Assessment of Two Automated Oscillometric Monitors for Indirect Blood Pressure Measurement in Neonatal Foals and the Effect of Site of Cuff Placement

Indirect oscillometry using a cuff placed over the coccygeal artery or dorsal metatarsal artery is an acceptable method for measuring arterial pressure in foals. Arterial blood pressure is a poor indicator of blood flow in anesthetized foals when vascular resistance is altered.

1. Introduction

Arterial blood-pressure monitoring is routine practice in equine neonatal intensive care units, which allows recognition of some cardiovascular derangements and titration of therapy with intravenous fluids, vasopressors, and inotropic agents [1]. Several automated oscillometric blood-pressure devices are currently available for use in human and veterinary medicine. However, the effects of the type of monitor and site of cuff placement on the accuracy of indirect oscillometric blood-pressure measurements have never been critically investigated in neonatal foals.

Blood flow rather than blood pressure is the driving force for tissue perfusion, and measurement of cardiac output is required for calculation of global oxygen delivery and consumption. Because measurement of cardiac output is currently considered impractical for routine use in foals, indirect arterial blood pressure is commonly used as an indication for potential blood flow [2]. The objectives of this study were to assess the accuracies of two automated indirect oscillometric monitors for measurement of mean arterial pressure (MAP) in foals, to determine the optimal site of cuff placement for MAP monitoring, and to determine the relationship between arterial blood pressure and cardiac output in anesthetized foals.

2. Materials and Methods

Ten neonatal foals were anesthetized and instrumented with a catheter in the metatarsal artery for direct MAP monitoring and measurement of cardiac output by lithium dilution. Concurrent MAP measurements were obtained with Cardell [a] and Dinamap [b] oscillometric monitors with cuffs placed at three different sites (coccygeal, metatarsal, and median arteries). Blood pressure was manipulated by varying the depth of anesthesia and the administration of dobutamine or phenylephrine. For each monitor and site of cuff placement, agreement between indirect and direct MAP measurement was determined using the method for repeated measurements reported by Bland and Altman [3]. The bias was calculated as the mean difference between direct and indirect mean arterial pressures. A positive bias reflected underestimation of direct MAP by a given oscillometric monitor, and a negative value indicated overestimation of direct MAP.

3. Results

Sixty-two pairs of direct/indirect MAP readings for each monitor at each cuff placement site were taken from the 10 foals. Direct MAP ranged between 23 and 95 mmHg (52.0 ± 15.6 mm Hg). There was a statistically significant (p = 0.025) interaction between the type of monitor and cuff-placement site. With the Cardell monitor, placement of the cuff over the coccygeal artery resulted in a significantly lower bias than placement of the cuff over the median or dorsal metatarsal artery (p < 0.0001 and p = 0.0149, respectively). There was no significant difference in bias with cuff-placement site using the Dinamap monitor. There was no significant difference in bias between the Cardell and Dinamap monitor with cuff placement over the coccygeal or metatarsal artery. Cuff placement over the median artery resulted in a significantly lower bias with the Dinamap than with the Cardell monitor (p = 0.0007). At constant end-tidal isoflurane concentration, systemic vascular resistance was significantly (p < 0.001) higher when foals were given phenylephrine (867 ± 49 dynes.s.cm-5) than when the same foals were given dobutamine (418 ± 36 dynes.s.cm-5). The correlation between cardiac index and MAP was significantly (p = 0.025) higher when data collected during phenylephrine administration were excluded (r = 0.77) compared with when the complete data set was used (r = 0.47).

4. Discussion

The present study did not identify statistically significant differences in the performance of the Dinamap and Cardell monitors when data collected from all three sites were considered. However, there was a significant difference between type of monitor and site of cuff placement. With the Cardell monitor, cuff placement over the coccygeal artery resulted in a significantly lower bias than cuff placement over the median and metatarsal artery. Although statistically significant, the small difference in mean bias between the coccygeal and metatarsal MAP readings (3.6 mm Hg) is unlikely to be clinically relevant. However, the difference between the coccygeal and median artery readings (10.3 mm Hg) could lead to misinterpretation of the patient's status. In humans, the American Association for the Advancement of Medical Instrumentation has recommended a mean difference between the gold standard and a test device of ±5 mm Hg or less with SD ≤ 8 mm Hg [4]. Thus, from a clinical perspective and by standards used in human medicine, both Cardell and Dinamap monitors with cuffs placed over either the coccygeal or dorsal metatarsal arteries would be acceptable for MAP measurement in foals. Measurement of blood pressure does not correlate well with cardiac output in anesthetized foals, especially when vascular resistance is increased.

The authors thank the Florida Thoroughbred Breeders' and Owners' Association for their support of the equine research breeding herd.

Footnotes

[a] Cardell Veterinary Monitor 9402, CAS Medical Systems, Branford, CT 06405.
[b] Dinamap Pro 100, GE Medical Systems, Milwaukee, WI 53223.