Invasive blood pressure measurement in anaesthetised horses: a clinical and an experimental study

General anaesthesia of horses is associated with an increase in mortality and morbidity in comparison to other species. The association between the development of hypotension and major anaesthetic complications is well documented. Thus, the recommendation to monitor and treat hypotension is based on measuring pressure directly from a catheter inserted in a peripheral artery. Although invasive blood pressure (IBP) is considered the gold standard in blood pressure measurement, it is unknown whether pressure measured in different peripheral arteries is uniform across the various sites. It is also unknown whether pressure at these peripheral sites are indicative of the central pressures, which govern perfusion to the vital organs. Objectives 1. To determine agreement between invasive blood pressure measured in three peripheral arteries in anaesthetized horses undergoing elective surgery. 2. To determine the agreement between invasive blood pressure measured in the facial and the metatarsal artery with the carotid artery and to evaluate the effects of two haemodynamic conditions on this agreement in anaesthetised horses. Methods The first objective was achieved using clinical cases undergoing anaesthesia for elective surgery. Invasive blood pressure was measured simultaneously in one of the following three combinations: i) transverse facial and facial artery; ii) transverse facial and metatarsal artery and iii) facial and metatarsal artery. The agreement in blood pressure measured for each combination was performed in six horses, three positioned in dorsal recumbency and three positioned in lateral recumbency as determined by a balanced incomplete block design. At each sample time, systolic (SAP), mean (MAP) and diastolic (DAP) arterial pressures were measured concurrently in each artery and the mean of three consecutive measurements was recorded. Position of horse, heart rate and the use of dobutamine were also recorded. Bland Altman analysis was used to assess agreement between sites. The second objective was achieved using a non-recovery experimental model. Horses were anaesthetised and positioned in dorsal recumbency. Invasive blood pressure was measured simultaneously via catheters placed in the facial, metatarsal and carotid artery. Cardiovascular function and agreement between arteries was assessed before and during administration of phenylephrine and sodium nitroprusside. Phenylephrine and sodium nitroprusside were administered until carotid mean pressure (MAPc) increased or decreased from baseline (65 ± 5 mmHg) to > 90 mmHg or < 50 mmHg, respectively. The order of phenylephrine and sodium nitroprusside was balanced and allocated randomly by selecting the protocol from sealed envelopes on the day of the study. Data recorded at each sample time included systolic, mean and diastolic pressure for carotid (c), facial (f) and metatarsal (m) artery as well as cardiac output (Qgt) and systemic vascular resistance (SVR). Bland-Altman analysis was used to assess agreement between peripheral and central sites and regression analysis was used to determine influence of Qgt and SVR. Results In the clinical study, a total of 54 paired measurements were obtained, with 18 paired measurements from each combination. All paired measurements showed poor and haphazard (non-systematic) agreement. The widest limit of agreement (LOA) was 51mmHg for SAP measured in the facial artery and metatarsal artery with a bias of -11 mmHg. The smallest limit of agreement was 16 mmHg for MAP measured in the transverse facial and the metatarsal artery with a bias of 1 mmHg. In the experimental study, a total of 96 paired measurements were obtained between peripheral arteries and the carotid artery. The largest difference was observed in the SAP of the carotid and the metatarsal arteries with a bias (LOA) of 2 (-15 to 19) mmHg. The bias (LOA) for MAP between the carotid and the facial arteries was 2 (-4 to 9) mmHg and for MAP between the carotid and metatarsal arteries was 5 (-4 to 14) mmHg. The best agreement for DAP was seen between the carotid and the facial arteries with a bias (LOA) of 1 (-3 to 5) mmHg. Regression analysis indicated marginal influence of Qt on agreement between MAPc and MAPf and little influence of systemic vascular resistance. Conclusion and clinical relevance There was poor and haphazard agreement for SAP, MAP and DAP measured in each pair of peripheral arteries in the clinical study. This was supported by the results of the experimental study. These results show that blood pressure measured in different peripheral arteries cannot be used interchangeably. This has implications for studies that use IBP as an outcome variable and studies determining agreement between non-invasive blood pressure and IBP measurements in horses under general anaesthesia. In the experimental study MAP and DAP of the carotid was generally higher compared to the peripheral arteries. Thus, measurement of blood pressure in peripheral arteries may lead to overzealous treatment of hypotension, albeit maintaining central pressures. The best agreement observed with the carotid artery was the facial artery. Cardiac output and systemic vascular resistance did not largely influence the difference between sites.