Discussion
The primary goal of the present study was to determine whether MAP measured via NIBP was equivalent to invasive MAP during normovolemic normotension and hypovolemic hypotension in dorsally-recumbent anaesthetised horses. The NIBP was measured at the base of the unshaved tail using a goal cuff width-to-tail circumference ratio of 0.25 because previous studies have shown that this was the only anatomic site and cuff width to be equivalent to invasive MAP.12,13,20Invasive MAP was measured at three peripheral arteries (FA, TFA, MT) based on their clinical relevance and because MAP might vary substantially among different peripheral arteries in different animals.8,20 By demonstrating equivalence between MAP measured via NIBP and invasively in dorsally-recumbent isoflurane anaesthetized horses over a wide range of systemic arterial pressures encompassing normovolemic normotension and hypovolemic hypotension, the current results corroborated and extended previous findings in normotensive, laterally recumbent, isoflurane-anaesthetized horses and ponies.11,12 However, equivalence between NIBP and invasive MAP was obtained for the FA but it was not equivalent for the TFA in the dorsally-recumbent horses of the present study. This result differs from the findings using the TFA in laterally-recumbent horses of a previous study,11 suggesting that body positioning during anaesthesia might affect equivalence between NIBP and invasive MAP at least in the TFA. A goal cuff width-to-tail circumference ratio of approximately 0.25 is critical for accurate measurements,11,20 with larger ratios of 0.4-0.6 being consistently found to yield non-equivalent results in anaesthetised and awake standing horses under normotensive, hypotensive and hypertensive conditions.6,7,11,20 Taken together, the current results and the available literature supports the conclusion that NIBP can be used to accurately monitor arterial blood pressure in anaesthetised horses during normovolemic normotension and hypovolemic hypotension provided that it is measured at the tail base with an appropriate goal cuff width-to-tail circumference ratio of 0.25.
The sizable vertical distance between the middle coccygeal artery and the heart base requires measurement adjustment to account for differences in gravity for horses in dorsal recumbency. If this is not done, readings from the NIBP on the tail will be falsely elevated by 0.735 mmHg for each centimetre of vertical distance. The distribution of vertical distances in the horses studied was fairly homogeneous around a median of 27 mmHg, despite the range of breeds and sizes. Equivalence between NIBP and FA was still demonstrated when the NIBP data was corrected using a standardised 27 mmHg for all horses. Using this approach, equivalence was also demonstrated between NIBP and MT. While ideally the correction should be based on each horse’s individual vertical distance, these results demonstrate that a standard correction of 27 mmHg might be acceptable if the vertical distance is not known or cannot be accurately determined as may be the case in a clinical setting.
Calculation of sensitivity and specificity based on MAP from the FA that would prompt treatment for hypotension (MAP < 70 mmHg) demonstrated that the NIBP had a false negative rate of 12% (sensitivity of 88%) and a false positive rate of 27% (specificity 73%) when corrected to heart level using each horse’s vertical distance. When NIBP was corrected to heart level using the standardised 27 mmHg, the false negative rate increased to 20%, and the false positive rate decreased to 22%. Thus, although equivalence was demonstrated during a wide range of arterial pressures, NIBP measurements would cause true hypotension to go untreated in 12-20% of instances depending on the heart level correction method used. Since in the 12% false negative instances the NIBP overestimated invasive MAP by an average of 11 mmHg, hypotension was redefined as NIBP MAP < 80 mmHg and < 90 mmHg while using a standardised heart level correction of 27 mmHg and sensitivity, specificity, false negative and false positive data were recalculated. Using this approach, a definition of hypotension of NIBP MAP < 80 mmHg (corrected to heart level using 27 mmHg for all horses) or < 107 mmHg (uncorrected to heart level) resulted in the best balance between false negatives (5% of hypotensive horses not treated) and false positives (35% of normotensive horses being treated). Successive measurements at individual time points could offer additional protection for cases where values are borderline. Collectively, these results suggest that, when using NIBP at the tail base with an appropriately sized cuff (goal: 0.25 ratio), hypotension should be defined as NIBP < 80 mmHg (corrected to heart level) or < 107 mmHg (uncorrected to heart level) in dorsally-recumbent, anaesthetized horses. However, the limitations to this approach must also be recognized such that invasive arterial pressure monitoring should remain the standard in equine anaesthesia whenever possible.
The variation in blood pressure measurements between different peripheral arteries found in the current study align with results from previous studies.20-22 A second goal of the present investigation was to describe the variability of the differences between MAP invasive measured in FA, TFA and MT in relation to that measured centrally in the thoracic aorta, seeking to gain insight into whether or not there is an ideal or preferred peripheral site for invasive measurement of arterial blood pressure in anaesthetised horses. The FA was found to have the least variability, followed by the MT and the TFA, suggesting that the FA might be the preferred peripheral artery site for invasive MAP measurement. It is unclear why the variability was greatest in the TFA. The finding of a mean positive MAP gradient between Ao and TFA of 2 mmHg (95% CI 0, 4), which is not physiologically possible, suggests that there were external factors, such as neck/head orientation during dorsal recumbency, that influenced the accuracy of invasive MAP determination at this site. The above results suggest that the FA and TFA are, respectively, the most and least desirable sites for invasive MAP determination in dorsally recumbent, isoflurane-anaesthetised horses. These observations along with the finding of equivalence between NIBP and FA, provide further assurance regarding the usefulness of measuring MAP via NIBP at the tail base with a goal cuff width-to-tail circumference ratio of 0.25 in dorsally recumbent, isoflurane-anaesthetised horses.
Several limitations in this study should be considered. Invasive blood pressure measurements from the Ao were only available for 4 horses in the study, which may underpower those results. Invasive blood pressures in the middle coccygeal artery were not concurrently measured during the experiment. Our results, similar to other studies,20,22,23 indicate that MAP varies among peripheral arterial sites, and therefore NIBP may or may not be an accurate reflection of the true pressure of the artery it is measuring.8 Another limitation of this study is that the experiment was performed on adult horses and may not be translatable to draft breeds, foals, or ponies.
In conclusion, NIBP measured at the tail (goal cuff width-to-tail-circumference ratio of 0.25) is equivalent to invasive MAP from the FA in anaesthetised horses in dorsal recumbency. The results suggest that hypotension with NIBP should be considered as MAP less than 80 mmHg (when 27 mmHg is subtracted as a heart level correction) or less than 107 mmHg when no heart level correction is made. Based on comparisons between invasive MAP in peripheral locations and MAP from the Ao, the invasive MAP from the FA had the lowest coefficient of variation, suggesting it is the peripheral artery that is most reflective of aortic pressures. This study evaluated hypotensive and normotensive blood pressure readings in adult horses and further research is needed to guide use in draft breeds, juveniles, or hypertensive states.
Table 1: Population data, measurements for 0.25 cuff width-to-tail circumference ratio, cuff selection, and heart level adjustment based on the vertical distance between the tail and the scapulohumeral joint (approximation of the right atrium; 1 cm = 0.735 mmHg) for invasive and non-invasive oscillometric blood pressure (NIBP) comparisons in horses anaesthetised in dorsal recumbency during normotension and in a hypovolemic haemorrhage model of hypotension (n = 9).