In order to understand and approach hypotensive patients, one must first understand blood pressure. Although not a true measure
of perfusion, blood pressure is one of the most non-invasive means the veterinary field has of measuring whether or not the
tissues are getting enough blood and ultimately enough oxygen. Blood pressure is defined by the following equation:
Blood Pressure (BP) = Cardiac Output (CO) x Systemic Vascular Resistance (SVR)
Cardiac output is the amount of blood pumped by the heart per minute. Systemic vascular resistance is the amount of tone
of the peripheral blood vessels. Cardiac output is not easily measured and is defined as stroke volume multiplied times heart
rate. Heart rate is easily measured, but stroke volume is not easily measured in the clinical setting which is why cardiac
output is not generally measured. Sympathetic and parasympathetic tone regulate heart rate. Four factors effect stroke volume:
preload, myocardial relaxation, myocardial contractility, and afterload. Systemic vascular resistance is effected by the
adrenergic receptors, dopamine, and vasopressin receptors in the vasculature, which cause alterations in resistance of the
blood vessels. Many neurohormonal responses to hypotension also play a role in maintaining blood pressure, including the
renin-angiotensin-aldosterone system, antidiuretic hormone, atrial natriuretic peptide, stretch receptors, and baroreceptors.
The body's response to hypotension is very complex and therefore, a general understanding of the factors involved in blood
pressure regulation is necessary in order to properly approach the hypotensive patient.
Indirect blood pressure monitoring
In general, the mean pressure should be kept above 60 mmHg and the systolic pressure above 80 mmHg to ensure adequate organ
perfusion in the anesthetized patient. In the awake patient, mean pressure should be kept above 80 mmHg and the systolic
pressure above 100 mmHg. Doppler blood pressure monitors and oscillometric blood pressure machines have been validated in
small animals and give tremendous insight into the blood pressure status of an animal. There is controversy over whether
the Doppler pressure reading in anesthetized cats is closer to systolic or mean blood pressure. Traditionally, oscillometric
blood pressure monitors have been unreliable in the cat. However, the recent validation of the Cardell® oscillometric blood pressure monitor in cats has proven to be extremely useful.
Appropriate cuff size is necessary in order to get a proper blood pressure reading. Too large of a cuff or a too loosely
placed cuff will frequently give falsely low readings and too small of a cuff or a too tightly placed cuff will frequently
give falsely high readings. Many oscillometric blood pressure monitors display a heart rate reading in addition to the blood
pressure display. Ensuring that the heart rate on the monitor matches the actual heart rate is a good way to determine if
the results displayed are believable. Poor perfusion and hypotension are two common causes of aberrant blood pressure readings.