Abstract
Studies were undertaken using transesophageal Doppler echocardiography to
monitor left ventricular systolic function in anaesthetised horses. A 3.5 MHz
transoesophageal probe was specifically developed in collaboration with Vingmed
Sound for equine use. The indices of systolic function investigated were maximum
acceleration of aortic blood flow (dv/dt^J, maximum blood flow velocity, (V^J,
cardiac output (CO), left ventricular pre-ejection period (PEP) and left ventricular
ejection time (ET).
The feasibility of the technique was demonstrated in a group of 8 healthy
Thoroughbred horses anaesthetised using a standard protocol. It was established that
two dimensional transoesophageal echocardiography provided a reference view of the
left ventricular outflow tract and aorta that consistently allowed high quality Doppler
echocardiographic measurement of aortic blood flow velocity. The flow envelopes
obtained were suitable for measurement of indices of left ventricular systolic function.
The repeatability of the measured indices was similar to that of the maximum rate of rise
of left ventricular pressure (LVdp/dtmax), obtained simultaneously by cardiac
catheterisation.
Cardiac output estimations made using transoesophageal Doppler
echocardiography were compared with those obtained by thermodilution in the same
group of horses under general anaesthesia. Cardiac output was altered by infusions of
the sympathomimetic amine, dobutamine. Aortic velocity spectra obtained both by high
pulse repetition frequency and continuous wave insonation modes were used to obtain
the velocity time integral for calculation of cardiac output. The measurements derived
from transoesophageal echocardiography agreed well with those obtained by
thermodilution. Both correlation coefficients and limits of agreement between the two
techniques were better than those obtained from similar studies in standing horses using
transthoracic echocardiography.
The sensitivity of the Doppler derived indices of left ventricular function to
inotropic intervention was assessed in the final sequence of studies. As these indices
are derived during the ejection period they are load dependent, so their response to
changes in ventricular loading was also assessed and compared with the most
commonly used index of myocardial contractility in horses, LVdp/dtmax. Three drugs
were administered to the anaesthetised horses in a randomised sequence during three
separate anaesthetic episodes. The drugs, dobutamine, dopamine and dopexamine were
selected because of their relatively different effects on afterload, preload and
contractility. Maximum acceleration of aortic blood flow was as sensitive to the
changes in ventricular performance as LVdp/dtmax. Maximum aortic blood velocity
showed the same qualitative response to infusion of the drugs but the changes were
quantitatively less than in dv/dtmax and LVdp/dtmax. The systolic time intervals, PEP and
ET, were also responsive to drug infusion; pre-ejection period shortened with each
drug, whilst ET increased after dopamine and dopexamine, but was reduced by
dobutamine.
These studies have shown that dv/dtmax is as sensitive as the invasive index
LVdp/dtmax for detecting changes in left ventricular performance. In addition dv/dtmax
and Vmax appear to be no more affected by changes in ventricular loading conditions than
the isovolumic index LVdp/dtmax. It is concluded that transoesophageal Doppler
echocardiography provides a minimally invasive technique for assessment of left
ventricular systolic function in anaesthetised horses.
