Oxygen-Saturation is among the most widely used observation modes in medicine. It is easy to apply, non-invasive, cost-effective and yields information about peripheral pulse rate.
Multiple saturation probes are available which differ in their accuracy of approximating the arterial saturation by means of transcutaneous light absorption for oxygenated and de-oxygenated haemoglobin using different spectra.
There is an increasing body of literature showing a trend towards increased mortality with hyperoxia on admission in PICU patients. On the other hand, the ICU-ROX trial found no difference in ventilator-free days in adult ICU patients between conservative and usual oxygen therapy.
The graph on the left shows the O2 dissociation curve and how it is shifted by different influences.
Important number pairs include:
- paO2 28mmHg --> SpO2 50%
- paO2 40mmHg --> SpO2 75%
- paO2 60mmHg --> SpO2 90%
- paO2 70mmHg --> SpO2 93%
- paO2 80mmHg --> SpO2 95%
- paO2 90mmHg --> SpO2 97%
- paO2 100mmHg --> SpO2 98%
This relationships become even more important in children with congenital heart disease that have lower saturations than normal.
The formula on the left describes the factors that influence the oxygen delivery to the body.
The oxygen saturations are far more important than the contribution of the arterial O2 partial pressure!
Ross, Patrick A., Newth, Christopher J.L., & Khemani, Robinder G. (2014). Accuracy of pulse oximetry in children.(Report). Pediatrics, 133(1), 22–229.
Ross et al. have shown that across a range of different saturation probes the biggest discrepancy between SpO2 and SaO2 lies in the range of 76-80% with a tendency of the SpO2 to OVER-estimate the SaO2.
This needs to be taken into consideration while looking after children with lower than normal saturations targets.
Raman et al. have demonstrated a U-shape correlation between paO2 on admission to PICU and mortality. Hypoxia is clearly associated with increased mortality while the relationship of hyperoxia and probability of death isles clear.
From a clinical standpoint one needs to assess the patient comprehensively. Maintenance of end organ function is paramount and deserves complex assessment of patient factors, oxygen delivery, oxygen consumption, oxygen extraction and surrogates such as NIRS.
Given that SaO2 contributes substantially to DO2 while the additional contribution of paO2 is negligible there is no evidence to support maintaining paO2 values in excess of what is necessary to achieve target saturations.
Oxygen needs to be seen as a drug which - in Paracelsus' thinking - warrants administration in its optimal dose.
Saturation is a more direct measurement of tissue oxygen delivery than PaO2
ABG is a painful and expensive test
One ABG typically leads to a cascade of ABGs, multiplying costs and blood loss
Venous BG have with venous blood.
Obtaining an ABG may delay management
ABG only measures oxygenation at a single time point
Aspects favouring SpO2 over Blood gas analysis
Suggested Spo2 targets
General ICU population without pulmonary disease
Given an expected alveolar pAO2 of 100mmHg in room air and a normal alveolo-arterial gradient.
Children with lung disease (as per PALICC guidelines)
alarm settings in ventilated patients
Patients without O2 requirement should have no upper limit for saturations
Patients with a additional oxygen requirement should have an upper alarm set according to their targets (see above)
The nursing staff should be instructed to treat "high saturation" alarms with the same vigour as desaturation alarms.