In which scenario is the use of oxygen contraindicated?

The use of oxygen is contraindicated in the scenario of paraquat poisoning when the patient's oxygen saturation (SpO2) is greater than 90%. Paraquat is a highly toxic herbicide and its effects on the lungs can be exacerbated by supplemental oxygen in certain situations. In patients with paraquat poisoning, providing supplemental oxygen can lead to increased production of reactive oxygen species, worsening lung injury and potentially leading to more severe outcomes. Therefore, patients who have SpO2 levels above 90% should not be given oxygen, as the risks outweigh the benefits.

In the other scenarios presented: hypoxia generally necessitates oxygen administration to improve tissue oxygenation; diving emergencies often require careful management but can include oxygen therapy; and in acute coronary syndrome, oxygen may be beneficial if the patient is hypoxic. Each of these cases involves the principle of providing oxygen to sustain or improve patient outcomes, contrasting with the specific contraindications associated with paraquat poisoning.