Silent Hypoxemia of COVID-19 Pneumonia: Typical Courses With Implications for Outpatient Care
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In some cases of COVID-19, profound life-threatening hypoxia has been observed without the patient perceiving symptoms. This phenomenon, referred to as silent hypoxemia, is not unknown in other settings, but was first described in the context of COVID-19 (1, 2) and occurred in our COVID-19 patient population with increased frequency. Some patients initially present with mild symptoms, but begin to show signs of decompensation after some hours. There is still a lack of systematic studies; the pathophysiology is only partly understood. Consequently, diagnostic interpretation and outpatient care are challenging; the absence of symptom-related distress increases the risk that severe courses of the disease are not recognized. Thus, performing only a brief clinical examination may lead to complications later in the course of the disease.
The pilot project “Medical Emergency Team” (MET, “Medizinisches Einsatzteam”) of the Municipal Fire Brigade Cologne (“Berufsfeuerwehr Köln”) was established for medical interventions in patients with COVID-19. The MET consists of experienced emergency physicians. If the routine follow-up by the health authorities detects a crisis situation based on the patient’s risk profile and the deterioration of symptoms, the MET intervenes on site. This case series describes typical clinical courses of silent hypoxemia in five patients (Table), using outpatient and clinical data, and contributes to risk stratification in COVID-19 patients. The clinical course of patient 1 is described in detail below as an example.
During the daily phone contact, patient 1 (Table) reported relative well-being. The patient had no history of pre-existing cardiopulmonary disease and was not on long-term medication. As an isolated risk factor, moderate nicotine abuse was noted (15 cigarettes/d). By the second week of illness, the patient started to report dry cough, sleep problems and occasional headache. Dyspnea at rest or on exertion was negated and the patient declined to be contacted by a physician. On day 10, the patient was first visited by the MET. He reported mild subjective impairment, felt only mildly ill and experienced mild dyspnea on exertion; auscultation of the lungs was unremarkable. Besides anosmia, a cardinal symptom of COVID-19, lung ultrasound showed early B-lines in the areas 4 and 6, bilaterally, indicating increased accumulation of fluid in pleural alveoli. On day 12, the patient presented in mildly reduced general condition without increase in dyspnea. Lung ultrasound detected increased numbers of partly confluent B-lines in posterior views. Given the deterioration of breathing rate and oxygen saturation (SpO2) and the progressive development of B-lines, hospital admission was recommended. However, the fully oriented patient declined to be admitted to hospital, even after discussion of the risks. On day 13, he presented in unchanged general condition and subjective wellbeing with mild dyspnea on exertion. Lung ultrasound showed progressive, confluent, prominent B-lines in the dorsobasal lung regions with pleura thickening and pleural fragmentation. Because of this deterioration, in-patient treatment could no longer be avoided; however, the patient was not able to understand this, because, subjectively, he was feeling well. During ambulance transfer to hospital, the patient received oxygen. On admission (day 13, 1 pm), increased levels of lactate dehydrogenase, ferritin, D-dimer, and C-reactive protein (CRP) were noted together with lymphocotopenia. Chest radiography showed basal infiltrates typical of COVID-19 infection. Over the course of the day, SpO2 levels dropped below 85 % (4 L/min O2) and the patient was transferred to the intensive care unit where he soon required intubation for respiratory decompensation. Thoracic computed tomography (day 14) revealed marked bilateral pulmonary infiltrates and consolidations. Because of persistent hypoxemia and a Horovitz index <150, intermittent prone positioning of the patient had to be started on day 16. The patient underwent tracheostomy because of prolonged mechanical ventilation on day 21. Subsequent weaning was uncomplicated; on day 29, the tube was removed and on day 30 the patient was transferred back to the regular ward. On day 43, he was discharged in good general condition.
Because of the associated risk of dynamic deterioration and peracute respiratory decompensation, silent hypoxemia is a critical factor in the outpatient care of patients with COVID-19. Characteristically, the patients subjectively feel symptom-free without shortness of breath—despite, in some cases, profound hypoxemia. The sensation of dyspnea is pathophysiologically complex and multifactorial. Besides chemoreception (hypercapnia, hypoxia), mechanical afferent signals of stretch-sensitive mechanoreceptors in the respiratory muscles play on important role. In COVID-19 patients, a feeling of relative well-being, an unremarkable general condition and the absence of symptom-related distress increase the risk that a life-threatening situation is misjudged by health professionals. In our experience, established score systems designed to predict the prognosis of patients with community-acquired pneumonia (CURB-65) are insufficient for estimating the risk in patients with silent hypoxemia. Immediate clinical assessment of cognitive impairment as an evaluation parameter is frequently not feasible. It remains unclear whether the absence of perceived symptoms is the result of a specific pathophysiology of the SARS-CoV-2 infection or the manifestation of a hypoxia-induced alteration of cognitive abilities (1, 2). Currently, pathophysiological similarities between acute mountain sickness (AMS) and silent hypoxemia are being postulated (3, 4). Applied to silent hypoxemia, the cognitive impairment of the patient’s ability to judge in a way that is adequate to the situation should be assessed in a structured fashion, e.g. using the Mini-Mental State Examination (MMSE). In this way, cognitive alterations of the patient and their effect on the subjective evaluation of the need for treatment can be assessed. However, a normal MMSE score does not necessarily mean that the ability to judge is intact. The MET project requires in high-risk patients (pre-existing conditions, age, BMI) the assessment of the general condition (dyspnea at rest and on exertion, body temperature), oxygen saturation, respiratory rate and breathing mechanics by a physician as well as an additional lung ultrasound. In our patient sample, lung ultrasound enabled the early detection of finding patterns indicative of a critical deterioration before these have manifested as an objectively measurable vital sign deterioration, as described by Vetrugno et al. (5). Ultrasound findings are certainly dependent on the skill of the examiner; however, in our experience physicians can achieve reproducible findings already after a brief training. In the MET project, the patient sample was characterized with regard to predictive clinical, patient history and biometrical data to be able to better adapt the outpatient diagnostic assessment to the individual risk constellation of the patient.
Conflict of interest
The authors declare no conflict of interest.
received on 30 September 2020; revised version accepted on 15 December 2020
Translated from the original German by Ralf Thoene, MD.
Cite this as:
Miller C, Stangl R, Adler C, Strohm M, Bernardo C, Lechleuthner A, Viethen A: Silent hypoxemia of COVID-19 pneumonia—typical courses with implications for outpatient care. Dtsch Arztebl Int 2021; 118: 8–9. DOI: 10.3238/arztebl.m2021.0115
Professional Fire Brigade Cologne, Institute for Security Science and Rescue Technology (Miller, Stangl, Adler, Strohm, Bernardo, Lechleuthner, Viethen), email@example.com Department III of Internal Medicine, Heart Center of the University of Cologne (Adler) Evangelisches Krankenhaus Kalk gGmbH, Center of Internal Medicine (Bernardo)
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