Information Derived from the Arterial Pressure Waveform

Historically, the arterial line waveform has appeared in the exam in several forms. The trainees have at one stage been expected to discuss broadly what sort of information can be derived from it (Question 30.2 from the second paper of 2013). Questions regarding the change of the waveform depending on its position in the vascular tree have also appeared (Question 11.1 from the first paper of 2010). More often, the college will produce an arterial waveform tracing with some abnormality (eg. AF with loss of atrial kick, or respiratory "swing" ) and then ask the trainee to identify the abnormality and give four causes.

Information from arterial line amplitude

  • Systolic pressure
  • Diastolic pressure (coronary filling)
  • Mean arterial pressure (systemic perfusion)
  • Pulse pressure (high in AR, low in cardiac tamponade or cardiogenic shock)
  • Changes in amplitude associated with respiration (pulse pressure variation): this was the topic of  Question 5.2 from the first paper of 2012, which presented the trainees with a picture of a "swinging" arterial trace, and asked about the implications thereof.

A systolic pressure variation of over 10mmHg is thought to be significant.

An increase in respiratory pulse pressure variation could be due to any of the following:

  • Inadequate right heart filling, for example:
    • Hypovolemia (thus, it can imply a degree of fluid responsiveness)
    • Vasodilated shock state (central venous venodilation)
  • Excessive right heart afterload, for example: 
    • Acute severe asthma with gas trapping and hyperinflation
    • Tension pneumothorax
    • Massive pulmonary embolism
  • Decreased right ventricular compliance, for example:
    • Cardiac tamponade or large pericardial effusion
    • RV failure due to infarction
    • Post-radiotherapy changes or infiltrative disease, eg. amyloid
    • LV failure with LV dilatation (causing RV diastolic failure)

Information from arterial line frequency

  • Heart rate
  • Rhythm
  • Effect of rhythm on MAP

Information from arterial waveform shape

  • Slope of anacrotic limb represents aortic valve and LVOT flow
  • Slurred wave in AS
  • Collapsing wave in AS
  • Rapid systolic decline in LVOTO
  • Bisferiens wave in HOCM
  • Low dicrotic notch in states with poor peripheral resistance

Information from the Square Wave test

The square wave dynamic response test receives more attention elsewhere.    In one previous SAQ (Question 11.2 from the first paper of 2010) the candidates were presented with an underdamped square wave trace, which looked something like this:

Underdamped arterial line waveform

The candidates were then asked to comment on  "the fidelity of the arterial system". What did the college mean by this?

  • "Fidelity" of the system is its ability to faithfully reproduce the arterial pressure.
  • The fidelity of a fluid-coupled pressure transducer system is constrained by damping and natural frequency.
    • Damping describes the natural tendency of the fluid (or air bubbles) in the system to extinguish motion. Some damping is essential to absorb the harmonic resonance reverberating though the system.
    • Natural frequency is the frequency at which the system is most prone to resonance. If the frequency of the harmonics in the arterial pressure waveform is similar to the natural frequency of the measuring system, the signal becomes exaggerated by constructive interference (hence, the peaks will be more "peaky" and troughs will be deeper; the systolic will be overestimated and the diastolic underestimated).
  • The fidelity of an arterial system is ideal when both damping and natural frequency are optimised to respond to the normal range of arterial frequencies, between 1 and 30Hz.
  • The square wave test or "fast flush test" demonstrates whether the system is optimised for this role. A system with optimal dynamic response characteristics will oscillate only once or twice (and at least once) after a flush.

Influence of catheter position on the arterial waveform 

The shape of the arterial waveform changes according to its position in relation to the aortic valve. The college has previously used this picture:

The candidates were then asked to "list the likely sites A-E". The college answer identified A as the central aortic measurement, B as proximal upper limb, and so forth in order of increasing distance. This phenomenon is discussed in greater detail somewhere in the Haemodynamic Monitoring section, probably in the chapter on "Normal arterial line waveforms".
In brief:

The further you get from the aorta,

  • The taller the systolic peak (i.e. a higher systolic pressure)
  • The further the dicrotic notch
  • The lower the end-diastolic pressure (i.e. the wider the pulse pressure)
  • The later the arrival of the pulse (its 60msec delayed in the radial artery)

The MAP doesn't change very much because, from the aorta to the radial artery, there is little change in the resistance to flow. MAP only really begins to change once you hit the arterioles; this is called distal systolic pulse amplification.

 

References

McGhee and Bridges Monitoring Arterial Blood Pressure: What You May Not Know (Crit Care Nurse April 1, 2002 vol. 22 no. 2 60-79 )

Thomas, Gary, and Victoria Duffin-Jones. "Monitoring arterial blood pressure." Anaesthesia & Intensive Care Medicine 16.3 (2015): 124-127.

For those who like hardcore physics, this excellent resource will be an enormous source of amusement. It appears to be a free online textbook of anaesthesia. Nowhere else was this topic covered with a greater depth, or with a greater attention to mathematical detail.