Blood Pressure (BP) in Cats
Blood pressure is a test done to determine the pressure of the blood against the walls of blood vessels. This pressure is determined by how the heart pumps, the resistance to blood flow in the small arteries, the elasticity of the walls of the main arteries, how much blood is present and the blood viscosity or thickness.
Blood pressure may be measured with either direct or indirect methods. Direct methods utilize an indwelling catheter that goes into an artery and is attached to a mechanical pressure transducer. The transducer is frequently part of a machine that can also monitor heart rate, respiratory rate, oxygen tension, etc. Indirect blood pressure measurement is performed using an inflatable cuff and an instrument to detect arterial blood flow. This technique is similar to what our physicians do on us to determine our blood pressure. This article focuses on the "indirect" blood pressure measurement method.
Blood pressure is checked on any pet that is suspected of having high or low pressure. This includes pets with heart disease, kidney disease, sudden onset of blindness, recent trauma, or other severe illness. Other causes of abnormal blood pressure include diseases of the endocrine system, tumors of the adrenal gland, medications and central nervous system disorders. Blood pressure is often monitored while a pet is under anesthesia. Although not measured as commonly as in people, blood pressure assessment is becoming a more important monitoring tool in veterinary medicine.
There are no real contraindications to performing this test in any animal.
What Does a Blood Pressure Reveal?
A blood pressure reveals the pressure within the vessels. High blood pressure (hypertension) and low blood pressure (hypotension) can lead to significant illness that often needs medical treatment.
Most commonly, blood pressure measurements are used to diagnose and monitor hypertension. They are also useful in detecting hypotension, such as that occurring during shock states, blood or fluid loss (volume depletion), or cardiac arrest. Blood pressure is also monitored during surgery and during the treatment of critically ill animals to evaluate the pet's circulatory status.
How Is a Blood Pressure Done?
Your veterinarian may need to clip a small amount of hair on the underside of the paw of either the front or back leg over the site of an arterial pulse. Conductive gel is then placed on the shaved part of the paw, and a sensor is then placed on the paw, with the gel being used to conduct the vibrations of the pulse. The technique for measuring pressure varies somewhat depending upon the instrument used. A blood pressure cuff is placed above the sensor, which is attached to a machine used to amplify the sound of the pulse. Animal legs are too small for a stethoscope, so the amplifier and sensor is used instead.
As when you get your own blood pressure taken, the cuff is inflated until the beating pulse is no longer heard. Then air is slowly allowed to escape the cuff. At the point the beating pulse is heard again, that is the blood pressure. In people, the first sound of the pulse is called the systolic pressure. As more air is let out of the cuff, there is a point when the sound of the pulse suddenly becomes louder. This is the diastolic pressure. However, when using a doppler in animals, it is often difficult to detect this volume change. For this reason, the blood pressure is often expressed as a single value. Other non-doppler blood pressure machines use electronic methods to detect the blood pressure. These machines can determine both systolic and diastolic pressure.
Most normal systolic values range from 110 to160 mmHg and most diastolic values range from 55 to100 mmHg. Hypertension is defined as pressures > 170/110 mmHg. Hypotension is defined as pressures < 90/50 mmHg in the awake animal. Normal MAP pressure is usually calculated as the average of systolic and diastolic pressures. Trends in repeated pressures are more significant than results of single measurements.
Is a Blood Pressure Painful?
There is no pain involved in obtaining a blood pressure. However, some pets resent the restraint necessary to perform the test.
Is Sedation or Anesthesia Needed for a Blood Pressure?
Neither sedation nor anesthesia is needed; however, some pets have a blood pressure performed during anesthesia or sedation. Sedation is generally undesirable when measuring blood pressure for the purpose of diagnosing hyper- or hypotension, because most forms of chemical restraint alter blood pressure. Unless blood pressure measurement is being performed in the surgical patient, animals are usually awake and a minimal amount of restraint is used. If possible, the pet should be fairly relaxed.