The purpose of optimizing preload it’s to improve, when needed, the cardiac output. However a preload assessment measure will not tell when a further increase on preload will lead also to an increase in cardiac output.
When the patient isn’t preload optimized the stroke volume (SV) with positive pressure ventilation will increased on inspiration . This is registered by the SV Variation (SVV) between inspiration and expiration or it’s proxies, Pulse Pressure Variation (PPV) and, on a less degree, systolic pressure. If the ventilation it’s being done in a standard way, this measure will give an accurate prediction of preload responsiveness.
It can be done automatically by a monitor (for SVV and PPV) or manually (for PPV). Manually the PPV can be determined:
- Record the minimum pulse pressure on a respiratory cycle (PPmax)
- Record the maximum pulse pressure on a respiratory cycle (PPmin)
- Calculate the PPV using the following formula:
The SVV can be determined on the same way, nevertheless all the monitors that can display a beat by beat SV can calculate it.
Conditions and caveats
- The patient must be in volume controlled ventilation, without spontaneous breathing
- Tidal volume must be at least 8 ml/Kg
- If using SVV the SV must be measured beat to beat
- Should’t be used on a patient with an arrhythmia, right heart failure or pulmonary hypertension, as those will lead to an increased variation
Dynamic changes in arterial waveform derived variables and fluid responsiveness in mechanically ventilated patients: a systematic review of the literature;Marik PE, Cavallazzi R, Vasu T, Hirani A; Crit Care Med; 2009;37(9):2642–7 ↩