When would pulse oximetry be particularly unreliable?

Pulse oximetry can be particularly unreliable in cases of severe asthma due to the physiological changes that occur during an asthma attack. In severe asthma, the patient's airway may become significantly obstructed, leading to inadequate gas exchange. This can cause the oxygen saturation levels to fluctuate more than normal, potentially resulting in falsely low readings from the pulse oximeter.

In severe asthma, factors such as poor perfusion or increased work of breathing can also affect the accuracy of the device. This makes it difficult for the pulse oximeter to provide a true reflection of the oxygen saturation in the blood. Therefore, while pulse oximetry is a useful tool in many situations, its reliability can diminish in severe asthma cases due to the compromised respiratory function.

The other scenarios do not typically impact the reliability of pulse oximetry as significantly. A calm patient, regardless of age, is likely to have stable readings. While children may present challenges in obtaining a reading due to movement or anxiety, it doesn't inherently affect the device’s reliability as dramatically as severe asthma does. Having a fever can influence physiological responses but is not directly correlated with inaccurate pulse oximetry results.