BACKGROUND

There has been an exponential increase in the prevalence of obesity worldwide.(1) Consequently, there has been an increase in the number of obese individuals referred for pulmonary function tests (PFTs) prior to bariatric surgery, for example, as well as because of chronic wheezing, chronic breathlessness, and multiple comorbidities potentially explaining disproportionate dyspnea.(2) The pulmonologist in charge of interpreting the results of a PFT, as well as the physician requesting the test, should be acquainted with the peculiar effects that obesity has on lung function.

OVERVIEW

A 72-year-old male-smoking history = 32 pack-years; height = 159 cm; and body mass index (BMI) = 48.2 kg/m2-was referred for a full PFT in a tertiary health care facility due to worsening dyspnea despite maximal therapy for suspected COPD. Office spirometry was, according to the referral note, "unremarkable". In fact, spirometry, whole-body plethysmography, and DLCO were all within the normal range. However, because the patient experienced severe dyspnea and distress after the tests, he was referred to the emergency department by the respiratory therapist. Upon arrival in the emergency department, the patient had a respiratory arrest. After endotracheal intubation, chest CT angiography showed bilateral massive pulmonary thromboembolism, as well as severe emphysema and diffuse airway plugging. After a prolonged stay in the ICU, the patient eventually died of ventilator-associated pneumonia. How could such dramatic, life-threatening abnormalities be missed by the PFTs?

Obesity may increase the expiratory flows due to increased lung/chest wall elastic recoil. FVC may underestimate slow VC because FVC is precociously "amputated" by early small airway closure in the forced maneuver, i.e., the FEV1/FVC ratio tends to increase. (3) Although functional residual capacity decreases in comparison with that in the earlier stages of obesity,(4) volume "extremities"-RV and TLC-are only mildly affected (unless obesity is massive). It follows that expiratory reserve volume decreases and inspiratory capacity increases in tandem with BMI.(4) These changes are in opposite direction to those caused by obstruction with air trapping, leading to underestimation of or a false negative for airway disease. DLCO increases for a given alveolar volume (VA) because lung perfusion and intrathoracic blood volume increase; moreover, VA decreases more than does DLCO as the lung deflates. Therefore, carbon monoxide transfer coefficient (KCO = DLCO/VA) increases exponentially as VA decreases. (5) Consequently, signs of impaired gas exchange efficiency (low DLCO and KCO) might be obscured. A short height and abdominal obesity, as in our patient, tend to potentiate these effects of obesity. Chart 1 provides a non-exhaustive list of the most common pitfalls in the interpretation of PFTs in obese patients.
 



CLINICAL MESSAGE

BMI must be available in every PFT report: it is the third variable to look at (after age and sex) before any attempt to interpret the tests. This case illustrates that PFTs in obese patients can be relatively unaltered even in the presence of life-threatening conditions in the airways, lung parenchyma, or both. Special caution is advisable if little is known about the pre-test probability of abnormality (as is frequently the case). The final report should acknowledge these "shades of gray" rather than giving a rigid dichotomous "verdict": recognizing uncertainty always meets the best interests of the patient.

REFERENCES

1. Finkelstein EA, Khavjou OA, Thompson H, Trogdon JG, Pan L, Sherryet B, et al. Obesity and severe obesity forecasts through 2030. Am J Prev Med. 2012;42(6):563-570. https://doi.org/10.1016/j.amepre.2011.10.026
2. O'Donnell DE, Milne KM, Vincent SG, Neder JA. Unraveling the Causes of Unexplained Dyspnea: The Value of Exercise Testing. Clin Chest Med. 2019;40(2):471-499. https://doi.org/10.1016/j.ccm.2019.02.014
3. Saint-Pierre M, Ladha J, Berton DC, Reimao G, Castelli G, Marillier M, et al. Is the Slow Vital Capacity Clinically Useful to Uncover Airflow Limitation in Subjects With Preserved FEV1/FVC Ratio? Chest. 2019;156(3):497-506. https://doi.org/10.1016/j.chest.2019.02.001
4. O'Donnell DE, Deesomchok A, Lam YM, Guenette JA, Amornputtisathaporn N, Forkert L, et al. Effects of BMI on static lung volumes in patients with airway obstruction. Chest. 2011;140(2):461-468. https://doi.org/10.1378/chest.10-2582
5. Neder JA, Berton DC, Muller PT, O'Donnell DE. Incorporating Lung Diffusing Capacity for Carbon Monoxide in Clinical Decision Making in Chest Medicine. Clin Chest Med. 2019;40(2):285-305. https://doi.org/10.1016/j.ccm.2019.02.005