Noninvasive Blood Pressure Monitoring

Blood Pressure Measurement: Invasive vs. Noninvasive

• Noninvasive blood pressure monitoring:
  • Indications: Per the American Society of Anesthesiologists guidelines, “Every patient receiving anesthesia shall have arterial blood pressure…determined and evaluated at least every five minutes”.¹
  • Contraindications: conditions in which repeated mechanical compression may cause traumatic injury (local bone fracture), open wounds to areas the cuff is intended to cover, presence of arteriovenous dialysis fistula, and indwelling peripherally inserted central lines.² Automated blood pressure cuffs require pulsatile blood flow, making them inaccurate in cases requiring cardiopulmonary bypass and in patients with ventricular assist devices (VAD).³
  • Complications: pain, petechiae and ecchymoses, limb edema, venous stasis and thrombophlebitis, peripheral neuropathy, and compartment syndrome
• Patients who have undergone axillary lymph node dissection were thought to be at an increased risk of lymphedema if blood pressure was repeatedly measured on the ipsilateral arm. However, these reports have been mostly anecdotal, and a large prospective cohort study showed no increase in lymphedema associated with blood pressure cuff usage.^4,5
• Invasive blood pressure monitoring:
  • Indications: anticipated rapid changes or extremes of blood pressure, patient inability to tolerate brief hemodynamic instability, and need for frequent blood sampling to assess respiratory function, oxygenation, ventilation, or metabolic derangements
  • Contraindications: ongoing limb ischemia, local infection, or compromised collateral arterial supply (e.g., Raynaud phenomenon, thromboangiitis obliterans)3
  • Complications: limb ischemia, hemorrhage, thrombosis, embolism, aneurysm or fistula formation, skin necrosis, and infection²

Noninvasive BP: Measurement Techniques

• Palpation
  • SBP can be determined by palpating a peripheral pulse, inflating a blood pressure cuff proximal to the location until the flow is occluded, releasing cuff pressure slowly (2-3 mmHg per heartbeat), and finally measuring the cuff pressure at which the pulse is first felt again.
  • Limitations of this method include no DBP or MAP measured and underestimation of SBP.
• Doppler
  • A Doppler probe transmits an ultrasonic high-frequency sound across the lumen of an artery and may be substituted for the physician’s finger in the prior method to measure blood pressure.
  • A shift in sound frequency (the Doppler effect) is produced by the movement of blood as pressure is released from an inflated blood pressure cuff.
  • The inflation pressure at which flow is detected can reliably determine the SBP.
  • Limitations of this method include no diastolic pressure or MAP measured.
• Auscultation
  • Korotkoff sounds are frequencies created by turbulent flow within an artery caused by partial vessel compression by a blood pressure cuff. These sounds can be heard with the aid of a stethoscope over an artery as pressure is released from an inflated cuff.
  • The pressure at which the first Korotkoff sounds are heard corresponds with the SBP.
  • The pressure at which the muffling or loss of sound can be recorded as the DBP.
  • This method is best used to measure the SBP and DBP but is unable to measure the MAP directly.
  • Measurement of the SBP and DBP can then be used to estimate the MAP with the equation:

• Oscillometry:
  • An automated blood pressure cuff is inflated until a pressure is reached, which occludes arterial flow to the limb. Pressure is then slowly released, and small fluctuations in cuff pressure are closely monitored by the machine.
  • At first, the machine will only detect the pulseless decrease in pressure. However, as pressure is decreased below SBP, arterial flow resumes, which produces a detectable vibration of the arterial wall. This vibration produces oscillating changes in cuff pressure, which are measured by the computer.6
  • As the cuff pressure is decreased to SBP and then further decreased to DBP, the frequency of oscillations increases and then decreases as flow through the vessel eventually becomes smooth, producing no oscillations in cuff pressure. These changes in oscillatory frequency roughly correlate with Korotkoff sounds heard with manual auscultation (see Figure 1).
  • From these measurements, the point of maximal oscillation is measured as the MAP.
  • SBP and DBP are estimated using proprietary algorithms that vary by manufacturer.7,8
  • As SBP and DBP are not directly measured, they are therefore deemed to be less reliable than the MAP.3
  • A comparison of oscillometric and direct arterial blood pressure measurements have shown that oscillometric blood pressure cuffs will often underestimate SBP and overestimate DBP. They also tend to underestimate values during periods of hypertension and overestimate during periods of hypotension.9
    • This discrepancy in measurement may lead to a significant underestimation of pulse pressure.3
  • DBP is the least accurate value measured by oscillometry, as this value is estimated using the minimum cuff pressure at which pressure fluctuations can be perceived. This value is difficult to measure as oscillations can still be present, even when the cuff is below the actual DBP.2
  • Adequate blood pressure cuff sizing is important for accurate measurement.
    • Blood pressure cuffs should have a bladder that extends to 40% of the arm circumference and 80% (or 2/3) of the distance between the elbow and shoulder.3
  • Blood pressure measurements may be inaccurate in patients with any motion (shivering, tremors, seizure, etc.), severe hypotension, arrhythmias (e.g., atrial fibrillation), or mechanical disruption of the cuff or tubing.10
  • In addition to the previously noted automated (oscillometric) measurement techniques, palpation and auscultation also require pulsatile blood flow to measure the components of blood pressure. Therefore, these methods are inaccurate in the measurement of blood pressure for patients on cardiopulmonary bypass (CPB) and those primarily dependent on VADs, which produce nonpulsatile flow.3