This article was originally published in the January/February 2015 “Today’s Technician” column of Today’s Veterinary Practice. Today’s Veterinary Practice is the NAVC’s official, peer-reviewed journal for veterinarians. You can find the full list of “Today’s Technician” articles on the Today’s Veterinary Practice website.
Systemic arterial blood pressure is created by the pumping action of the heart, circulating arterial blood volume, and smooth muscle tone of blood vessel walls.
Arterial blood pressure is essential for adequate perfusion of tissues, providing oxygen delivery for energy demands. The ability to measure and monitor blood pressure trends provides important data about patient cardiovascular status and may help define approach to treatment.
Blood pressure monitoring has become a vital part of small animal practice, and is often the responsibility of the nursing staff. Veterinary nurses should:
- Have the technical skills to accurately assess blood pressure
- Understand the advantages and disadvantages of different monitoring techniques
- Recognize abnormal values and understand their implications
- Know when to alert the clinician who is managing the patient.
Part 1 of this article series discusses terms associated with blood pressure measurement, indications for measurement, and normal and abnormal blood pressure values. Part 2 will describe types of blood pressure monitors and provide step-by-step instructions on how to perform blood pressure measurement.
BLOOD PRESSURE DEFINITIONS
Blood Pressure Values
When the left ventricle contracts, blood is pushed into the aorta creating systolic arterial pressure (SAP); then the left ventricle empties, relaxes, and begins to fill again, and aortic pressure falls, creating diastolic arterial pressure (DAP). Mean arterial pressure (MAP) is calculated from the systolic and diastolic values:1
MAP = DAP + 1/3 (SAP–DAP)
Types of Monitoring
Arterial blood pressure can be monitored in 2 ways:
- Direct arterial blood pressure monitoring—considered the gold standard for monitoring—uses an arterial catheter and allows continuous monitoring of patient SAP, DAP, and MAP (Figure 1).2-4
- Indirect arterial blood pressure monitoring relies on detection of arterial blood flow or vessel wall movement in a peripheral artery, using Doppler or oscillometric methods (Figures 2 and 3).
INDICATIONS FOR MEASUREMENT
Specific indications for blood pressure monitoring include:
- During anesthesia, when patients are anticipated to become cardiovascularly compromised due to the effects of anesthetic drugs, type of surgery performed, and nature of the patient’s disease
- In an emergency/critical care setting, in which the condition of the patient changes from minute to minute
- In general practice, to obtain and help identify early markers of disease during baseline health checks
- In patients with known or suspected hypotension or hypertension due to underlying diseases.
Blood Pressure Monitoring: Looking at the “Big Picture”
Veterinary nurses should think critically about results obtained in their patients, never looking at 1 value independent of the rest of the patient.
In other words, think about your patient globally and consider the patient’s blood pressure reading as part of the bigger picture. For example:
- If the patient is hypertensive, is it also tachycardic?
- Is the patient in pain, anxious, or both?
- Is the patient’s hypertension a result of being restrained to obtain a blood pressure reading?
- If the patient is hypotensive, is it also tachycardic or bradycardic?
- Is the patient hypothermic?
- How responsive is the patient? Is it dull or stuporous?
These are the types of questions that veterinary nurses should ask. By taking time to consider the “whole picture” rather than only looking at isolated pieces of information, veterinary nurses are better able to process what may be happening with their patients and pass significant concerns on to clinicians.
NORMAL ARTERIAL BLOOD PRESSURE VALUES
Blood pressure measurement provides important information about the cardiovascular status of a patient, and the importance of knowing normal values in different species cannot be overstated (Table 1).
Hypotension is defined as lower than normal blood pressure. Hypotension results in decreased tissue perfusion and can result in compromised blood flow to vital organs. Common causes of hypotension are listed in Table 2.
A diagnosis of hypotension is made based on the patient’s presentation, physical examination, and blood pressure measurement.
In dogs and cats, MAP is the preferred value used for making a diagnosis of hypotension. A patient is considered hypotensive when:
- MAP is < 60 mm Hg.3,4
- SAP is < 90 to 100 mm Hg.2,4
A hypotensive blood pressure reading should be followed by brief physical assessment of the patient, and the clinician should be notified of the patient’s status.
Hypotensive patients should be monitored on a regular basis and their cardiovascular status should be re-evaluated, at minimum, every 30 minutes. Frequent reassessment is necessary to determine whether therapy is working or if more aggressive intervention is necessary.
