Introduction Hypercapnia in the acute phase of COPD exacerbation is common, with CO₂ >6 kPA in 44% of patients at some point during their admission. Little data exists on the prevalence of hypercapnia in stable COPD patients, and even less in those with AATD. As emphysema is more predominant in the lower lobes of AATD patients, this is likely to contribute to hyperinflation and hence potentially increase CO₂ levels.

Methods The Birmingham AATD database (ADAPT) is a registry of with over 1000 patients with AATD. The registry has basic demographics, detailed spirometric parameters as well as baseline blood gases. Hypercapnia is defined as CO₂ greater than 5.5 kPa.

Results The blood gas results of 766 (PiZZ genotypes) individual patients were available for analysis. 93 patients (12.14%) had a type 1 respiratory failure, defined as a PO₂ <8 kPa, 69 had hypercapnia (9.01%) and 16 (2.09%) patients fulfilled both criteria. There is a statistically significant difference seen in the hypercapnic vs non-hypercapnic population with regards to FEV₁ (1.07 vs 1.46, p = 0.01), FVC (3.45(CI 3.1–3.81) vs 3.82, p = 0.02) and BMI (27.1 vs 24.9, p = 0.02). There is no difference in the amount of upper zone emphysema (29.54 vs 30.50 (CI 29.12–33.01)) or lower zone emphysema (40.66 vs 49.13 (CI 42.64–47.32)). Chi-squared analysis of lower zone predominance (lower zone – upper zone) showed no statistical difference either (p = 0.76). Factors clinically significant in univariate analysis were taken forward to logistic regression analysis where BMI was the only clinically significant (p = 0.008) predictor.

Conclusion Hypercapnia is relatively common amongst AATD patients, but Type 2 respiratory failure is uncommon. There is an increased risk of hypercapnia with worse FEV₁, FVC and higher BMI. The presence or location of emphysema did not seem to influence the CO₂ levels.