Pressure

Invasive pressure monitoring is an essential part of critical care, and aims to guide assessment of the delivery of oxygen to tissues.

Pressures

Commonly monitored haemodynamic pressures include:

  • Systemic arterial pressures
  • Pulmonary artery pressures
    • PCWP
  • Central venous pressure

Central Venous Pressure

The CVP, JVP, and RAP may vary in the setting of some pathologies, such as SVC stenosis causing SVC obstruction.

The CVP is the pressure in the vena cava, and by extension the RA. The CVP is:

  • Normally:
    • 0-5mmHg in the spontaneously breathing (well) person
    • <10mmHg in the positively-pressure ventilated person
    • Correlated with the PAWP
      This relationship is often lost in illness.
  • Downstream pressure for the circulation
  • The filling pressure for the right heart

Pulmonary Capillary Wedge Pressure

This is the left atrial back-pressure measured from the PAC tip when the balloon is inflated such that it fully occludes the vessel. The wedge pressure is:

  • A surrogate for the LAP
    In absence of flow (because of balloon occlusion), there will be no pressure component due to resistance and therefore the measured pressure should be ~LAP. This assumes:
    • A continual column of blood between the catheter and the LA
      Catheter should be in West Zone 3.
  • Not measured routinely
    Routine wedging is the cause of much of the morbidity of PACs, due to the risk of PA rupture.

Measuring:

  • Gradually inflate the balloon until the normal PAP waveform disappears and is replaced with a (rather damped) looking version of the CVP waveform

Pressure Monitoring Devices

Arterial pressure monitoring evaluates:

  • SBP
    • Generally, the determinate of bleeding
  • MAP
    • Determinant of perfusion for most organs
    • Least dependent on measurement site or technique
    • Least affected by damping
  • DBP
    • Determinant of myocardial perfusion pressure

Intermittent Oscillometric Blood Pressure

Detection of pulsation in a pneumatically inflated cuff over a proximal limb, which occludes and then reinflates to provide an indication of pressure in that vessel.

Pros:

  • Easy
  • Rapid
  • Uncomfortable

Cons:

  • Not always clinically appropriate
    • PVD
    • AV fistula
    • Lymphoedema
    • Skin and muscular injury
    • IV site location
  • Inaccurate in dysrhythmia
  • Overestimates low pressures and underestimates high pressures
  • May lead to pressure injuries with repeated inflations

Continuous Non-Invasive Blood Pressure

Bladder placed around a digit, which compresses the artery to a constant size (a volume-clamp). Arterial pulsation changes the volume of the clamp, which correlates to systemic blood pressure.

Pros:

  • Non-invasive
  • Continuous

Cons:

  • No blood gas sampling
  • Poor at detecting rapid changes in haemodynamic state
  • Intermittent pause in monitoring to allow finger perfusion

Invasive Arterial Measurement

Direct transduction of the pressure of a column of fluid that is connected to an intra-arterial catheter.

Pros:

  • Continuous, accurate monitoring of pressure
  • Gas sampling

Cons:

  • Invasive
    • Painful
    • Infective
    • Positional
  • Requires fluid infusion to maintain patency
    Usually 3mL/hr of saline (sometimes heparinised).

References

  1. Bersten, A. D., & Handy, J. M. (2018). Oh’s Intensive Care Manual. Elsevier Gezondheidszorg.