Sustained hypotension is a life threatening situation where the body's major organs (kidney, liver, brain, and heart) can
experience irreversible damage from inadequate perfusion pressure. Veterinary technicians may encounter hypotension frequently
when caring for emergency and critical care patients, as well as anesthetized or post operative patients who are frequently
at risk of systemic hypotension.
Hypotension is not a disease process in itself but rather a failure of normal regulatory mechanisms. Regulation of blood pressure
is controlled by neural response, hormonal actions, and local metabolite release such as baroreceptor activation, catecholamines
and anti-diruretic hormone, and nitric oxide respectively. Hypotension can result from many underlying conditions and causes
are generally referred to as one of three categories: decreased preload, decreased cardiac output, and decreased systemic
vascular resistance. Some of the most common causes are hypovolemia, cardiogenic shock, sepsis and anesthesia respectively.
Physiology of blood pressure
Blood pressure is the product of cardiac output and systemic vascular resistance. Cardiac output is the product of heart rate
and stroke volume. Vascular resistance refers to the ability of blood to flow through peripheral vasculature and is affected
by vasomotor tone of vessels and the viscosity of the blood. Stroke volume is affected by preload, contractility, and afterload.
Blood pressure is broken down into three different measurements and is measured in mmHG. Systolic arterial pressure (SAP)
is the pressure generated by the left ventricle in full contraction (systole). Diastolic arterial pressure (DAP) represents
the lowest pressure of arteries during cardiac filling (diastole). Mean arterial pressure (MAP) is the average of pressure
in the arteries during the entire cardiac cycle. MAP is more affected by DAP because most of the cardiac cycle is spent in
diastole. Adequate perfusion of the body's organs is judged by MAP, but coronary artery perfusion actually takes place during
diastole. MAP can be roughly approximated by adding one third of SAP value to two thirds DAP value.
Hypotension is typically defined as MAP <60mmHg or SAP <80mmHg. At lesser pressures, the kidney and brain are known to be
compromised in glomerular filtration rate and cerebral perfusion. Periods of hypotension greater than 15-30 minutes (prolonged
hypotension) results in damage to nephrons and cellular death.
Table 1: Normal Blood Pressure Values