Pulse oximetry allows non-invasive monitoring of oxygenation (SpO2), it is simple and reliable in many areas of clinical practice. SpO2 has a confidence rate of 95% ± 4%, so readings ranging between 70% and 100% are considered reliable.(5) In patients with mechanical ventilation, the objective is to identify hypoxemia.
It is important to remember that oximeters do not measure arterial oxygen pressure (PaO2), for this reason, they cannot directly diagnose hypoxemia or hyperoxemia (PaO2 <60 mmHg and PaO2> 120 mmHg respectively).(6) What they do is “estimate” hypoxemia when SpO2 falls <90%, which would correspond to a PaO2 <60 mmHg according to the oxyhemoglobin dissociation curve (Table 1). (7) However, it must be taken into account that changes in temperature and pH cause changes in this relationship. As the pH increases (alkalosis) or the temperature decreases (hypothermia), the shift of the curve is to the left since hemoglobin binds more strongly with oxygen, delaying its release to the tissues. Acidosis and fever shift the curve to the right as the hemoglobin molecule decreases its affinity for oxygen, facilitating the release of oxygen to the tissues.