ECMO-/ECLS

Continuous hemoglobin oxygen saturation is a critical piece of data
to assess the effectiveness of ICU interventions. The arterial partial pressure of
oxygen (PaO 2 ) may be a more direct measure of the efficiency of gas exchange
at the alveolar/capillary junction and is, therefore, an important tool to assess the
degree of pulmonary dysfunction. However, it is a trivial contributor to overall
oxygen content in the blood. Using the oxygen content equation (shown below),
it becomes readily apparent that changes in SaO 2 have substantially greater
impact on oxygen content. While a decline in PO 2 for a given inspired oxygen
concentration and mean airway pressure may signal a decline in pulmonary
function or anatomic shunt and may be useful for prognostic purposes, increased
P0 2 is not the primary goal of subsequent therapeutic manipulations to achieve
improved patient oxygenation.
O 2 content = 1.36(SaO 2 )(Hgb) + PO 2 (0.003)
Notably, the oxygen content equation also demonstrates the linear
relationship between hemoglobin saturation and blood oxygen content.
Therefore, there is no mathematical threshold of oxygen saturation that signals
patient risk. Many intensivists empirically target 90% or greater as the desirable
values though this is neither supported by available data nor by the mathematical
principles of the equation. In circumstances such as respiratory failure, tolerance
of lower arterial oxygen saturation may be an essential component of strategies
to avoid ventilator-induced lung injury (VILI).
An important, but often overlooked additional value of pulse oximetry is
the presence and quality of the waveform. An astute clinician recognizes that