Diffusion capacity for carbon monoxide in patients recovered from COVID-19: A useful parameter to assess pulmonary function.

Dear Editor,

Diffusion capacity of the lungs for carbon monoxide (DLCO), in combination with spirometry and lung volumes, is indicated to evaluate the parenchymal and non-parenchymal lung diseases. The preoperative DLCO is not routinely measured in patients in most cardiac surgery units. DLCO is used to assess the severity of obstructive and restrictive lung diseases, pulmonary vascular disease, and preoperative risk. The patients surviving coronavirus disease-2019 (COVID-19) are frequently reported to have pulmonary sequelae.[1]

We recently encountered five patients admitted for cardiac surgery, who had contracted COVID-19 about 6–8 months ago. Preoperative assessment of the pulmonary function included chest X-ray, high-resolution computed tomography of the chest, arterial blood gas analysis at room air, and spirometry with DLCO measurement. The DLCO was measured by SPIRO AIR® volumetric PFT (pulmonary function test) System (Medisoft, Sorinnes, Belgium) using a single-breath technique. The patients’ details and PFT values are shown in Table 1. There was severe impairment of DLCO and alveolar volume (Va) values (37% and 45% of predicted, respectively) in patient number one. The patient was referred for pulmonary rehabilitation and optimization of medical therapy, and was asked for a follow-up at 3 months. The remaining four patients with normal DLCO values underwent cardiac surgery uneventfully.

Table 1

Patient Characteristics and Pulmonary Function Test Values

	Patient 1	Patient 2	Patient 3	Patient 4	Patient 5
Age	62	68	51	63	57
Gender	Male	Male	Male	Female	Male
Height (m)	1.71	1.66	1.75	1.52	1.64
BSA (m2)	2.07	1.74	2.04	1.78	1.74
BMI (kg/m2)	30.8	24.0	28.1	32.5	25.3
H/o Smoking	No	Yes	No	No	No
Diagnosis	Severe MR	CAD	Severe MS	Severe MR, Severe TR	CAD
COVID-19 Positive	Sept 2020	Aug 2020	Oct 2020	Sept 2020	Sept 2020
HRCT Chest	Subpleural GGO, Traction- bronchiectasis	Thickening of interlobular septae	Fibroatelectatic bands in RML	Thin atelectatic bands (both lungs)	GGO, Prominent interlobular septae (upper lobes, RML)
PaCO2, PaO2 (room air) mmHg	45.8, 60.5	34.5, 96.8	36.2, 79.0	31.7, 95.1	33.3, 79.7
6 MWT (m)	Not performed	504	483	379	558
SpO2% (Pre, Post 6MWT)	91/--	98/96	95/94	97/94	96/94
Spirometry
FVC (L, % pred)	1.93 (55)	3.24 (105)	4.06 (102)	2.34 (118)	2.75 (85)
FEV1 (L, % pred)	1.66 (60)	2.45 (99)	3.24 (102)	1.89 (115)	2.35 (90)
FEV1/FVC Ratio (%, % pred)	85.72 (113)	73.38 (98)	79.95 (103)	80.41 (104)	85.26 (111)
PEF (L/s, % pred)	7.11 (89)	8.59 (116)	7.72 (89)	4.63 (86)	6.81 (88)
MEF (L/s, % pred)	2.69 (80)	2.59 (87)	3.27 (84)	2.17 (81)	3.13 (92)
Lung Volumes
He-VC (L)	3.64	3.2	4.12	2.94	3.36
He-FRC (L)	3.47	3.41	3.47	2.47	3.27
He-RV (L)	2.37	2.45	2.20	1.76	2.19
He-TLC (L)	5.92	5.57	6.21	3.82	5.42
RV/TLC (%)	38.1	40.7	34.1	40.4	36.5
Diffusion Test
DLCO (ml/mmHg/min, % pred)	9.67 (37)	21.80 (93)	26.47 (89)	15.10 (77)	20.85 (75)
Va (L, % pred)	2.94 (45)	5.64 (91)	5.62 (81)	3.06 (72)	5.14 (76)
Kco (ml/mmHg/min/L, % pred)	3.29 (82)	3.87 (102)	4.71 (109)	4.94 (106)	4.05 (98)
BHT (%)	10.5	10.5	10.7	11.8	10.83

BSA: body surface area, BMI: body mass index, MR: mitral regurgitation, CAD: coronary artery disease, MS: mitral stenosis, TR: tricuspid regurgitation, HRCT: high resolution computed tomography, GGO: ground-glass opacities, RML: right middle lobe, 6 MWT: six-minute walk test, FVC: forced vital capacity, pred: predicted, FEV1: forced expiratory volume in 1 s, PEF: peak expiratory flow, MEF: maximum expiratory flow, He: Helium, FRC; functional residual capacity, RV: residual volume, TLC: total lung capacity, DLCO: Diffusion capacity for carbon monoxide, Va: alveolar volume, Kco: Transfer coefficient for carbon monoxide, BHT: breath holding time

Recent studies have shown that the lung is the most commonly affected organ by COVID-19.[1] In the initial reports from Wuhan, China, up to one-third of patients with COVID-19 developed severe pneumonia and acute respiratory distress syndrome.[2] The exact mechanism of lung dysfunction in recovered COVID-19 patients remains unknown. Endothelial injury and alveolar-capillary micro-thrombosis have been described as underlying mechanisms of pulmonary vascular disease in these patients. Due to the thickening of the alveolar-capillary membrane, there is reduction in both DLCO and Kco (transfer coefficient) in patients with interstitial lung disease and pulmonary fibrosis.[3] An isolated reduction in DLCO suggests loss of the pulmonary capillary bed from early parenchymal lung disease. In contrast to the spirometric values, DLCO is less affected by the patient’s effort. Unpublished data from a recent study by Qin et al.[4] have shown that 44 (54%) of 81 patients had abnormal DLCO (<80% predicted) after 3 months of COVID-19. They also found that the patients with impaired DLCO had a higher percentage of pulmonary interstitial damage. In rehabilitating patients of severe acute respiratory syndrome (SARS) and the Middle East respiratory syndrome (MERS), impairment of DLCO was the most commonly seen abnormality followed by a restrictive ventilatory defect. A recent prospective, multicenter study has shown that DLCO % predicted at 4 months after COVID-19 was the most important, independent correlate of a more severe initial disease.[5]

In conclusion, our case series reveals that COVID-19 survivors presenting for cardiac surgery can have impaired DLCO. Long-term studies are needed to address whether the assessment of DLCO, in addition to spirometry, should be considered in routine clinical follow-up of COVID-19 survivors.

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Conflicts of interest

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