Chapter 12 – “Hypoxic”
Welcome to the twelfth installment of “Inconceivable: medical terms that don’t mean what you think they mean.”
You are taking care of a 32-year-old woman with a week of fever, cough, and progressive dyspnea on exertion. Her chest x-ray shows a large lobar infiltrate and you correctly diagnose her with community acquired pneumonia. Her pulse oximeter is showing persistent readings in the low 90s, her heart rate is persistently in the 110s, but she has a normal blood pressure and lactate, and you decide to admit her for IV antibiotics and close monitoring. You sign-out to the medical team that despite her young age and lack of co-morbid conditions, her hypoxia justifies an admission. But is she hypoxic? Let’s find out.
Hypoxia is defined as inadequate oxygen delivery to the tissues. This can be either global body hypoxia or to a specific part of the body. Hypoxemia, on the other hand, refers to a low level of oxygen in the blood.
Although hypoxia and hypoxemia can co-exist, they are not synonymous, and one does not necessitate the other. For example, a patient with COPD may have a pulse oximeter reading only 90%, however their body may have compensated for the chronic hypoxemia by increasing the amount of oxygen-delivering hemoglobin to prevent hypoxia. Similarly, as in our aforementioned patient with pneumonia, she has increased her cardiac output – as evidenced by her tachycardia – and her tissues are likely not hypoxic despite her hypoxemia.
Conversely, a patient may not be hypoxemic but may have hypoxia. Take, for instance, a patient presenting to the ED with right leg claudication upon walking 50 yards. This patient would be experiencing tissue hypoxia in his right leg despite having a normal level of oxygen in his blood (i.e. not hypoxemic).
Hypoxemia – abnormally low oxygen in the blood.
Hypoxia – inadequate oxygen delivery to the tissue(s).
Hypoxemia vs. hypoxia. N Engl J Med. 1966 Apr 21;274(16):908-9.
Sarkar M, Niranjan N, and Banyal PK. Mechanisms of hypoxemia. Lung India. 2017 Jan-Feb; 34(1): 47–60.
Dr. Krueger (@HoyaBadger) is currently an emergency medicine attending physician at Advocate Lutheran General Hospital, clinical instructor at UIC, and a former UIC EM Chief Resident.