Under Pressure - Blood Pressure Explained

Animals have the risk of developing high blood pressure. High blood pressure in animals is usually associated with a critical health issue, because hypertension is typically the result of an existing underlying disease.

High blood pressure in dogs is often associated with Cushing’s disease or hyperadrenocorticism. Cushing’s disease is when a disorder, usually a benign tumour, allows an excess of cortisone to be released into the blood stream.

Cats often develop high blood pressure as a result of having hyperthyroidism and/ or kidney disease. Hyperthyroidism is an overproduction of a thyroid hormone that results in an increased metabolism.

So What Exactly Is Blood Pressure?

Blood pressure is the driving force for blood flow (perfusion) through capillaries that supply oxygen to organs and tissue beds of the body. Blood pressure is needed to propel blood through high resistance vascular beds, including those of the brain, heart, lungs and kidneys. Blood pressure values are expressed in millimetres of mercury (mm Hg) and as three measurements: systolic, mean and diastolic.

The systolic pressure is the pressure generated when the left ventricle is fully contracted. Diastolic pressure is the pressure measured when the left ventricle relaxes. Mean arterial pressure (MAP) is calculated as one third the systolic pressure plus two thirds the diastolic pressure. Mean blood pressure determines the average rate at which blood flows through the systemic vessels. It is closer to diastolic then systolic because, during each pressure cycle, the pressure usually remains at systolic levels for a shorter time than at diastolic levels. Most times, under anaesthesia, a patient's mean pressure is what the anaesthetist focuses on. A mean arterial pressure of at least 60 mm Hg (70 in horses) is needed to properly perfuse the heart, brain and kidneys. Mean arterial blood pressures consistently below 60 mm Hg can lead to renal failure, decreased hepatic metabolism of drugs, worsening of hypoxemia, delayed recovery from anaesthesia, neuromuscular complications and central nervous system abnormalities, including blindness after anaesthesia.

Prolonged hypotension (> than 15-30 minutes) can lead to nephron damage. Although the effects may not be immediately apparent since 65-75% of nephrons need to be damaged before renal disease becomes clinically observable, the effects may play a role in the onset of renal disease later in a pet's life. Severe untreated hypotension can lead to cardiac and respiratory arrest. Hypertension, or excessively high blood pressure, can lead to problems as well. Ideally, any animal under anaesthesia should have regular blood pressure monitoring because most anaesthetic drugs affect blood pressure in some way.

Indirect Blood Pressure Monitoring

Oscillometric devices work by picking up pulsation under an occlusion cuff placed over an artery. The cuff is connected to a monitor that can be programmed to measure blood pressure at specific intervals of time. These devices deliver systolic, mean and diastolic readings as well as the heart rate. Most have alarms that can be set to alert when readings are out of the accepted range. The cuff size should be approximately 40% of the circumference of the limb (or tail) around which it will be placed. Cuffs that are too large will lead to artificially low readings and too small a cuff will give false high readings. Ideally, cuffs should be placed on a limb that is close to heart level (the level of the right atrium is the zero mark for blood pressure). Limbs well above the heart may give artificially low readings. Legs hanging well below the heart will give false highs. The cuffs are usually marked with the proper placement over the artery. They must not be applied too tightly as this may occlude flow and cause inaccurate readings as well as swelling distal to the cuff. Poor pulse signals from poor flow (the rear limbs during a severe GDV or large abdominal mass), or any movement of the limb during a reading will interfere with the device and may cause it to fail or deliver an inaccurate reading. These devices do not usually work consistently or at all on very small patients, although there are some newer, veterinary specific monitors out that claim to work accurately on small dogs and cats.

Doppler Ultrasonic Flow Detector

Doppler flow detectors involve the placement of an ultrasonic probe (crystal) over an artery. The frequency of the sound reflected from the moving arterial blood differs from that of the sound from the crystal and the shift in frequency is converted into an audible sound. Doppler probes may be placed over any artery, but the most useful for measuring blood pressure are the palmer arterial arches of the forelimb and hindlimb. The area should be shaved and a generous amount of ultrasonic or lubricating gel applied to the area or the crystal. The crystal is secured in place snugly (but not too tightly) with tape. A properly sized occlusion cuff is placed above the crystal. The cuff is inflated using a sphygmomanometer. The cuff is allowed to deflate slowly until the flow returns. The first audible sound heard is the systolic blood pressure. In some patients it may be possible to detect a second sound, this is said to be the diastolic pressure.

In the absence of a mean arterial pressure, systolic pressure should be maintained above 80-90 mm Hg. Doppler flow detectors are very useful monitors, with one of the biggest advantages to them being the audible sound of blood flow. Changes in flow can be heard as well as changes in the regularity of the pulse. These monitors have the advantage of being useful in all sizes of patients, from the tiny exotic species to large equine and zoo animals. Another advantage is that they are relatively inexpensive. Disadvantages of this method of blood pressure monitoring include inaccuracies at low pressures, systolic only readings and the importance of proper cuff size on accuracy.

Direct Method

Direct blood pressure measuring is the gold standard for blood pressure measurements. It involves placing a catheter into an artery and attaching the catheter to a transducer. The transducer (placed at heart level) is connected to an oscilloscope that gives systolic, mean and diastolic readings continuously. In addition, most monitors also display a wave form that corresponds with the pulse. This provides important information about the patient and the blood pressure. The arterial wave form should match the ECG complexes. Catheters may be placed in any artery. The possible complications of arterial catheter placement should be considered when choosing a site. Hematomas, air embolism, thrombosis and infection are the most common, yet rare, complications. Sterile technique should be considered when placing these catheters to reduce complications. Arterial catheters should be clearly marked so that nothing but heparinised saline is ever injected into them. The catheter needs to be flushed at regular intervals with hepanised saline to prevent clotting. Drugs should never be given through an arterial catheter. Unfortunately, direct arterial blood pressure measuring technology can add $1,000 or more to the price of a monitor making cost a big factor when choosing blood pressure measuring.

Troubleshooting Blood Pressure

Normal systolic blood pressures in the conscious patient are 100-160 mm Hg, normal diastolic pressures are 60-100 mm Hg and normal mean arterial blood pressure ranges are 80-120 mm Hg. Hypotension is classified as MAP of less than 60 mm Hg. It is important to be able to identify the cause of a blood pressure abnormality to know how to begin treatment for it. There are generally three things to consider when looking for causes of hypotension. Look for drugs or physiological/pathological factors that may reduce systemic vascular resistance (SVR), look at heart rate, and look for things that affect stroke volume (preload/contractility). Many of the drugs used in anaesthesia cause some degree of hypotension, and less often, hypertension. Knowing the side effects of these drugs and how they work will help in determining treatment.

Blood pressure should be routinely measured on any patient undergoing general anaesthesia. The best way to prevent hypotension is to detect changes in blood pressure as soon as they begin.