It is important to remember that a patient’s cardiovascular status can decline rapidly. Renal tissue perfusion diminishes when MAP declines below approximately 65 mm Hg.4 In patients with sustained MAP < 65 mm Hg, the patient’s urine output should be monitored closely because decreased urine output can signify impaired renal perfusion.
Hypertension is typically divided into 2 categories: primary and secondary.
Primary hypertension is caused by an imbalance between cardiac output and systemic vascular resistance that has no known cause; therefore, it is sometimes referred to as idiopathic hypertension.6,7 It is rare in small animals.
A diagnosis of primary hypertension is made when reliable blood pressure readings demonstrate a sustained increase in blood pressure, paired with a normal complete blood count, serum biochemical profile, and urinalysis.6
Secondary hypertension is elevated blood pressure that occurs secondary to concurrent disease (Table 3), or due to administration of medications, including certain therapeutic agents, such as glucocorticoids, mineralocorticoids, erythropoietin, sodium chloride, phenylpropanolamine, and nonsteroidal anti-inflammatory drugs.6,7
The following blood pressure values—if measured on 3 separate occasions during the same visit—are consistent with hypertension:6,7
- Dogs: SAP/DAP > 150/95 mm Hg
- Cats: SAP > 150 mm Hg.
Persistent elevations in blood pressure can have serious consequences. Sustained hypertension is associated with damage to the eyes, ears, brain, and kidneys.6,7 A hypertensive emergency occurs when the patient has a marked elevation of blood pressure and is at risk for development of end-organ damage or vascular incidents, such as cerebral hemorrhage causing neurologic signs or intraocular hemorrhage causing blindness.7
It is imperative that severely hypertensive patients are treated at a facility that can provide 24-hour critical care monitoring.
When “Normal” Does Not Mean All is Well
It is important to note that even if blood pressure is normal, it does not necessarily mean that tissue perfusion is adequate.3 In simple terms, the body attempts to maintain blood flow to the brain and myocardium at all costs:3
- As part of this homeostatic mechanism, the initial response of the body to compensate for decreased MAP is to bring it back up to normal by increasing systemic vascular resistance, by vasoconstriction.
- Vasoconstriction, in turn, can diminish blood flow to peripheral tissues, resulting in decreased tissue perfusion despite the patient remaining normotensive.
- Often, the patient does not become hypotensive until the compensatory mechanisms have failed.
ACCURATE BLOOD PRESSURE MEASUREMENT
Stress and anxiety in the clinic setting, coupled with stress during handling to obtain a blood pressure measurement, can cause elevated readings, resulting in a false diagnosis of hypertension. The following steps should be taken to ensure an accurate measurement:
- Prior to blood pressure measurement, patients should be allowed to remain calm in a quiet examination room for 5 to 10 minutes.6
- Blood pressure measurements should be taken only after the patient has had time to acclimate to the clinic setting.6
- Heart rate should always be recorded at the time of blood pressure measurement; presence of tachycardia in association with hypertension should prompt consideration of white coat hypertension.
Minimizing the risk of an incorrect diagnosis of hypertension in the fearful or painful patient relies on interpretation of a pet’s body language during the examination, use of a little creativity, and a lot of patience:
- Provide a safe, quiet, comfortable environment and slowly introduce yourself, allowing the pet to feel at ease and build a sense of trust.
- Consider taking the pet into a quiet room, attempting to make the cage/carrier more comfortable, using a box as a retreat sanctuary for a cat, or engaging in gentle play using a dog’s favorite toy.
- If you are successful and can measure blood pressure, take 3 readings to obtain an average; then consider repeating the process later when the pet is more relaxed.
White Coat Syndrome
Some patients may fall into the category of white coat syndrome, which should not be confused with true hypertension. Unfamiliar faces, scents, and sounds, followed by a close encounter with a stranger in a white coat can inadvertently cause stress, anxiety, and fear, hindering our ability to obtain accurate blood pressure readings.
In human medicine, this is a well-recognized phenomenon. Blood pressure surges occur in patients with this condition, making it difficult to ascertain valid blood pressure measurements, and potentially resulting in an inaccurate diagnosis of hypertension.8
Veterinary nurses play a fundamental role in patient care. We are often on the front lines, assessing patients as they are initially presented and then assessing them repeatedly after the clinician leaves. This gives us unique insight to detect subtle variations that may otherwise go unnoticed. Documenting that information, interpreting it, and communicating changes to the clinician provides the veterinary team an opportunity to proactively manage potential problems.
DAP = diastolic arterial pressure; MAP = mean arterial pressure; SAP = systolic arterial pressure