How to… measure and interpret blood pressure
Measuring blood pressure is a skill performed frequently by the veterinary nurse. But what IS blood pressure, what is it affected by, how is it monitored, and what does all of this mean for our patient?
If you want to know which blood pressure monitoring methods to use in practice, and when and how we treat hyper and hypotension, keep reading for my top tips on blood pressure monitoring in veterinary patients.
What is blood pressure?
Blood pressure is the force exerted by blood on the arterial walls as it flows through them. It is measured in millimetres of mercury (mmHg) and is determined by the patient’s cardiac output (CO) and systemic vascular resistance (SVR).
BP = Cardiac Output x Systemic Vascular Resistance
It is a key parameter to monitor in many of our patients, particularly medical patients, critical patients and anaesthetised patients, since oxygen delivery, removal of metabolic waste products and energy/nutrient delivery to tissues will all be affected by blood pressure.
There are two points in the cardiac cycle where blood pressure can be measured:
Systolic blood pressure is the pressure within the artery during systole - where the ventricles are contracting and forcing blood into the arteries. This is the highest (peak) blood pressure measurement we obtain.
Diastolic blood pressure is the pressure within the artery during diastole - where the heart muscle is in a state of relaxation, and blood is returning to the atria ready for the next contraction. This is the lowest (trough) blood pressure measurement we obtain.
The mean arterial pressure is the average pressure within the arteries. It is calculated from the systolic and diastolic results using the following formula:
MAP = (2 x diastolic pressure) + systolic pressure / 3
What is normal blood pressure?
Normal blood pressure values in cats and dogs are:
Cats: Systolic = 80-140mmHg; Diastolic = 55-75mmHg; Mean = 60-100mmHg
Dogs: Systolic = 90-140mmHg; Diastolic = 50-80mmHg; Mean = 60-100mmHg
What affects blood pressure?
Because we said that blood pressure is calculated based on the cardiac output and systemic vascular resistance, anything which affects cardiac output or vascular resistance will also affect our patient’s blood pressure.
Common factors which affect blood pressure in veterinary patients include:
Tachycardia
Bradycardia
Hypovolaemia
Haemorrhage
Vasodilation
Vasoconstriction
Cardiac disease
Cardiac arrhythmias
Neurological dysfunction
Renal disease
Hyperthyroidism
Pain
Stress
Catecholamine release (e.g. adrenaline)
General anaesthesia
How is blood pressure measured?
Several methods are used the measure blood pressure; these can be classified into invasive (direct) or non-invasive (indirect) methods.
Invasive blood pressure monitoring
Invasive monitoring techniques require the placement of an arterial catheter. This is connected to a transducer via a length of non-compliant (stiff) drip tubing. The transducer also connects to a drip line, which is attached to a bag of saline flush within a pressure inflator. The drip delivers a very small volume of flush through the catheter continually to avoid clot formation, as arterial catheters are much more prone to clotting than intravenous catheters.
The transducer detects the pressure within the line and transmits this signal to a multiparameter monitor, where a pressure waveform is displayed alongside real-time systolic, diastolic and mean arterial pressure readings.
Invasive monitoring techniques are considered the gold standard as they provide the most accurate measurements, but the requirement for an arterial catheter and specialised monitoring equipment limits their use in practice. They are often used in referral settings in critically unwell or anaesthetised dogs.
Non-Invasive blood pressure monitoring
Non-invasive monitoring methods are used most commonly in practice. There are three non-invasive methods that can be used:
The Doppler method;
The oscillometric method;
The high-definition oscillometric method.
Doppler Measurement
Doppler measurement is performed using ultrasound waves to detect pulsatile flow in a peripheral artery, which is detected and converted to sound using a piezoelectric crystal and speaker.
The probe is placed over a peripheral artery such as the dorsal pedal artery, and a cuff is placed above the probe, attached to a sphygmomanometer. The cuff is inflated to around 20mmHg past the point where the pulse can no longer be heard before the cuff is gradually deflated until the pulse is audible again. The cuff pressure at which the pulse first returns is the patient’s blood pressure.
The Doppler method only provides a measurement of systolic arterial pressure in small animals, though some studies have shown that Doppler measurements may more closely reflect mean arterial pressure in anaesthetised cats.
Doppler measurement is quick and easy to perform and provides accurate readings in smaller patients, mobile patients, hypotensive or hypovolaemic patients, and patients with cardiac arrhythmias. As it relies on the veterinary nurse hearing the pulse return as the cuff is deflated, it minimises the risk of machine error or inaccurate results. Doppler monitors are generally inexpensive and readily available in practice.
