Blood pressure monitoring and arterial catheterization

1

What methods are available to measure blood pressure?

Blood pressure (BP) measurements can be divided into direct and indirect methods. Indirect methods include using a cuff or palpation of a pulse, whereas direct methods use an arterial catheter. The most common indirect or noninvasive BP (NIBP) measurement uses a BP cuff, usually on the arm over the brachial artery. An NIBP can be measured using either a stethoscope, listening for Korotkoff sounds (manual method), or by using an oscillometer (automated method). Direct or invasive BP (IBP) monitoring requires the use of a catheter placed into a central or peripheral artery.

2

How can palpation of a pulse be used to measure blood pressure?

Palpation of pulse can also be used in an emergent situation but is not an accurate method. The traditional teaching relies on the 60/70/80 rule, which suggests a minimum systolic BP that is necessary to palpate a pulse based on anatomic location (60 mm Hg for carotid, 70 mm Hg for femoral, and 80 mm Hg for radial). Unsurprisingly, studies show this method does not correlate well with direct measurements. Regardless, these same studies also show that in cardiac arrest, pulses are consistently lost in the following order (radial > femoral > carotid).

3

How does an oscillometric blood pressure cuff determine blood pressure? Which blood pressure parameters (i.e., systolic, diastolic, mean) are calculated versus measured?

The oscillometric (automated) BP measurement is the most common method to measure BP in a hospital setting. It is classified as an indirect or NIBP that works by measuring mean arterial pressure (MAP) and calculating systolic and diastolic BPs. The BP cuff first inflates to a pressure greater than systolic BP. Then it slowly “bleeds” air and, as the cuff pressure approaches MAP, oscillations caused by the pulse are transmitted to the cuff and measured. The oscillations increase in amplitude as the cuff pressure gets closer and closer to MAP. The oscillations are the greatest in amplitude when the cuff pressure equals MAP. As the cuff pressure decreases below MAP, the oscillations become smaller and smaller in amplitude, until they disappear. The cuff pressure corresponding to when oscillations are greatest in amplitude is recorded as MAP.

Manufacturers have various proprietary methods to calculate systolic and diastolic BP from the MAP measured by the cuff. For example, a common method is to calculate the systolic BP where the ascending slope of the oscillations is maximal or when the ascending oscillations are 50% of the maximum oscillation amplitude. The diastolic pressure is then calculated using the systolic pressure and MAP.

4

How can an oscillometric NIBP calculate diastolic blood pressure using MAP and systolic blood pressure?

Once the MAP and systolic BP are known, the diastolic BP can be calculated using the following equation and solving for diastolic BP:

MAP , mean arterial pressure; sBP , systolic blood pressure; dBP , diastolic blood pressure

5

How do NIBP measurements differ when obtained from auscultation of Korotkoff sounds (manual) versus the oscillometric method (automated)?

Auscultation (manual) NIBP measurements rely on Korotkoff sounds caused by turbulent flow that correspond to systolic and diastolic BP. The cuff is inflated above systolic BP and slowly bled. Using a stethoscope, the first Korotkoff sounds correspond to the systolic BP. The cuff is continuously bled more, and the Korotkoff sounds disappear at the diastolic BP. Therefore auscultation NIBP measures systolic and diastolic BP and MAP would need to be calculated. In contrast, oscillometric NIBP measures MAP and calculates systolic and diastolic BPs. Auscultation NIBP measurement is subject to interobserver variability, which is not the case for oscillometric NIBP.

6

What are the indications for invasive arterial blood pressure monitoring (i.e., arterial line)?

• Cardiovascular instability

• Need for continuous infusions of titratable medications (i.e., vasopressors or antihypertensive agents)

• Clinical situations at risk for significant blood loss or fluid shifts, such as intracranial, vascular, or thoracic surgery

• Preexisting cardiovascular disease, such as severe heart failure or valvular heart disease

• Concerns that NIBP monitoring may be inaccurate, as in patients with morbid obesity, atherosclerosis, and essential tremor

• Need for frequent blood samples (e.g., arterial blood gases)

7

What anatomic locations are available for direct or invasive blood pressure measurement?

IBP monitoring can be divided into peripheral and central locations. Peripheral anatomic locations include radial, brachial, and dorsalis pedis arteries, whereas central anatomic locations include axillary and femoral arteries.

8

What does it mean to level and zero a transducer? Are they the same thing?

Leveling and zeroing a transducer are separate processes that are often completed at the same time. Zeroing or calibrating a transducer involves opening the stopcock on the transducer to atmospheric pressure and selecting “zero” on the monitor. This sets the atmospheric pressure as the reference point to 0 mm Hg, which implies that BP measurements will be relative not absolute to atmospheric pressure. Leveling involves setting the vertical position of the transducer with respect to what it is supposed to measure. For example, in the supine position, the transducer is generally leveled at around 5 cm posterior to the sternum (i.e., mid-axillary) to approximate aortic root pressure (arterial line) or right atrial pressure (central venous pressure). While in the sitting position, the arterial line transducer is often leveled to the external auditory meatus to approximate BP within the circle of Willis.

9

What would happen to the blood pressure if the transducer is inadvertently positioned above or below the patient?

If the transducer is vertically positioned above the patient, the patient’s measured BP would be falsely decreased. If the transducer is vertically positioned below the patient, the patient’s measured BP would be falsely elevated.

10

If the patient’s true MAP is 100 mm Hg and the transducer is lowered 10 cm below the patient, what would the invasive blood pressure now display as the MAP? Why?

The relationship between cm H ₂ O and mm Hg is approximately 10:7. Therefore the displayed MAP is approximately 107 mm Hg. This is because mercury (Hg) is approximately 13.6 times denser than water (H ₂ O), and the catheter tubing of the arterial line contains normal saline (which is approximately the same density as H ₂ O). In other words, a 13.6 cm column of H ₂ O would exert a pressure that is equivalent to a 1 cm column of Hg (or 10 mm Hg), which has an approximate ratio of 10:7.

11

What are reasons for discrepancies between noninvasive and invasive blood pressure monitoring?

Causes of error with NIBP measurements include inappropriate cuff size and positioning, patient obesity, and decreased peripheral blood flow (e.g., septic shock or subclavian artery stenosis). Errors with IBP monitoring can be caused by a problem within the system (e.g., kink in the catheter or tubing, poor calibration, or air in the tubing) or certain patient characteristics (e.g., hypothermia, arterial spasm, subclavian artery stenosis). In practice, the most common cause for BP differences between IBP and NIBP monitoring is an incorrectly leveled transducer.