Blood Pressure Recording
After initiation of monitoring, 18-gauge, 1.5-inch catheters (Becton, Dickinson and Company) were placed in three locations in all 9 horses: the facial artery as it crosses over the ventral mandible (FA), the transverse facial artery (TFA), and the metatarsal artery (MT). Four horses had a 100-cm-long thermodilution catheter (Swan-Ganz CCOmbo, Edwards Lifesciences) inserted via the carotid artery with ultrasound guidance for thoracic aorta pressures (Ao). Pressure waveform analysis and external distance from percutaneous insertion site confirmed thoracic aortic placement. Arterial cannulas were connected to transducers via a low compliance extension set filled with heparinized saline. Transducers’ calibrations were verified against a column of water over a range of expected pressures (0 - 150 mmHg) prior to use and zeroed to atmospheric pressure at the level of the scapulohumeral joint to approximate the level of the right atrium.15 Before each recording, a dynamic pressure response test (“fast-flush” test), was performed to confirm adequate damping.
Non-invasive blood pressure (NIBP) was obtained via oscillometric technique using an automatic monitor (Mindray DPM 5 Vital Signs Monitor, Mindray Global) with an inflatable blood pressure cuff placed at the base of the unshaved tail. The tail circumference was measured in cm using a flexible measuring tape and cuff size was chosen to reflect a goal cuff width-to-tail circumference ratio of 0.25 as recommended for accurate MAP measurement in anaesthetised horses in previous studies.11,12 The vertical distance between the tail and the scapulohumeral joint was determined for each horse, aided by the use of a 25.4 cm multi-function standard digital level (Husky; The Home Depot) affixed to a straight wooden rod, and NIBP measurements were corrected for the hydrostatic pressure difference (1 cm = 0.735 mmHg).