Obesity affects blood gas results

Increasing obesity is associated with worsening arterial blood gases (pO₂(a) and pCO₂(a)). That is the headline finding of a recent cross-sectional study of morbidly obese individuals with apparently normal lung function and no respiratory disease. 

The study population comprised 114 women and 35 men with a mean (SD) age of 43 (11) years and mean (SD) body mass index (BMI) of 45 (6.3) kg/m². (Healthy body weight defined as BMI 18.5-29.9 kg/m²; overweight as BMI 25-29.9 kg/m² and obesity as BMI >30 kg/m².) 

Although all study participants were suffering at least one obesity-related illness, none had a respiratory illness; indeed, a qualification for entry to the study was normal lung function. Arterial blood was sampled for blood gas analysis from each study participant. 

In addition to height and weight measurements necessary for BMI calculation, three further obesity-related anthropometrics were recorded for each subject: neck circumference (NC), waist circumference (WC) and hip circumference (HC). Waist: hip ratio (WC divided by HC) was calculated. Study participants were submitted for a battery of respiratory-function (spirometry, lung volume) tests. 

All were normal with the exception of expiratory reserve volume (ERV), which was on average 49 % of the predicted value. (ERV is the extra volume of air that can be forcibly blown out at the end of normal tidal expiration).

Individual pO₂(a) ranged from 7.7 kPa (58 mmHg) to 15.3 kPa (115 mmHg) and pCO₂(a) ranged from 3.6 kPa (27 mmHg) to 6.5 kPa (49 mmHg). Forty-two of the 149 study participants (28 %) had abnormally low pO₂(a) (<10.7 kPa [80 mmHg]) and 8 (5 %) had abnormally high pCO₂(a) (> 6.0 kPa [45 mmHg]). 

All anthropometric measures (BMI, NC, WC, HC, WC:HC) correlated significantly (p<0.001 for all correlations) with decreasing pO₂(a) and increasing pCO₂(a). Multiple linear regression analysis showed that increasing BMI, NC and WC were significantly (p<0.001) associated with decreasing pO₂(a) after adjustment for age, gender and cigarette consumption (pack years). 

The same statistical methodology showed that increasing BMI, NC and WC were significantly (p<0.023) associated with increasing pCO₂(a). In short, the worse the obesity, the lower is the pO₂(a) and the higher is the pCO₂(a). Morbid obesity can be associated with hypoxemia and hypercapnia despite essentially normal lung function.