Question 3.1

College Answer

a) What pathophysiological abnormality is illustrated by the arterial waveform?

Systolic pressure variation > 10 mm Hg

Pulse pressure variation

b) What is the clinical significance of the abnormality illustrated above?

Often implies a degree of fluid responsiveness

c) List 3 conditions in which such a scenario can occur.

Hypovolaemia, tamponade, bronchospasm, pneumothorax, raised intra-abdo pressure, raised intra- thoracic pressure, LV dysfunction, Dynamic hyperinflation

Discussion

This is an example of the haemodynamic effects of positive pressure ventilation.

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 a significant haemodynamic benefit from the afterload reduction associated with positive pressure ventilation
  • LV dilatation causing RV diastolic failure

The degree to which this arterial line "swing" correlates with fluid responsiveness is discussed elsewhere.

The conditions in which PPV can occur are a fairly straightforward bunch, with the exception of LV dysfunction. This one is weird. PPV should predict good LV function rather than bad. Turns out, cardiac resynchonisation therapy has been shown to increase PPV, which leads one to correctly conclude that a ventricle with poor contractility will not be briskly responsive to changes in preload.

However, severe LV failure with LV dilatation can still produce increased pulse pressure variation by two mechanisms:

• LV dysfunction results in an amplification of the afterload-reducing effects of positive pressure ventilation; thus the systolic pressure increases  in early inspiration, giving rise to pulse pressure variation.
• LV dysfunction decreases the compliance of the right heart. This is a purely mechanical effect: the RV cannot fill effectively if the LV is a huge dilated slob, occupying most of the pericardial sack with its bulk.

Michard, Frédéric, Marcel R. Lopes, and Jose-Otavio C. Auler. "Pulse pressure variation: beyond the fluid management of patients with shock." Critical Care11.3 (2007): 131.

He, Huai-wu, and Da-wei Liu. "The pitfall of pulse pressure variation in the cardiac dysfunction condition." Critical Care 19.1 (2015): 1-1.