Definitions

BLOOD PRESSURE (BP): an indirect indication of the amount of blood exiting the heart and the resistance that meets it throughout the body, composed of two measurements, two numbers in fraction representation.

1. SYSTOLIC: the number placed on top, always larger, which reflects the amount of blood exiting the heart by which pressure is exerted against the arterial walls.
2. DIASTOLIC: the number placed on the bottom, always smaller, which reflects the peripheral resistance (the "pushback") from the baseline pressure in the tissues against which the systolic forces are pushing.

They are named Systolic and Diastolic because the top number documents what is happening during systole, and the bottom number documents what is happening during diastole.

SPHYGMOMANOMETER: The traditional device (gauge) used to measure blood pressure in conjunction with a stethoscope.

The sphygmomanometer includes a blood pressure cuff that wraps around the arm or leg and can be inflated or deflated manually, offering good manual control of the process. There are also automatic devices, whose results should always be confirmed manually when the reading is suspicious.

PALPATION: A non-traditional way to take blood pressure in time-critical or mass casualty scenarios. By feeling for certain pulses you can estimate systolic (but not diastolic) blood pressure. A palpable radial pulse implies a systolic of 80mmHg at minimum, femoral 70mmHg, and carotid 60mmHg. 

According to this: Palpatory Method of Measuring Diastolic Blood Pressure

"Palpatory method - Inflate the cuff rapidly to 70 mmHg, and increase by 10 mm Hg increments while palpating the radial pulse. Note the level of pressure at which the pulse disappears and subsequently reappears during deflation will be systolic blood pressure." 

 

Other Blood Pressure Calculations

MEAN ARTERIAL PRESSURE: determined either directly by catheterization or estimated with a formula:

Diastolic Pressure + 1/3 x (Systolic Pressure - Diastolic Pressure).

EXAMPLE: with 120/80, the mean arterial pressure is 80 + 1/3 (40) = ~93.

PULSE PRESSURE: the Systolic Pressure minus the Diastolic Pressure.

EXAMPLE: with 120/80, the pulse pressure is 40.

As can be seen, an increase in pulse pressure can result from either

1. an increase in systolic pressure or
2. a decrease in diastolic pressure.

(A decrease is the opposite.)

A narrowing pulse pressure indicates a relative increasing diastolic pressure and/or relative decreasing systolic pressure. Perfusion will stop once both pressures come together--that is, once systolic pressure = systemic vascular resistance; that is, if the pulse pressure narrows to 0. 

• Widened (high) Pulse Pressure (>40 mmHg)

1. Isolated Systolic Hypertension (i.e., without diastolic hypertension)
2. Aortic Regurgitation
3. Thyrotoxicosis
4. Arteriovenous fistula
5. Aortic Coarctation
6. Anemia
7. Emotional state
8. Cushing's Triad: seen with head injuries when increased intracranial pressure (ICP) significantly decreases the cerebral blood flow

• Narrowed Pulse Pressure (<30 mmHg)

1. Hypovolemic Shock--sympathetic response to decreased circulating Blood Volume
2. Diastolic Blood Pressure increase without a systolic Blood Pressure increase
3. Tachycardia
4. Severe Aortic Stenosis
5. Constrictive Pericarditis
6. Pericardial Effusion
7. Ascites

Whether these types of blood pressure determinations are useful for predicting risk of cardiovascular disease makes no difference In the Field, where they are not helpful.

 

The Relationship Between BP and Pulse Rate

Due to the feedback systems using baroreceptors and osmoreceptors and the autonomic nervous system (ANS), there is a dynamic compensatory relationship between BP and pulse rate.

Typically, if the BP falls, the pulse rate rises to compensate so that tissue perfusion can remain constant.

If the pulse rate rises and the relative amounts of blood coursing through the arteries increases, the same feedback systems can result in vasodilation to compensate for the increase in volume, resulting in a lower BP.

Compensation can only go so far, and when the compensation fails to achieve perfusion maintenance, such as in shock from hemorrhage or heart failure, the entire system fails and cardiovascular collapse occurs.

 

Technique

Determining SYSTOLIC pressure:

1. The stethoscope is placed over the antecubital fossa to auscultate the brachial artery.
2. The cuff is wrapped around the upper arm and pumped up to 180-200, which is enough to prevent blood from passing (tourniquet). This is uncomfortable, and since even elevated systolic pressures typically are below 160,
3. the pressure can be released quickly (but smoothly) until the systolic pulse is heard through the stethoscope (= the systolic pressure reading).
4. The systolic pressure is when the tourniquet effect of the cuff is no longer strong (inflated) enough to fight the force of ejected blood through the artery, and the pulsations return.
5. Release of the pressure should be very slow at this time because catching the diastolic pressure is a much more subtle process.

