Topics Discussed in the Podcast: Provider bias and the pulse oximeter. CO oximetry. Pulse oximeter lag. Approach to the well-appearing patient with a low SpO2 reading. Relation of vascular tone to pleth wave amplitude and variability. Using the pulse ox waveform to confirm mechanical capture during transcutaneous pacing. Odds and ends...
Pulse oximetry is more than just a measurement of oxygenation:
The pulse oximetry plethysmograph is a pulsatile waveform that can be thought of as an arterial waveform. However, this waveform is influenced by cardiac output and systemic vascular resistance.
Thinking of the SpO2 pleth as a pulsatile waveform, variations in the patients cardiac output should be transmitted to the fingertip arteries, which results in variations in the pulse ox waveform amplitude.
The implications of this are far reaching. In patients who are mechanically ventilated, cyclical changes in intrathoracic pressures can produce measurable variations in the pulse oximetry waveform of a preload dependent patient, which suggesst a potentially fluid-responsive patient.
Do we really need to remove fingernail polish?
You can save your department some money because it's unlikely to affect the SpO2 measurement to any clinically relevant degree. However, if you are concerned, just turn it sideways.
Is this the silver bullet of measuring fluid responsiveness, non-invasively?
No. At least not yet. The majority of studies evaluating pulse oximetry utilization in this application were conducted under controlled conditions, such as mechanically ventilated patients with consistent respiratory changes in a proper sedation. Additionally, pulse oximetry waveform variations can be difficult to measure during increased systemic vascular resistance (How many patients on mechanical ventilation also require vasopressor support?).
Then there's the issue with variations in vascular tone. The fingertip is much more susceptible to these changes as opposed to the vessels in the forehead, nose, or ear.
To visualize what pulse wave variability looks like, here's a helpful picture:
[caption id="" align="alignnone" width="600"] Extreme respiratory variation in a mechanically ventilated patient[/caption]