Effect of Hypotension and Dobutamine on Gastrointestinal Microcirculations of Healthy, Anesthetized Horses

Simple Summary When horses undergo abdominal surgery, there is a risk that tissues may not receive enough blood. One way to decide if this is happening is to measure blood pressure. The assumption is that if blood pressure is normal, then tissue blood flow is fine. A technique called dark-field microscopy has been used to look at the blood flow to tissues. We measured blood flow at three different places (the gums, the rectum, and the outside of the colon) of six healthy horses undergoing general anesthesia. Additionally, we measured how blood flow to these tissues changed when the patient's blood pressure was lowered by giving an increased amount of gas anesthesia, which was subsequently raised with a drug called dobutamine. We found that blood flow to these tissues was present when the patients had normal blood pressure. When the blood pressure decreased, the blood flow to the tissues was unaltered. Finally, when blood pressure returned to normal, the blood flow to the tissues remained unaltered. Because blood flow to the tissue was unchanged despite clinically relevant changes in blood pressure, we concluded that using blood pressure as the only method to assess tissue blood flow may be inaccurate.Abstract Horses undergoing abdominal exploratory surgery are at risk of hypotension and hypoperfusion. Normal mean arterial pressure is used as a surrogate for adequate tissue perfusion. However, measures of systemic circulation may not be reflective of microcirculation. This study measured the mean arterial pressure, cardiac index, lactate, and four microcirculatory indices in six healthy, anesthetized adult horses undergoing elective laparotomies. The microcirculatory parameters were measured at three different sites along the gastrointestinal tract (oral mucosa, colonic serosa, and rectal mucosa) with dark-field microscopy. All macro- and microcirculatory parameters were obtained when the horses were normotensive, hypotensive, and when normotension returned following treatment with dobutamine. Hypotension was induced with increases in inhaled isoflurane. The horses successfully induced into hypotension did not demonstrate consistent, expected changes in systemic perfusion or microvascular perfusion parameters at any of the three measured gastrointestinal sites. Normotension was successfully restored with the use of dobutamine, while the systemic perfusion and microvascular perfusion parameters remained relatively unchanged. These findings suggest that the use of mean arterial pressure to make clinical decisions regarding perfusion may or may not be accurate.