Objective: Study the effect of body position on spirometry in obese patients as defined by waist to hip ratio (WHR) of greater than 0.85 in women, and 0.9 in men

Background: Spirometry is routinely ordered to work up dyspnea in obesity. The most common abnormality is a restrictive disease pattern. The underlying mechanisms of this pattern are not completely understood. One plausible explanation is diaphragmatic weakness or skeletal muscle weakness. The change in forced vital capacity (FVC) from sitting to supine is a very sensitive and specific test for detecting diaphragmatic weakness. The effect of body position on spirometry in obesity has not been extensively studied, and there are no studies that look at this when obesity is measured by waist to hip ratio. Effect of body position has been studied in normal patients, and it is expected the FVC can decrease as much as 10% when changing from sitting to supine. The investigators do not know what would be considered 'the normal' amount for FVC to decrease by in the obese population, and thus would like to test patients with increased WHR both in sitting and supine position. The investigators also want to do muscle strength test by measuring the maximal inspiratory and expiratory pressures (MIPs and MEPs).

Anticipated results: the investigators anticipate that our study population will replicate the restrictive disease pattern usually seen in obesity. The investigators also anticipate for the FVC to decrease when in the supine position compared to sitting. The amount by which it decreases will likely fall between 10 -25%. The investigators anticipate to not find any abnormalities in MIPs and MEPs in obesity.

The design for this study will be a cross-sectional study. This study will be conducted in the pulmonary function laboratory at Rush University Medical Center. A sample of subjects who meet the inclusion criteria will be selected from the adult population scheduled for CPFT lab appointments. The details of the study will be discussed by the respiratory therapist prior to the end of CPFT study. Information normally gathered in routine CPFTs includes oxygen saturation, height, and weight. Additional measures for subjects who accept the invitation to participate in this study will include measurements of BMI, waist circumference, and hip circumference. Inclusion criteria includes individuals of both genders, age of 18 years or older and individuals with abdominal obesity as defined by WHO as WHR greater than 0.85 in women, and 0.90 in men. Exclusion criteria include patients who cannot understand or comply with the spirometry test, known lung disease, obstructive ventilatory defect on CPFT, chest wall abnormalities/disorders, known neuromuscular disease, pregnant patients or prisoners.

The BMI (kg/m2) will be calculated as weight (in kilograms) divided by square of the height (in meters). Waist-hip ratio (WHR) will be measured in the standing position using a stretch-resistant tape. Waist circumference will be measured at a midpoint between the lowest rib and the middle of the iliac crest. The hip is defined as the maximal circumference around the gluteal muscles below the iliac crests.8,9 These measurements will be obtained by respiratory therapists who have been trained in obtaining WHR measurements using a standardized patient to assure measurement fidelity.

Spirometry measurements of FEV1 and FVC will be made in two testing positions for this study; the patients seated 90o upright (sitting position), and the patient fully supine position (0º horizontal decubitus position). FVC (forced vital capacity) is the volume of air in liters that can be forcibly and maximally exhaled after taking in the deepest breath. FEV1 is the volume of air that can be forcibly exhaled from the lungs in the first second of a forced expiratory maneuver and is reported in liters. Lung volumes will also be measured in the sitting position using either body plethysmography or nitrogen washout method as part of the routine CPFT testing. Maximal inspiratory pressure (MIP) and maximal expiratory pressure (MEP), will also be measured in the sitting position and reported in centimeters of water (cm H2O). These reflect the maximum pressures generated by the patient on inhalation and exhalation, respectively and test respiratory muscle strength. All tests will be conducted in accordance with 2005 American Thoracic Society/European Respiratory Society guidelines13 using the Sensormedics Vmax pulmonary function system. The Vmax system is calibrated daily and reports all measurements at body temperature, pressure and water vapor. All tests will be performed by a team of respiratory therapists who have demonstrated annual competence following the recommendations of the American Thoracic Society/European Respiratory Society.14

Individuals who meet the study criteria and are able to successfully complete a CPFT will be recruited to participate in the study. After completing a consent form, the waist and hip measurements will be made. MIP and MEP measures will be made in the sitting position. Testing of spirometry will be repeated in the supine position. Sitting FEV1, FEV1 % predicted, FVC, FVC % predicted, FEV1/SVC, lower limit of normal for FEV1/FVC, supine FEV1, FEV1 % predicted, FVC, FVC % predicted, MIP, MEP, ERV, RV, FRC and TLC will be extracted from the pulmonary function test results and entered in REDCap for data tracking. Age, race, gender, and diagnosis code will be taken from the information routinely gathered for a pulmonary function test. The question related to smoking history will be asked directly of the subject. All data will be entered into REDCap.

Procedures done for research purposes and procedures done for routine clinical management:

A routine complete pulmonary function test (CPFT) as ordered by their referring provider includes measurements of slow vital capacity by spirometry, forced vital capacity by spirometry, lung diffusion by DLCO, and measurement of lung volume by body plethysmography. All of these measurements will be performed while the patient in sitting position. The additional testing for this study includes spirometry while the patient is in the supine position; and MIPs, and MEPs in the sitting position.

Inclusion Criteria:

• Individuals of both genders, age of 18 years or older
• Individuals with abdominal obesity with WHR > 0.85 in women, > 0.9 in men
• Ability of patients to transfer themselves into a cardiac chair
• Able to understand and comply with testing instructions

Exclusion Criteria:

• Patients who are less than 18 years old
• Patients who are unable to perform an acceptable and repeatable forced vital capacity
• Airflow limitation as evidenced by sitting FEV1/VC < lower limit of normal
• Patients who have a WHR < 0.85 in women, or < 0.9 in men
• Patients who become lightheaded during sitting spirometry
• Patients who cannot transfer themselves independently to a cardiac chair
• History of lung disease (known obstructive or restrictive lung disease)
• Chest wall abnormalities or kyphoscoliosis
• Neuromuscular disease
• Active hemoptysis or recent angina

The target population is 30 medically stable obese patients with WHR > 0.85 in women, or > 0.9 in men, coming to outpatient clinic at Rush University Medical Center for complete pulmonary function testing.