88
46TH
ANNUAL
CONGRESS
OF
THE
SAEVA
SKUKUZA
16-‐20
FEBRUARY
2014
The cardiovascular system relies on active pumping (heart) of blood to provide flow
through the pulmonary - (CO2 released and O2 absorbed) and systemic circulations
(CO2 absorbed and O2 released). There is a dynamic equilibrium between the heart
and blood vessels that ensure adequate continuous flow. Briefly, there must be
enough volume and tone (diameter) in the blood vessels to provide adequate load
conditions (preload and afterload) and adequate ventricular filling (stroke volume) to
maintain adequate cardiac output (Tranquilli et al. 2007). Cardiac output (CO) is
dependent on stoke volume (SV) and heart rate (HR); where CO = SV x HR. Stroke
volume is dependent on preload, heart stage of contractility (inotropy and lusitropy),
and afterload. Mean arterial blood pressure (MAP) is dependent on the CO and the
systemic vascular resistance (SVR: in other words blood vessel tone); where MAP =
CO x SVR (Tranquilli et al. 2007). Cardiac output is an indicator or how much
volume of blood is being circulated within the blood vessels; while MAP indicates the
perfusion pressure within the blood vessels. A MAP value of 60 mmHg is suggested
to be the lowest allowable perfusion pressure to ensure adequate perfusion to vital
organs (brain, heart, kidney and liver) (Tranquilli et al. 2007). The relationship
between CO and MAP is not always linear where a drop in CO causes a drop in
MAP. For example, patients that are dosed with an alpha2-adrenoreceptor agonist
(detomidine, xylazine, romifidine) often have a raised MAP but a dramatic decrease
in CO (Sinclair. 2003). The dynamic relationship between the ventricles and
vasculature (ventricular-vascular coupli ng) is thus imperative to understand.
In a health horse, the oxygen delivery (DO2) is not flow-dependant when matched to
normal oxygen consumption (VO2). This supply/demand relationship can be
demonstrated by calculating the oxygen extraction ratio (OER). Equations typically
used to calculate DO2 may help explain the link between these values and Hb, CO
and MAP (Table 1). In a normal healthy horse, the OER is around 21% of the oxygen
supplied to the metabolically active tissue (Cambier et al. 2008). This indicates that
there is more than enough oxygen available to meet the metabolic demand, thus the
delivery is not flow dependant.
Horses suffering from colic have a number of physiological disturbances that alter
the balance of oxygen delivery and demand.
Factors that decrease DO2 in colic horses:
1. Hypovolaemia (relative and/or absolute; decreased preload)
2. Distended abdomen may occlude major veins (decreases cardiac return and
therefore preload) and splinting of the diaphragm (decreases ventilation)
therefore increasing ventilation/perfusion (V/Q) mismatching in the lungs
3. Acidosis (decreases inotropic effect of heart)
4. Anaemia (decreased Hb and therefore decreased CaO2)
5. General anaesthesia (hypoventilation, cardiovascular depression, decreased
sympathetic tone predispose to V/Q mismatch and hypotension)
6. Intermittent positive pressure ventilation (IPPV: can decrease cardiac return
due to thoracic venous collapse, especially during a prolonged inspiratory
phase)
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