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NIV: COPD, neuromuscular disease and obesity
P276 The value of vital capacity and daytime pulse oximetry to predict hypercapnia in obese patients
  1. E Boleat1,
  2. S Mandal1,
  3. E Suh1,
  4. N Hart1,2
  1. 1Lane Fox Respiratory Unit, Guy's St Thomas NHS Foundation Trust, London, UK
  2. 2Guy's and St Thomas' NHS Foundation Trust and King's College London, National Institute of Health Research Comprehensive Biomedical Research Centre, London, UK


Introduction The Health Survey for England reported that 25% of UK adults are obese with a 10% rise over 15 years. Consequently, clinicians are faced with a rising number of obese patients referred for bariatric and non-bariatric surgery. Previous data indicates a 50% incidence of obstructive sleep apnoea in patients with a BMI >40 kg/m2 with obesity hypoventilation syndrome present in up to a third. These patients have higher risk of peri-operative complications. A screening tool to predict hypercapnic respiratory failure (PaCO2 >6 kPa) based on simple clinic tests would be useful. Correlations were performed to determine which tests may be useful.

Methods Data from all obese patients (BMI >30 kg/m2) with evidence of sleep-disordered breathing on oximetry initiated on home ventilatory support between August 2005 and December 2010 were obtained from a discharge summary database.

Results 205 patients were included for analysis. The group mean age was 54.9 (SD 14.2) years, daytime clinic oxygen saturations (SpO2clinic) 91.0% (5.8%), FEV1 1.8 l (0.96 l), FVC 2.2 l (1.11 l), weight 132.8 kg (28.5 kg), BMI 47.6 kg/m2 (9.6) and Epworth sleepiness score 8.9 (5.6). Mean daytime PaCO2 was 6.68 kPa (1.31). Significant correlations were found between PaCO2 and BMI (r=0.20; p<0.005), FEV1% predicted (r=−0.20; p<0.005), FVC% predicted (r=−0.20; p<0.005) and SpO2clinic (r=−0.52; p<0.005). Receiver operating characteristics (ROC) analysis was used to determine the utility of SpO2clinic and FVC to predict hypercapnia. The area under the curve (AUC) for SpO2clinic was 0.81 (p<0.001); a cut-off of SpO2clinic of <92% demonstrated a sensitivity of 86% and specificity of 52% in predicting hypercapnia. The AUC for FVC was found to be 0.77 (p<0.0001); a cut-off of <1.94 l demonstrated a sensitivity of 77% and specificity of 61% in detecting hypercapnia (see Abstract P276 figure 1).

Abstract P276 Figure 1

Receiver operating characteristics for FVC and SpO2clinic in predicting hypercapnia.

Conclusion These data have significant clinical utility for clinicians involved in providing respiratory support services for obese patients undergoing bariatric and non-bariatric surgery. In particular, it could form the foundations of a screening algorithm including simple measures such as home oximetry, spirometry and clinic pulse oximetry, to identify the highest risk patients that need to be reviewed by sleep and ventilation clinicians.

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