Hyperventilation as one of the mechanisms of persistent dyspnoea in SARS-CoV-2 survivors

To the Editor:

There are increasing reports of persistent dyspnoea several months after the onset of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection [1]. In most cases, functional disability seems out of proportion compared to residual pulmonary function impairment [2]. To date, knowledge about the functional limitations following a SARS-CoV-2 infection remains limited.

The aim of our study was to evaluate pulmonary, cardiac and functional capacity of SARS-CoV-2 survivors by performing cardiopulmonary exercise testing (CPET). We performed a prospective single-centre study of 114 consecutive patients at 3±1 months after the initial diagnosis of SARS-CoV-2. All patients underwent pulmonary function tests, chest computed tomography (CT) and trans-thoracic echocardiogram, as well as symptom-limited CPET. Non-opposition was obtained for all patients, according to French law.

Incremental exercise testing was performed using cycle ergometer (Ergometrics 800S; GE). The peak oxygen uptake (peak V′_O2), minute ventilation (V′_E) and heart rate were selected as the highest values obtained from any 30-s measurement period. The ventilatory equivalent for carbon dioxide (V′_E/V′_CO2) slope was determined using linear regression analysis of V′_E and V′_CO2 obtained throughout the exercise period. The Wasserman equation was used to determine the normal predicted peak O₂ value [3], in accordance with recent recommendations [4].

The approach to interpretation of the main exercise limitation was multifactorial and in accordance with current guidelines [4, 5]. Cardiovascular limitation at exercise was defined when one or more of the following criteria were present: evidence of left ventricular systolic or diastolic dysfunction, reduced exercise capacity with low peak V′_O2 (<85% of predicted value), reduced oxygen pulse with a flattening curve during incremental exercise, chronotropic insufficiency or reduced peak circulatory power. Respiratory limitation was defined when one or more of the following criteria were present: 1) a significant reduction of pulmonary function at rest, i.e. forced expiratory volume in 1 s <70% of predicted and/or diffusing capacity of the lung for carbon monoxide (D_LCO) <70%; 2) breathing reserve (defined as peak V′_E/maximum voluntary ventilation <30%); or 3) hypoxaemia (peripheral capillary oxygen saturation (S_pO2) ≤88%) during exercise. Peripheral limitation was defined as a reduced peak V′_O2 with a premature anaerobic threshold below 40% of predicted in the absence of pulmonary or cardiac limitation. Inadequate exercise hyperventilation was defined as V′_E/V′_CO2 slope >40, increased ventilatory equivalents for CO₂ and O₂, as well as higher V′_E at anaerobic threshold in the absence of clear pulmonary or cardiac limitation.

The baseline characteristics of enrolled patients are reported in table 1. The median (range) age was 57 (48–66) years and one third were women. 12% of patients had a chronic pulmonary disease, such as COPD or asthma. Most patients (91%) required in-hospital treatment at the onset of the SARS-CoV-2, with 18% of the patients having required mechanical ventilation.

At 3-month follow-up, half of all patients were still symptomatic, dyspnoea (40%) and fatigue (32%) being the most common symptoms. 40% of patients had altered diffusion capacity, which was mild or moderate in all cases (D_LCO >40% of predicted value [6]). Mild to moderate residual lung involvement on chest CT was found in 65% of patients. Trans-thoracic echocardiogram showed a normal left ventricular ejection fraction in the majority of the patients as well as a preserved global longitudinal strain. Pulmonary hypertension (systolic pulmonary artery pressure (sPAP) >40 mmHg) was detected in one patient.

