Variations in Blood Pressure

Describe the physiological factors that may contribute to pulse variations in blood pressure

Blood pressure is not uniform throughout the circulation. Ventricular ejection generates two waves:

  • A blood flow wave
    Travels at ~20cm.s-1.
  • An arterial pressure wave
    Distends the elastic walls of the large arteries during systole, which then recoil during diastole to facilitate continual blood flow. This is the Windkessel effect.
    • This wave travels at 4m.s-1
    • This is what is felt when pulses are palpated

Changes by Site of Measurement

Measured pressure changes predictably at more distal sites:

  • All gradients are increased
    Arterial upstroke and falloff are both steeper.
  • The SBP increases
  • DBP decreases
  • MAP is constant
  • The dicrotic notch occurs later and becomes less sharp
    This occurs due to reflections in arterial pressure waves.

Physiological Changes

Changes in circulatory function:

  • Inotropy
    The rate of systolic upstroke is related to , and therefore contractility.
  • SVR
    The gradient between the peak systolic pressure and the dicrotic notch gives an indiciation of SVR. E.g., a steep downstroke suggests a low SVR, as the pressure in the circulation rapidly falls when ejection ceases.
  • Preload
    A beat-to-beat variation is seen with the respiratory cycle, due to the change in preload occurring with changes in intrathoracic pressure.

Pathological Changes

Some pathological causes include:

  • Aortic Stenosis
    Causes a reduction in:
    • Pulse pressure
      Due to reduced stroke volume.
    • Gradient of upstroke
      Due to reduced stroke volume and reduced velocity of ejection.
  • Aortic Regurgitation
    • Wide pulse pressure
      Combination of:
      • Increased SBP due to the increased force of ejection due to increased preload (Starlings Law), which occurs due to high ESV
      • Decreased DBP due to part of the stroke volume flowing back into the ventricle through the incompetent valve

References

  1. Chambers D, Huang C, Matthews G. Basic Physiology for Anaesthetists. Cambridge University Press. 2015.
Last updated 2017-09-23

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