Excessive variations in the plethysmographic waveform during spontaneous ventilation: An important sign of upper airway obstruction

The respiratory variations in the plethysmographic (PLET) waveform of the pulse oximeter during mechanical ventilation can be automatically quantified as the PLET variation index (PVI®). Like other dynamic variables, the PVI may provide useful information about fluid responsiveness but only when the patient is receiving fully controlled mechanical ventilation with no spontaneous breathing activity. However, a growing number of monitors that automatically measure and display the values of the PVI and other dynamic variables are being introduced into clinical practice. Using these monitors in spontaneously breathing patients may cause inadequately trained personnel to make erroneous decisions or may eventually lead to a total disregard of dynamic parameters altogether. The aim of this study is to call attention to the fact that excessive variations in the PVI during spontaneous ventilation, termed sPVI, should not be regarded as artifactual since they may be an early important sign of upper airway obstruction (UAO). Among the monitor screen shots that were stored for educational purposes, I have identified 4 screen shots of patients who were clinically diagnosed as having significant UAO. In all instances, UAO was associated with prominent variations in the PLET waveform. These variations were calculated as the difference between the maximal and minimal amplitudes of the PLET signal divided by either the maximal amplitude (sPVI) or by the mean of the 2 values (δPOP). The ranges of the measured δPOP and sPVI values during UAO were 28% to 42% and 25% to 39%, respectively. These values are 2 to 3 times higher than the range of 9.5% to 15% that was repeatedly found as the best threshold for the identification of fluid responsiveness in mechanically ventilated patients. In 2 of these cases, simultaneously measured values of the pulse pressure variation were high as well (19% and 34%), while the calculated pulsus paradoxus was 28 and 40 mm Hg. In 2 cases, the analog signals of impedance plethysmography and capnography persisted, despite the presence of clinically significant UAO. It is, therefore, suggested that monitoring the sPVI may be of great clinical importance in spontaneously breathing patients who are susceptible to develop UAO.