“Never in my life have I had to ask a patient to get off the telephone because it was time to put in a breathing tube,” said Dr. Richard Levitan, who recently spent 10 days at Bellevue Hospital Center in Manhattan.There are two simple explanations for the observed phenomenon of silent hypoxia: first is that hypoxia is not nearly as potent a stimulus of respiration as is hypercarbia, as I took great pains to explain in the Applied Physiology lectures, using the example of shallow water breath hold divers who do not pre-oxygenate and emerge with sats on the order of 50%, the same as climbers on Mt Everest. (See also this post on the boy in the wheel well, unpressurized, from California to Hawaii.) I thank Matt Wong (@EM_phile on Twitter) for pointing me to this video which provides a striking illustration of lack of dyspnea with sudden and severe hypoxemia. This video is compulsory viewing:
Dyspnea is driven far more by ventilation demands and the work of breathing than it is by hypoxia alone. This is underrecognized. I wager this stems in part from the fact that in most diseases that cause hypoxia, there is significantly elevated work of breathing because of parenchymal disease/infiltration (elastic loads) and airway secretions (resistive loads), combined with minute ventilation loads from high CO2 production, dead space ventilation (Vd/Vt) and metabolic acidosis. If COVID does not have all of those loads, patients may not experience substantial subjective dyspnea despite low oxygen.