Measurement Of Blood Pressure

MEASUREMENT OF BLOOD PRESSURE

• Blood pressure can be measured either directly & indirectly.

1. DIRECT METHOD/DIRECT MANOMETRY/INTRAARTERIAL METHOD:

• A catheterized artery connected to a mercury manometer/ pressure transducer/any other pressure-sensitive device.
• Accurate method.
• Yet, suitable only for experimental purposes.
• For clinical BP measurement, an indirect method preferred.

2. INDIRECT METHOD (USING SPHYGMOMANOMETER):

• Fairly accurate measurement.
• Non-invasive method.

• An inflatable rubber/Riva-Rocci cuff attached to a mercury manometer.
• Wrapped around arm about an inch above cubital fossa.
• Cuff is rapidly inflated.
• Until pressure is well above expected systolic BP, so that brachial artery is occluded.
• Cuff pressure is lowered slowly at rate of 2-3 mm/sec.
• When it goes below systolic pressure, blood spurts through artery with each heartbeat.
• This marks systolic BP.
• On further lowering, blood flows through brachial artery relatively smoothly.
• Yet, still passes in spurts (since BP is at its lowest).
• Finally, when cuff pressure falls to level of diastolic pressure, 
• Blood flow in brachial artery becomes entirely free from turbulence.
• This marks diastolic BP.
• Blood pressure measured by an auscultatory method using sphygmomanometer tends to be higher than true intra-arterial pressure measured by arterial cannulation.
• Because of some cuff pressure dissipating between cuff & artery in soft tissue.

• While measuring BP, patient should be in resting position (seating or lying down).
• Patient’s arm with attached cuff shows be at heart level.
• Cuff should be applied to upper arm.
• Cuff should encircle at least 80% of arm.
• I.e. length of bladder should be at least 80% arm circumference.
• Cuff width should be at least 40% of circumference.

• Three different methods of performing sphygmomanometry –
• Oscillatory.
• Palpatory.
• Auscultatory.
• Difference is in criteria for identifying systolic & diastolic pressure.

1. OSCILLATORY METHOD:

• Here, mercury column is observed.
• As mercury column falls to touch systolic pressure, it starts showing small oscillations.
• Oscillations are largest at mean BP & abruptly disappear at diastolic pressure.
• Rarely used methodology.

2. PALPATORY METHOD:

• Here, radial artery pulse is palpated.
• As cuff pressure falls, pulse appears at systolic point.
• An inaccurate method.
• Results are about 2-5 mm Hg lower than those obtained through auscultatory method.
• Diastolic point cannot be identified by this method.

3. AUSCULTATORY METHOD:

• Method uses stethoscope, placed over brachial artery in cubital fossa.
• “Korotkoff’s sounds” appear at systolic pressure & disappear at diastolic pressure.
• Sounds are attributed to turbulence.
• Caused by partial arterial occlusion.
• According to Americal Heart Association, Korotkoff’s sound occurs in five phases:
• Onset of phase I Korotkoff’s sound corresponds to systolic pressure.
• Disappearance of sounds (phase V) corresponds to diastolic pressure.

Phase 1:

• Faint, clear, tapping sounds. 
• This is systolic pressure.

Phase 2:

• Murmurs or swishing sounds.

Phase 3:

• Crisper, more intense sounds.

Phase 4:

• Distinct, abrupt muffle of sound. 
• In children, this is Diastolic Pressure.

Phase 5:

• No sounds. 
• This is diastolic pressure in adult.

AUSCULTATORY GAP:

• Phase I sounds sometimes disappears.
• Occurs when pressure is lowered from systolic pressure.
• Sounds reappear at a lower level.
• This, interval between systolic & diastolic when no Karotkof sound heard referred as “Auscultatory gap”.
• Mainly in hypertensive patients.
• If cuff pressure is not adequately elevated in beginning, lower limit of auscultatory gap can be mistaken as systolic BP.
• Hence, falsely low recording of systolic BP may occur.
• This error avoided by recording systolic BP through palpatory method.

CONDITIONS INCREASING BP:

1. Size of inflatable cuff: 

• Is of critical importance.
• Selection of proper cuff size:
• Determined by limb dimensions for BP measurement. 

Width of cuff:

• Should be 40% of circumference.
• Otherwise, 1.2 times diameter of extreme.

Length of inflatable cuff:

• Should be 80% of arm circumference.
• Ie., Length-to-width ratio of 2: 1.

• Seen with using too narrow cuff/small cuff size
• Also in obese patients.
• Seen with thick calcified arteries.
• E.g., in elderly, atherosclerosis, diabetes & Monkenberg’s sclerosis.
• Vessels are difficult to compress & higher cuff pressure is required to compress them.