Respiratory variation of plethysmography signal with a pulse oxymeter: new predictive parameters of fluid responsiveness?

Introduction

Respiratory variation of pulse pressure (ΔPP) and of time of pre-ejection (ΔPEP) obtained from an arterial catheter have been shown as reliable predictors of fluid responsiveness in mechanically ventilated patients on sinus rhythm. The curve of plethysmography obtained from a pulse oxymeter allows one to calculate respiratory variation of 'pulsed plethysmography' (ΔPlethP) and of pre-ejection time (ΔPEPPleth).

Aim

To examine whether ΔPlethP and ΔPEPPleth could predict volume responsiveness as well as ΔPP and ΔPEP calculated from an arterial catheter.

Patients

Nineteen patients with septic shock, fully adapted to the ventilator and on sinus rhythm, were studied before and after a fluid challenge.

Methods

Pulsed plethysmography was defined by the amplitude of the pulse oxymeter wave. The pre-ejection period was defined by the time between the onset of the QRS and the onset of the blood pressure curve for PEPKT and the onset of the plethysmography curve for PEPPleth. The ECG, blood pressures and plethysmography tracings were recorded simultaneously on a computer. The respiratory variation of the parameters were calculated and averaged from five respiratory cycles according to the following formula: ΔP = [P max - min / P max + min / 2] × 100. Responders were defined as patients who increased their cardiac output – measured by Doppler transthoracic echocardiography – by more than 15% after fluid challenge.

Results

A strong correlation between the baseline value of the studied indices and the changes in cardiac output was observed (r = 0.86 and 0.85 for ΔPPKT and ΔPEPKT and r = 0.82 for both ΔPPPleth and ΔPEPPleth). Table 1 confirms the reliability of ΔPPKT and ΔPEPKT to discriminate responders from non-responders. It also shows that the plethysmography indices give similar information with similar threshold values.

Table 1 Sensibility and specificity: patients are fluid responsive if their values are over the threshold value with a Se and Spe value shown

Conclusion

The use of respiratory variation of a digital plethysmography signal (ΔPlethP and ΔPEPPleth) is helpful to assess fluid responsiveness in mechanically ventilated patients.