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The following is a summary of “Testing preload responsiveness by the tidal volume challenge assessed by the photoplethysmographic perfusion index,” published in the September 2024 issue of Critical Care by Bruscagnin et al.

The tidal volume (Vt)-challenge is a method to detect preload responsiveness in patients with a Vt of 6 mL/kg predicted body weight (PBW), involving an increase of Vt to 8 mL/kg PBW and measuring the rise in pulse pressure variation (PPV), requires an arterial catheter. The perfusion index (PI), which reflects the amplitude of the photoplethysmographic signal, may reflect stroke volume, and its respiratory variation (pleth variability index, PVI) may estimate PPV.

Researchers conducted a retrospective study to assess whether Vt-challenge-induced changes in PI or PVI could reliably detect preload responsiveness as measured by a passive leg raising (PLR)-induced increase in cardiac index (CI) ≥10%.

They recorded haemodynamic (PICCO₂ system) and photoplethysmographic (Masimo-SET technique, sensor placed on the finger or forehead) data in critically ill patients ventilated with Vt = 6 mL/kg PBW and no spontaneous breathing during a Vt-challenge and a PLR test.

The results showed the study included 42 critically ill patients ventilated with Vt = 6 mL/kg PBW and no spontaneous breathing. During the Vt-challenge in the 16 preload responders, CI decreased by 4.8 ± 2.8% (percent change), PPV increased by 4.4 ± 1.9% (absolute change), PI_finger decreased by 14.5 ± 10.7% (percent change), PVI_finger increased by 1.9 ± 2.6% (absolute change), PI_forehead decreased by 18.7 ± 10.9 (percent change) and PVI_forehead increased by 1.0 ± 2.5 (absolute change). All these changes were more significant than in preloading non-responders. The area under the ROC curve (AUROC) for detecting preload responsiveness was 0.97 ± 0.02 for the Vt-challenge-induced changes in CI (percent change), 0.95 ± 0.04 for the Vt-challenge-induced changes in PPV (absolute change), 0.98 ± 0.02 for Vt-challenge-induced changes in PI_forehead (percent change) and 0.85 ± 0.05 for Vt-challenge-induced changes in PI_finger (percent change) (P= 0.04 vs. PI_forehead). The AUROC for the Vt-challenge-induced changes in PV_forehead and PVI_finger was significantly larger than 0.50 but smaller than the AUROC for the Vt-challenge-induced changes in PPV.

They concluded mechanically ventilated patients with no spontaneous breathing or atrial fibrillation, PI changes detected during the Vt-challenge reliably indicated preload responsiveness, with greater accuracy when measured on the forehead compared to the fingertip, while PVI changes also detected preload responsiveness but with lower accuracy.

Source: ccforum.biomedcentral.com/articles/10.1186/s13054-024-05085-w