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

Over the next 2 weeks, her hypoxemia, inflammatory markers, and the infiltrates on imaging resolved (Figure 2). The lowest daily awake room air pulse oximetry readings are reported, initially with consistent readings in the low 80% range, but on day 12, readings were > 90% and remained > 90% for the remainder of her hospitalization. Therefore, shortly after hospital day 12, she was clinically stable for discharge from acute care to a subacute facility, but this required documentation of the clearance of her viral infection. She refused to undergo a subsequent nasopharyngeal swab but allowed an oropharyngeal COVID-19 PCR swab, which was negative. She remained stable and unchanged for the remainder of her hospitalization, awaiting identification of a receiving facility and was able to be discharged to transitional housing on day 38.

Discussion

The initial reports of COVID-19 pneumonia focused on ARDS and respiratory failure requiring mechanical ventilation with less emphasis on those with lower severity of illness. This was heightened by health care systems that were overwhelmed with large number of patients while faced with limited supplies and equipment. Given the risk to patients and providers of crash intubations, some recommended early intubation strategies.³ However, the natural history of COVID-19 pneumonia and the threshold for intubation of these patients remain poorly defined despite the creation of prognostic tools.¹⁷ This patient’s persistent hypoxemia and elevated inflammatory markers certainly met markers of disease associated with a high risk of progression.

The greatest concern would have been her level of hypoxemia. Acceptable thresholds of hypoxemia vary, but general consensus would classify pulse oximetry < 90% as hypoxemia and a threshold for administering supplemental oxygen. It is important to recognize how pulse oximetry readings translate to partial pressure of oxygen (PaO₂) measurements (Table 1). Pulse oximetry readings of 90% corresponds to a PaO₂ readings of 60 mm Hg in ideal conditions without the influence of acidosis, PaCO₂, or temperature. While lower readings are of concern, these do not represent absolute indications for assisted ventilatory support as lower levels are well tolerated in a variety of conditions. A common example are patients with chronic obstructive pulmonary disease. Long-term mortality benefits of continuous supplemental oxygen are well established in specific populations, but the threshold for correction in the acute setting remains a case-by-case decision. This decision is complex and is based on more than an absolute number or the amount of oxygen required to achieve a threshold level of oxygenation.

The PaO₂/FIO₂ (fraction of inspired oxygen) is a common measure used to address severity of disease and oxygen requirements. It also has been used to define the severity of ARDS, but the ratio is based on intubated and mechanically ventilated patients and may not translate well to those not on assisted ventilation. Treatment with supplemental oxygen also involves entrained air with associated imprecision in oxygen delivery.¹⁸ For this discussion, the patient’s admission PaO₂/FIO₂ on room air would have been between 190 and 260. Coupled with the bilateral infiltrates on imaging, there was justified concern for progression to severe ARDS. Her presentation would have met most of the epidemiologic criteria used in initial case finding for severe COVID-19 cases, including a blood oxygen saturation ≤ 93%, PaO₂/FIO₂ < 300 with infiltrates involving close to if not exceeding 50% of the lung.

With COVID-19 pneumonia, the pathologic injury to the alveoli resembles that of any viral pneumonia with recruitment of predominantly lymphocytic inflammatory cells that fill the alveoli, derangements in ventilation/perfusion mismatch as the core mechanism of hypoxemia with interstitial edema and shuntlike physiology developing at the extremes of involvement. In later stages, the histologic appearance is similar to ARDS, including hyaline membrane formation and thickened alveolar septa with perivascular lymphocytic-plasmocytic infiltration. In addition, there also are findings of organizing pneumonia with fibroblastic proliferation, thrombosis, and diffuse alveolar damage, a constellation of findings similar to that seen in the latter stages of ARDS.²

Although these histologic findings resemble ARDS, many patients with respiratory failure due to COVID-19 have a different physiologic profile compared with those with typical ARDS, with the most striking finding of lungs with low elastance or high compliance. From the critical care standpoint, this meant that the lungs were relatively easy to ventilate with lower peak airway and plateau pressures and low driving pressures. This condition suggested that there was relatively less lung that could be recruited with positive end expiratory pressure; therefore, a somewhat different entity from that associated with ARDS.¹⁹ These findings were often noted early in the course of respiratory failure, and although there is debate about whether this represents a different phenotype or timepoint in the spectrum of disease, it clearly represents a subset that is distinct from that which had been previously encountered.

On the other hand, the clinical features seen in those patients with COVID-19 pneumonia who progressed to advanced respiratory failure were essentially indistinguishable from those patients with traditional ARDS. Other explanations for this respiratory failure have included a disrupted vasoregulatory response to hypoxemia with failed hypoxic vasoconstriction, intravascular microthrombi, and impaired diffusion, all contributing to impaired gas exchange and hypoxemia.^19-21 This can lead to shuntlike conditions that neither respond well to supplemental oxygen nor manifest the type of physiologic response seen with other causes of hypoxemia.