Blood Pressure Regulation

Blood pressure = cardiac output X total peripheral resistance
cardiac output = stroke volume X heart rate

Blood pressure is determined by total peripheral resistance (vascular tone) and cardiac output. Cardiac output is determined by heart rate and stroke volume, which is impacted by contractility and preload. Control of blood pressure is by a variety of mechanisms.

Measurement of blood pressure is a matter of much research and debate. Direct arterial pressure monitoring provides the most accurate reliable results, but is not conducive to outpatient measurement or repeated measurement over several different hospital stays, because of the need to cannulate an artery. Therefore, indirect pressure measurements are the method of monitoring outpatients. Oscillometric blood pressure monitors (e.g., Dinamap, Cardell, SurgiVet) have an air filled cuff that surrounds the vessel. The machine inflates the cuff and then measures the oscillations in the air as the pressure is slowly decreased. At the point at which the pressure in the vessel equals the pressure in the cuff, blood flow through the vessel will resume, increasing the oscillations in the cuff due to the pulsatile nature of blood flow. The machine detects this pressure, and will display the systolic, diastolic, and the mean pressure, as well as the heart rate. When using an oscillometric monitor, the first few readings should be discarded. Once 5-10 readings that are tightly clustered have been obtained, they can be averaged for a reasonably accurate measurement. If the heart rate registering on the machine is widely at variance with the measured heart rate, discard the reading.

Doppler ultrasound technology utilizes a Doppler crystal to amplify the sounds of blood flow through a vessel. The crystal is placed over the artery and positioned so that a strong signal is obtained. Common areas to use include just proximal to the metacarpal or metatarsal foot pad, the base of the tail, or the dorsal metatarsal artery on top of the hind foot. Shaving the hair decreases the trapped air that dampens the signal. In short coated breeds, alcohol may be sufficient without clipping. A cuff placed proximal to the crystal is inflated to occlude blood flow. As the pressure is released, the systolic pressure can be determined at the pressure at which blood flow resumes. Diastolic pressure measurement, determined by a change in the character of the blood flow (a dampening of the sound), is subjective and unreliable. A mean pressure is not obtained by this method. Doppler readings are more accurate in lower pressure ranges than oscillimetric machines.

The appropriate size cuff to use is one whose width is 40% of the circumference of the limb. Larger cuffs are needed for larger animals, and a larger cuff is used for measurements high on the hind leg compared to lower on the leg where the limb is smaller. Too large a cuff will lead to an artificially low blood pressure measurement; conversely, too small of a cuff will lead to inaccurately high readings.

Stress associated with the clinic visit ("whitecoat" effect) can transiently elevate the blood pressure, causing a diagnosis of hypertension to be made incorrectly. Minimizing the stress associated with measuring the blood pressure is critical in obtaining reliable results. The blood pressure should be measured in a quiet environment, such as the exam room, with the owner present. Ideally, the blood pressure is measured before the veterinarian starts the physical examination, and should definitely be done prior to taking the temperature or drawing blood samples. Because each brand of blood pressure monitor may give slightly different readings (for example, a reading of 120 mmHg on one machine may correspond to a 125 mmHg reading on another machine), consistency with repeated visits, including using the same type of machine, same environment, and hopefully even the same technician increases the reliability of measurements and makes analyzing trends more accurate.

Just as the reference range for creatinine varies with breed, the normal range for blood pressure varies between breeds. Reference ranges have been determined for some breeds.

Hypotension

Definition of hypotension