmography (PPG) have emerged as relevant alternatives when arterial catheterization is not feasible. While PPG is well established in human medicine, its application in animals remains scarcely explored. The aim of this study was to assess the accuracy of PPG in measuring SAP in dogs anesthetized with propofol, comparing it with DOP under vasoconstriction and vasodilation induced by norepinephrine and sodium nitroprusside, respectively.

Materials, Methods & Results: Eleven healthy male dogs (1-6 years-old; 7-15 kg) were enrolled in the study. Following fasting and pre-anesthetic evaluation, anesthesia was induced with propofol (7.53 ± 2.13 mg/kg) and maintained with continuous infusion of the same agent (0.55-0.59 mg/kg/min), with no significant differences between treatments. Baseline SAP was recorded during infusion of 0.9% saline solution (5 mL/kg/h). Subsequently, each dog received, in randomized order, norepinephrine (NOR; 0.3-0.7 µg/kg/min) or sodium nitroprusside (NIT; 1-7 µg/kg/min), interspersed with saline until hemodynamic recovery. Each phase was evaluated at 3 time points (T1, T2, and T3 at 5-min intervals), with SAP measured by DOP (ulnar artery) and PPG (interdigital sensor). Data were analyzed using ANOVA or Friedman tests, followed by post hoc comparisons, and correlations were estimated by Spearman’s rank test (P < 0.05). SAP increased significantly with NOR and decreased with NIT compared to SAL. In DOP, the SAP reduction with NIT was significant at T2 and T3 versus SAL, whereas PPG detected differences as early as T1. Correlations between PPG and DOP were very strong under NOR (r = 1.00; p = 0.0028), strong under NIT (r = 0.82), and weak to moderate under SAL (r = 0.50).

Discussion: PPG demonstrated performance equivalent to that of DOP under pharmacologically induced vasoconstriction, suggesting that it may reliably estimate SAP in this scenario. During vasodilation, although the correlation remained strong, a reduction in sensitivity was observed, possibly attributable to lower peripheral pulse amplitude and reduced pulsatile contrast, as previously described in humans under the effect of vasodilators. At baseline normotension, the correlation was only moderate, indicating greater variability between methods when no marked hemodynamic changes are present. Furthermore, the similarity in propofol infusion rates among groups suggests that blood pressure variations were primarily due to the action of vasoactive drugs rather than fluctuations in anesthetic depth. In conclusion, PPG represents a promising tool for estimating SAP in anesthetized dogs, showing greater reliability under vasoconstrictive conditions. However, its clinical application should be approached with caution in states of normotension or hypoperfusion, and further studies with larger sample sizes and validation against invasive methods are required to confirm its accuracy and broaden its applicability.

Keywords: pulse oximetry, SAP, Doppler, norepinephrine, sodium nitroprusside, PPG, total intravenous anesthesia, propofol, canine.