If you slow down the sweep speed of your monitor and examine the waveform of the pulse oximeter you will see the respiratory variation. Indeed, one could argue that this should be the default display. Particularly if a patient is paralyzed and ventilation is controlled, the variation with ventilation is related to left ventricular stroke volume. Greater variation is seen when heart is on the steeper portion of the Starling curve, as the change in ventricular filling over the ventilatory cycle causes a large change in cardiac output. If you give fluid, the heart fills more. The heart is now on the flatter portion of the Starling curve, and the stroke volume will change less over the ventilatory cycle. As a result, respiratory variations decrease. This is why respiratory variations are so useful in guiding fluid management: you can directly observe the Starling curve in action with the changes in ventricular filling that accompany controlled ventilation.
What if a patient is breathing spontaneously and there is a great deal of respiratory variation? If there is respiratory variation, is the variation only due to volume issues?
The answer is “no.” Dr. Azriel Perel, Department of Anesthesiology and Critical Care, Sheba Medical Center, Tel Aviv University, Tel Aviv, Israel, describes four examples of respiratory obstruction related to plethysmographic waveform variation in his article published in this month’s issue of Anesthesia & Analgesia titled “Excessive Variations in the Plethysmographic Waveform During Spontaneous Ventilation: An Important Sign of Upper Airway Obstruction.”
First, the amount of variation resulting from upper airway obstruction was 2-3 times greater than the 9-15% variation that is typically found when fluid responsiveness is identified. If respiratory obstruction is possible, it is important to also examine the capnography waveform. For example, in one of the patients with upper airway obstruction with associated large variations in the plethysmographic waveform, the capnographic waveform disappeared. For two of the patients, the arterial pressure waveform also showed significant variation. The reason for the variation seen with upper airway obstruction is that negative pleural pressure is transmitted to the left ventricle, which might result in a significant decrease in left ventricular stroke volume. Myocardial mechanics might also be affected.
Should we routinely decrease the sweep speeds of the plethysmographic waveform to better detect waveform variation? Probably! Since pulse oximetry is used anyway, might earlier recognition of airway obstruction (e.g., due to laryngospasm) while monitoring plethysmographic waveform variation lead to better management of airway obstruction? Some monitors already can provide a plethysmographic variation index (PVI®).
This article helps us to understand that large variation does not necessarily mean that more fluid should be administered, but may also indicate other problems such as airway obstruction.