During CPET, 75% of patients had exercise impairment with peak V′_O2 values <85% of predicted. The median (range) peak V′_O2 was 17.9 (14.7–20.6) mL·kg⁻¹·min⁻¹. Peripheral deconditioning was the main limiting factor in 43% of patients. 24% of patients had an elevated V′_E/V′_CO2 ratio at peak exercise (>40) and 32% had an elevated V′_E/V′_CO2 slope (>35). Exercise hyperventilation was the main limitation in 16% of patients. After adjustment for covariates, age (β=0.4, p=0.002), intensive care stay (β=−10.27, p=0.017), mechanical ventilation (β=−12.63, p=0.004) and length of hospital stay (β=−0.24, p=0.009) were independently associated with % predicted peak V′_O2.

Patients who had altered diffusion capacity (D_LCO ≤75%) reached a lower maximal load and had lower peak V′_O2 values at anaerobic threshold as well as at peak exercise. However, the prevalence of hyperventilation was similar in both groups. Interestingly, patients with reduced D_LCO did not report more persistent dyspnoea.

The prevalence of exercise limitation in our cohort was much higher than in a study by Rinaldo et al. [7]. In accordance with a previous report [7], reduced exercise capacity in our cohort was mostly due to peripheral deconditioning. However, our study also found an elevated V′_E/V′_CO2 slope in one third of the study participants, suggesting a high incidence of inadequate exercise hyperventilation. Our findings are supported by two previous studies [8, 9] which reported persistent ventilatory inefficiency in smaller cohorts of SARS-CoV-2 survivors who underwent CPET. Increased V′_E/V′_CO2 slope and reduced diffusion capacity might suggest pulmonary hypertension at exercise [10]. However, in our cohort only one patient had an elevated sPAP at rest. What is more, there was no statistically significant difference in V′_E/V′_CO2 according to D_LCO subgroups.

Exercise hyperventilation is a condition characterised by alveolar hyperventilation that is inappropriate considering metabolic needs and mechanical stress in the body [11]. The origin of this hyperventilation is unknown but may be related to an abnormality of central ventilatory control in the aftermath of pulmonary infection [12]. Increased output of the respiratory centre results in respiratory alkalosis [12], which in turn activates the autonomous nervous system, causing a variety of neurovegetative symptoms as well as arterial vasoconstriction and hypoperfusion [13]. The diagnosis of hyperventilation syndrome is usually established after exclusion of other cardiopulmonary diseases, such as heart failure, asthma or COPD. CPET is useful for the differential diagnosis of dyspnoea at exertion. Markers of hyperventilation include increased V′_E/V′_CO2 ratio and slope during exercise [14, 15], rapid increase in ventilatory equivalents for V′_O2 and V′_CO2, abnormally high respiratory rate, and higher V′_E at anaerobic threshold [4]. Hyperventilation-related symptoms can range from dyspnoea, palpitations and chest pain, to dizziness and fatigue, which have been reported by the so-called “Covid long-haulers” [1]. Identification of hyperventilation syndrome is important, because the variety of disabling symptoms might take SARS-CoV-2 survivors to a wide range of specialist consultations, numerous investigations and inappropriate treatment, whereas respiratory physiotherapy with an experienced therapist with a focus on patient education can help significantly. Spontaneous recovery is also possible.

Our study has several limitations. Firstly, several parameters that could be of interest, such as end-tidal carbon dioxide tension and individual S_pO2 data, were unavailable for statistical analysis. However, none of the patients desaturated at exercise. Moreover, the limitations of reference values for CPET must be taken into account, as % of predicted values are less accurate for obese patients. However, the reference values that were used are in accordance with international guidelines. Another limitation of the study is the absence of arterial blood gas measurement, which could confirm the presence of respiratory alkalosis and thus further strengthen the diagnosis of hyperventilation syndrome. The noninvasive assessment of pulmonary gas exchange during exercise is less reliable and is known to produce higher values of ventilatory equivalents in patients without significant lung disease.

In conclusion, our study confirmed that peripheral deconditioning is the main mechanism of exercise intolerance in the aftermath of SARS-CoV-2. However, exercise hyperventilation should not be overlooked while exploring the causes of dyspnoea in SARS-CoV-2 survivors.

Shareable PDF