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).