The Natural History of a Patient With COVID-19 Pneumonia and Silent Hypoxemia

The severity of hypoxemia manifested by this patient may have elicited additional findings of respiratory distress, such as dyspnea and tachypnea. However, in patients with severe COVID-19 pneumonia, dyspnea was not a universal finding, reported in the 20 to 60% range of cohorts, higher in those with ARDS and mechanical ventilation, although some report near universal dyspnea in their series.^1,4,8,22,23 Tachypnea is another symptom of interest. Using a threshold of > 24 breaths/min, tachypnea was noted in 16 to 29% of patients with a much greater proportion (63%) in nonsurvivors.^6,24 Several explanations have been proposed for the discordance between the presence and severity of hypoxemia and lack of symptoms of dyspnea and tachypnea. It is important to recognize that misclassification of the severity of hypoxemia can occur due to technical issues and potential errors involving pulse oximetry measurement and shifts in the oxyhemoglobin dissociation curve. However, this is more pertinent for those with mild disease as the severity of hypoxemia in severe pneumonia is beyond what can be attributed to technical issues.

More important, the ventilatory response curve to hypoxemia may not be normal for some patients, blunted by as much as 50% in older patients, especially in those with diabetes mellitus.^7,25,26 In addition, the ventilatory response varies widely even among normal individuals. This would translate to lower levels of minute ventilation (less tachypnea or respiratory effort) with hypoxemia. Hypocapnic hypoxemia also blunts the ventilatory response to hypoxemia. Subjects do not increase their minute ventilation if the PaCO₂ remains low despite oxygen desaturation to < 70%, especially if PaCO₂ < 30 mm Hg or alternatively, increases in minute ventilation are not seen until the PaCO₂ exceeds 39 mm Hg.²⁷ Both scenarios occur in those with COVID-19 pneumonia and provide another explanation for the absence of respiratory symptoms or signs of respiratory distress in some patients.

The observation of more compliant lungs may help in the understanding of the variable presentation of these patients. Compliant lungs do not require the increased pressure needed to achieve a specific tidal volume that, in turn, may increase the work of breathing. This may add to the explanation of seemingly paradoxical silent hypoxemia in those patients where the combination of a blunted ventilatory response, hypocapnia, shunt physiology, and normal respiratory system compliance is represented by the absence of increased breathing effort despite severe hypoxemia.

If not for the patient’s refusal of medical services, this patient quite possibly would have been intubated due to hypoxemia and health care providers’ concern for her risk of deterioration. Reported intubation and mechanical ventilation rates have varied widely from extremes of from < 5 to 88% in severely ill patients.^9,22 About 75% will need oxygen, but many can be treated and recover without the need for intubation and mechanical ventilation.

As previously mentioned, options for treatment include standard and high-flow oxygen delivery, noninvasive ventilation, and awake prone ventilation. Their role in patient management has been recently outlined, and instead of an early intubation strategy, represents gradual escalation of support that may be sufficient to treat hypoxemia and avoid the need for intubation and mechanical ventilation (Table 2).

In addition, the patient’s hospital course was notable for the decline in known markers of active inflammation that mirrored the resolution of her hypoxemia and pneumonia. This included elevated lactate dehydrogenase, D-dimer, ferritin, and C-reactive protein with all but the latter rising and decreasing over 2 weeks. These findings provide additional information of the time for recovery and supports the use of these markers to monitor the course of pneumonia.

The patient declined all intervention, including oxygen, and recovered to her presumed prehospitalization condition. This experiment of nature due to unique circumstances may shed light on the natural time course of untreated hypoxemic COVID-19 pneumonia that has not previously been well appreciated. It is important to recognize that recovery occurred over 2 weeks. This is close to the observed and expected time for recovery that has been reported for those with severe COVID-19 pneumonia.

Conclusions

Since the emergence of the COVID-19, evidence has accumulated for the benefit of several adjunctive therapies in the treatment of this type of pneumonia, with corticosteroids providing a mortality benefit. Although unknown whether this patient’s experience can be generalized to others or whether it represents her unique response, this case provides another perspective for comparison of treatments and reinforces the need for prospective, randomized clinical trials to establish treatment efficacy. The exact nature of silent hypoxemia of COVID-19 remains incompletely understood; however, this case highlights the importance of treating the individual instead of clinical markers and provides a time course for recovery from pneumonia and severe hypoxemia that occurs without oxygen or any other treatment over about 2 weeks.