Pulmonary vascular disease or pulmonary vascular obstructive disease (PVOD). is a catch-all term for conditions that affect
the pulmonary circulation. These conditions are of particular importance to as they may result in severe respiratory dysfunction.
As a review, the pulmonary and systemic circulation are in series, with the cardiac output of the left and right heart (in
the absence of cardiac shunt) being equal. However the pressure in the pulmonary vasculature is low in relation to the systemic
circulation, with the mean pulmonary arterial pressure averaging less than 15 mmHg and a systolic pressure of < 25 mmHg at
rest and less the 30 mmHg during exercise. Elevated pulmonary pressures result in syncope, weakness and fatigue, and may ultimately
result in right heart failure. Knowledge of normal intra-cardiac pressure is essential to recognize the effects of disease
NORMAL CARDIAC AND VASCULAR PRESSURES
Strain on the right heart is assessed by measurement of the pulmonary vascular resistance (PVR), which is defined as the [mean
pulmonary arterial pressure-mean left atrial pressure (or pulmonary capillary wedge pressure)]/cardiac output. Increasing
PVR is associated with increased work for the right ventricle. If R heart pressures exceed left heart pressure in the presence
of an intra-cardiac shunt, there will be right to left shunting.
In simplicity, two things can go wrong with the pulmonary vasculature that will affect the pressures. The first is a clot
or clots may lodge in the pulmonary vasculature, while the second is that there is vasoconstriction or hypertrophy of the
pulmonary arteries.
Pulmonary thromboembolism (PTE) is a more clinical recognized cause of pulmonary dysfunction in critically ill dogs and to
a much lesser extent, the critically ill cats. As an overview, in order for a thrombus to form, there should be disruption
in one or more aspect of the Virchow's triad which includes vascular damage, blood stasis and hypercoagulability. A thrombus
is defined as a blood clot that forms in situ, while an embolus is a clot that lodges somewhere after first forming in another
site. It is often unclear as to the initial source of the clot; thus, the term thromboembolism is used. In human medicine
it is widely appreciated that thrombi form in the deep veins, include the iliacs and occasionally the leg veins. It is by
far less clear where they come from in dogs and cats. It seems possible that they are forming in veins, however; despite the
advances in diagnostic imaging, it is rare that one is visualized.
A number of risk factors have been associated with PTE in dogs, but as a general rule, critical illness and trauma are common
sources.
