A Short Summary on Blood Pressure

Definition: Lateral pressure exerted by the flowing blood on the wall of the vessels

Blood Pressure Measurement:

A) Methods:

  • Direct: Cardiac catheter
  • Indirect: Sphygmomanometer

Factors influencing BP measurement:

  • Increase BP: Caffeine, Tobacco smoking, Exercise, Emotional responses, Medications
  • Decrease: Changes in posture, Medications

B) Sphygmomanometer parts:

  • Aneroid barometer or Mercury manometer
  • Riva-rocci cuff: width must be atleast 40% of arm circumference, should be 20% broader than the diameter of the patient’s upper arm and must be tied 1 inch above medial crease
  • Hand pump

Avoid parallax error by placing the mercury manometer at the level of the observer’s eye

C) Indirect Methods:

  • Palpatory method: cuff inflation → pulse disappears → pressure raised by 20 mmHg more → Gradual release of pressure (2mm/s) → Point of reappearance of pulse (Systolic pressure)
  • Auscultatory method: pressure raised 20mm more than obtained systolic pressure → gradual release of pressure (2mm/s) → Appearance of sound (systolic pressure) → Disappearance of sound (diastolic pressure)

Arterial BP rises in response to stress. Avoid diagnosing hypertension after only 1 or 2 BP measurements, particularly if there is no evidence of end-organ damage i.e. normal fundi and no left ventricular hypertrophy. When re-measuring the blood pressure, the cuff pressure must be completely released for 1 to 2 min.Otherwise venous pooling can mimic elevated diastolic pressure.

D) Korotkoff sounds: Change in sound quality at interval of every ~ 10mmHg BP drop during auscultation over brachial artery

  • Phase I (Systolic pressure marking): Tap sound
  • Phase II: Murmur
  • Phase III: Gong sound
  • Phase IV: Muffled sound
  • Phase V (Diastolic pressure marking): Disappearance of sound

E) Mechanism of measurement:

  • Cuff pressure > Systolic BP: Occlusion → No pulse, No sound
  • Cuff pressure slightly < Systolic BP: Blood flow and turbulence → Sound
  • Cuff pressure slightly < Diastolic BP: Normal blood flow (No turbulence) → No sound

F) Auscultatory gap:

  • During auscultatory method (initial tapping sound → gap → sound)
  • Eliminated by palpatory method
  • Common in ageing and atherosclerosis
  • Can cause: underestimation of systolic BP and over-estimation of diastolic BP
Follow the link for more about Ausucultatory Gap

G) To check postural hypotension:

  • BP in supine position → Let patient lie for 3 minutes (leave cuff) → BP in standing position
  • Significant if pressure drop > 20/10 mmHg

Common causes of postural hypotension:

  • Hypovolaemia
  • Diabetes
  • Peripheral neuropathy
  • Parkinson’s disease
  • Anaemia
  • Adrenal insufficiency
  • Drugs: Diuretics and Vasodilators

JNC 7 Classification of Hypertension:

Physiology of Blood Pressure:

  • SBP (Systolic Blood Pressure) depends upon stroke volume and compliance of arteries
  • DBP (Diastolic Blood Pressure) depends upon peripheral resistance
  • BP = Cardiac Output (CO) X Peripheral Resistance (PR)
  • Cardiac Output (CO) = Stroke volume X Heart rate
  • Mean Arterial Pressure (MAP) = DBP + 1/3 Pulse Pressure
  • Pulse Pressure (PP) = SBP – DBP

Significance of Mean Arterial Pressure:

Mean arterial pressure is generally regarded as a measure of cardiac output and peripheral resistance. It is considered to be perfusion pressure seen by organs in the body. A MAP of at least 60 is necessary to perfuse the coronary arteries, brain, and kidneys. Normal range is around 70 – 110 mmHg. It’s slightly less than the value half way between systolic and diastolic pressure because systole is shorter than diastole, so you put the diastole twice.

In a head injured patient, the brain is at risk of ischaemic injury due to insufficient blood flow if the MAP falls below 50. On the other hand, a MAP above 160 reflects excess cerebral blood flow and may result in raised intercrainial pressures.

 

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