Determining DIASTOLIC pressure:

1. The release of cuff pressure continues manually until the pulsation heard ceases again--the DIASTOLIC.

Most document the number when the rhythmic pulsations stop being heard again, but the more accurate reading is when there is a change in the quality of the systolic pulsations heard. Before the pulsations stop (to silence under the stethoscope), there is a subtle change in the sound, a slight dip in volume, followed quickly by the cessation of sounds altogether. It is this dip that is the real DIASTOLIC pressure. Calling it with the cessation of sounds is useful in noisy environments when subtle changes cannot be heard, but it is usually 5-10 mm Hg lower than what the diastolic pressure actually is. Frankly, most often this will not make any difference in care rendered.

SYSTOLE--what is happening:

Normally blood flows smoothly without resistance and, therefore, without turbulence. The tourniquet effect of the cuff, of course, stops the pulses altogether. But when it is deflated enough to allow blood to force itself past the partial tourniquet pressure, there is turbulence, and the pulses are noisier and therefore, auditory.

DIASTOLIC--what is happening:

When the cuff pressure is released to the point when pulsation sounds disappear, this indicates a lack of turbulence due to the pressure in the cuff being lower than the pressure exerted by the peripheral resistance of the cardiovascular system at the microscopic tissue level.

Systolic/Diastolic Blood Pressure technique =  PUMP: Silence → Pulses (systolic) → Silence (diastolic).

 

Errors in Determining BP

Improper Cuff Size: if a person is obese, this will require a larger cuff to achieve the tourniquet effect crucial in determining blood pressure.

Blood pressure can also be determined from the leg, and with a large arm it may be necessary to use a leg cuff. Similarly, a small arm requires a smaller cuff, relevant--especially--pediatric patients or cachectic patients.

Too small a cuff will give an erroneous systolic pressure--overestimation by as much as 10-55 mmHg!

Improper Position: BP should be taken with the patient seated, the back supported, and legs uncrossed. The arm should be at the level of the heart.

The diastolic pressure may be falsely read as higher by 6 mmHg without good back support. The systolic can be falsely raised by 2-8 mmHg with the legs crossed. An arm hanging down can raise the pressures by 10-12 mmHg due to hydrostatic pressure from gravity.

Although the standard position (above) is important for accuracy, sitting and standing BPs are useful for diagnosing orthostatic hypotension when up to 40% of the blood can pool in the legs upon rapid standing.

Apprehension: Ideally, the patient should be quiet for 5 minutes before a BP is taken, usually impractical in the field. A distended bladder, noise, and drama in the background, or fright, can raise the BP 20-30%.

Multiple Measurements: One BP reading is worthless, but a series of them to determine a general consensus is more accurate--the more readings the better. 

 

Noisy Environments

It is possible to get a partial blood pressure reading--systolic only--without a stethoscope by palpating the return of pulsations of the radial artery which had ceased when a blood pressure cuff was inflated to the point of acting as a tourniquet--with slow release of the pressure in the cuff, when the pulsations return is the systolic pressure.

Diastolic pressure cannot be documented with this technique. Nevertheless, it is useful in very noisy environments that overwhelm anything you are attempting to listen to with a stethoscope. (If you've ever been called upon to take a blood pressure during air travel or en route with a lot of road noise, traditional methods might be useless.)

 

BP and Clinical Application

According to the American College of Cardiology/American Heart Association (ACC/AHA):

• Normal blood pressure – Systolic <120 mmHg and diastolic <80 mmHg.
• "Elevated" blood pressure – Systolic 120 to 129 mmHg and diastolic <80 mmHg.
• Hypertension:

• Stage 1 – Systolic 130 to 139 mmHg or diastolic 80 to 89 mmHg

• Stage 2 – Systolic at least 140 mmHg or diastolic at least 90 mmHg

If only one measurement is elevated (systolic or diastolic), the diagnosis is based on the number that is higher than normal.

 

In the Field

The finer nuances of BP are often irrelevant in the field, and the EMS responder usually has only a Systolic/Diastolic BP upon which to rely. Alterations in BP can include drops in systolic with heart failure; and drops in diastolic due to a loss of peripheral resistance (due to drugs or hypovolemia/shock).

In the field, BP elevations can be sudden and severe, resulting in a hypertensive crisis that can risk organ damage; drops in BP can portend poorly in emergent life-threatening conditions due to blood loss or heart failure.