‘Happy Hypoxia’ Might Not Be Such A Mystery After All in COVID-19 Patients

When patients started showing up at the hospital with dramatically low oxygenation levels but few outward signs of dyspnea, dubbed ‘happy hypoxia,’ treating clinicians were flummoxed. Now, a new study provides some explanations for the phenomenon, with authors saying they hope the new information will keep patients from being unnecessarily intubated and ventilated. Here are more details.

MAYWOOD, IL — Earlier in the COVID-19 pandemic, physicians were baffled by patients who presented with extremely low levels of oxygen, life-threatening in some cases, but showed no obvious difficulty in breathing.

A new study published online by the American Journal of Respiratory and Critical Care Medicine provides a better explanation for silent hypoxemia, which was dubbed “happy hypoxia.” The authors from the Hines Veterans Affairs Hospital and Loyola University of Chicago Stritch School of Medicine suggest their information could help prevent unnecessary intubation and ventilation in patients during the current and expected second wave of coronavirus.

Lead author Martin J. Tobin, MD, an Edward J. Hines Jr. VA Hospital pulmonologist and critical care specialist, and professor at the Stritch School of Medicine, said the condition "is especially bewildering to physicians as it defies basic biology. In some instances, the patient is comfortable and using a phone at a point when the physician is about to insert a breathing (endotracheal) tube and connect the patient to a mechanical ventilator, which, while potentially lifesaving, carries its own set of risks."

The small study, which included 16 COVID-19 patients with very low levels of oxygen – in some cases as low as 50%, compared to normal blood oxygen saturation of between 95 and 100% -- but without dyspnea, determined that "several pathophysiological mechanisms account for most, if not all, cases of silent hypoxemia.

“It is possible that coronavirus has an idiosyncratic action on receptors involved in chemosensitivity to oxygen, but well-established pathophysiological mechanisms can account for most, if not all, cases of silent hypoxemia,” the authors write, adding that those mechanisms include:

• How dyspnea and the respiratory centers respond to low levels of oxygen,
• How prevailing carbon dioxide tensions (PaCO2) blunt the brain’s response to hypoxia,
• The effects of disease and age on control of breathing,
• Inaccuracy of pulse oximetry at low oxygen saturations, and
• Temperature-induced shifts in the oxygen dissociation curve.

“Without knowledge of these mechanisms, physicians caring for hypoxemic patients free of dyspnea are operating in the dark—placing vulnerable COVID-19 patients at considerable risk,” researchers conclude, adding that “features about COVID-19 that physicians find baffling become less strange when viewed in the light of long-established principles of respiratory physiology; an understanding of these mechanisms will enhance patient care if the much-anticipated second wave emerges.”

One focus was the initial assessment of a patient's oxygen level with a pulse oximeter.

"While a pulse oximeter is remarkably accurate when oxygen readings are high, it markedly exaggerates the severity of low levels of oxygen when readings are low," Tobin points out. "Another factor is how the brain responds to low levels of oxygen. As oxygen levels drop in patients with COVID-19, the brain does not respond until oxygen falls to very low levels--at which point a patient typically becomes short of breath.”

Another factor was that low levels of carbon dioxide, which was seen in more than half of the patients, can minimize the effects of extremely low oxygen levels.

"It is also possible that the coronavirus is exerting a peculiar action on how the body senses low levels of oxygen," Tobin suggests, nothing the high numbers of COVID-19 with taste and smell affected.

"This new information may help to avoid unnecessary endotracheal intubation and mechanical ventilation, which presents risks, when the ongoing and much anticipated second wave of COVID-19 emerges," Tobin said.