Oscillometric Measurement
Oscillometric blood pressure monitors contain a transducer which detects vibration within arteries and converts these to digital signals, in order to provide a blood pressure measurement. Oscillometric methods can determine systolic, diastolic and mean arterial pressure as well as heart rate, so provide more information than Doppler methods. They are, however, considered less accurate than Doppler methods in certain situations:
In very small patients
In conscious patients (due to movement artefact interfering with results)
In wriggly or very anxious/shaking patients (due to movement artefact).
High-Definition Oscillometric Measurement
More recently, high definition oscillometry has been introduced. This works in the same way as traditional oscillometric blood pressure measurement but includes a pressure waveform when the device is connected to a computer. This pressure waveform can be interpreted to determine whether the blood pressure measurement is accurate or not.
Hypertension
Hypertension (systolic pressure above 140mmHg) is frequently seen secondary to other conditions such as hyperthyroidism, diabetes mellitus and chronic kidney disease, and is associated with sometimes severe damage to specific target organs, namely the eyes, kidneys and central nervous system. In severe cases, retinal detachment and blindness can result.
In cases of severe or prolonged hypertension, treatment is most commonly achieved using calcium channel blockers such as amlodipine, though there are several different anti-hypertensive medications used in specific circumstances:
Calcium channel Blockers: These reduce the influx of calcium ions into cardiac muscle cells and vascular smooth muscle cells, through calcium channels. This causes vasodilation and reduces cardiac contractility, in turn reducing blood pressure.
Angiotensin-Converting Enzyme Inhibitors (ACE inhibitors): These act on the renin-angiotensin-aldosterone system (RAAS) to block the conversion of angiotensin I to angiotensin II. This causes vasodilation, reducing cardiac output and systemic vascular resistance.
Alpha-1 Adrenergic Receptor Blockers: These inhibit the effects of noradrenaline. Noradrenaline causes vasoconstriction, increasing SVR and in turn increasing blood pressure. These are used in the pre-surgical management of pheochromocytoma, a noradrenaline-secreting neuroendocrine tumour.
Aldosterone Inhibitors: These inhibit the effects of aldosterone. Aldosterone increases sodium and fluid retention in the kidney, in turn increasing blood pressure. Spironolactone, an aldosterone inhibitor, is used in the treatment of primary hyperaldosteronism (Conn's syndrome).
Hypotension
Hypotension is defined as a systolic arterial pressure (SAP) of less than 90mmHg, and a mean arterial pressure (MAP) of less than 60mmHg.
As we said earlier in this post, blood pressure is calculated using the equation BP = cardiac output (CO) x systemic vascular resistance (SVR). Our cardiac output is the stroke volume (the volume of blood pumped from the heart with each beat) multiplied by the heart rate. Stroke volume is in turn affected by many factors, such as preload (the volume of the blood getting to the heart), contractility (how effectively the heart can contract and 'pump' the blood out) and afterload or systemic vascular resistance (the pressure the heart needs to work against during systole, to eject blood). Any factor which reduces our cardiac output, stroke volume, heart rate or SVR will therefore reduce our blood pressure.
Most commonly in practice, we see this due to hypovolaemia, reduced venous return, or due to vasodilation (e.g. due to anaesthetic drugs or sepsis). Hypotension is treated by managing the underlying cause; either by providing fluid resuscitation in a hypovolaemic patient, minimising vasodilative anaesthetic drug doses (such as volatile agents!), correcting cardiac arrhythmias where present, and vasopressor or inotrope use where required. Vasopressor agents cause vasoconstriction, increasing SVR, whereas inotropes improve cardiac contractility, improving cardiac output.
As veterinary nurses, blood pressure monitoring is a HUGE part of our day-to-day role and we must understand why and how we see blood pressure abnormalities, as well as how they are treated, particularly in medical or emergency patients.
So there we go! That’s the fundamentals of blood pressure monitoring, which monitors we use and why, and how we manage hypotension and hypertension! Which blood pressure monitors do you use most commonly in practice? Let me know in the comments below!
References:
Aldridge, P. and O’Dwyer, L. (2013) Practical Emergency and Critical Care Veterinary Nursing. Chichester: Wiley-Blackwell.
Kirby, R. and Linklater, A. eds. (2017). Monitoring and Intervention for the Critically Ill Small Animal – The Rule of 20. Iowa: Wiley-Blackwell.
Merrill, L. ed. (2012). Small Animal Internal Medicine for Veterinary Technicians and Nurses. Iowa: Wiley-Blackwell.
Sierra, L. and Savino, E. 2015. Blood Pressure Monitoring From a Nursing Perspective, Part 2: Blood Pressure Monitoring Techniques. Today’s Technician [Online] Available from https://todaysveterinarypractice.com/todays-technician-blood-pressure-monitoring-from-a-nursing-perspective-part-2-blood-pressure-monitoring-techniques/
