title: Bioelectrical impedance phase angle relates to function, disease severity and prognosis in stable chronic obstructive pulmonary disease.
abstract: OBJECTIVE
  Bioelectrical impedance analysis (BIA) provides a simple method to assess changes in body composition. Raw BIA variables such as phase angle provide direct information on cellular mass and integrity, without the assumptions inherent in estimating body compartments, e.g. fat-free mass (FFM). Phase angle is a strong functional and prognostic marker in many disease states, but data in COPD are lacking. Our aims were to describe the measurement of phase angle in patients with stable COPD and determine the construct and discriminate validity of phase angle by assessing its relationship with established markers of function, disease severity and prognosis.
  
  
  METHODS
  502 outpatients with stable COPD were studied. Phase angle and FFM by BIA, quadriceps strength (QMVC), 4-m gait speed (4MGS), 5 sit-to-stand time (5STS), incremental shuttle walk (ISW), and composite prognostic indices (ADO, iBODE) were measured. Patients were stratified into normal and low phase angle and FFM index.
  
  
  RESULTS
  Phase angle correlated positively with FFM and functional outcomes (r = 0.35-0.66, p < 0.001) and negatively with prognostic indices (r = -0.35 to -0.48, p < 0.001). In regression models, phase angle was independently associated with ISW, ADO and iBODE whereas FFM was removed. One hundred and seventy patients (33.9% [95% CI, 29.9-38.1]) had a low phase angle. Phenotypic characteristics included lower QMVC, ISW, and 4MGS, higher 5STS, ADO and iBODE scores, and more exacerbations and hospital days in past year. The proportion of patients to have died was significantly higher in patients with low phase angle compared to those with normal phase angle (8.2% versus 3.6%, p = 0.02).
  
  
  CONCLUSIONS
  Phase angle relates to markers of function, disease severity and prognosis in patients with COPD. As a directly measured variable, phase angle offers more useful information than fat-free mass indices.
authors.affiliation: King's College London, Cicely Saunders Institute, London, UK; NIHR Respiratory Biomedical Research Unit, Royal Brompton and Harefield NHS Foundation Trust and Imperial College, London, UK. Electronic address: matthew.maddocks@kcl.ac.uk.
authors.forename: Matthew
authors.initials: M
authors.lastname: Maddocks
authors.affiliation: NIHR Respiratory Biomedical Research Unit, Royal Brompton and Harefield NHS Foundation Trust and Imperial College, London, UK. Electronic address: s.kon@rbht.nhs.uk.
authors.forename: Samantha S C
authors.initials: SS
authors.lastname: Kon
authors.affiliation: NIHR Respiratory Biomedical Research Unit, Royal Brompton and Harefield NHS Foundation Trust and Imperial College, London, UK. Electronic address: s.jones5@rbht.nhs.uk.
authors.forename: Sarah E
authors.initials: SE
authors.lastname: Jones
authors.affiliation: NIHR Respiratory Biomedical Research Unit, Royal Brompton and Harefield NHS Foundation Trust and Imperial College, London, UK. Electronic address: j.canavan@rbht.nhs.uk.
authors.forename: Jane L
authors.initials: JL
authors.lastname: Canavan
authors.affiliation: NIHR Respiratory Biomedical Research Unit, Royal Brompton and Harefield NHS Foundation Trust and Imperial College, London, UK. Electronic address: c.nolan@rbht.nhs.uk.
authors.forename: Claire M
authors.initials: CM
authors.lastname: Nolan
authors.affiliation: King's College London, Cicely Saunders Institute, London, UK. Electronic address: irene.higginson@kcl.ac.uk.
authors.forename: Irene J
authors.initials: IJ
authors.lastname: Higginson
authors.affiliation: King's College London, Cicely Saunders Institute, London, UK. Electronic address: gao.wei@kcl.ac.uk.
authors.forename: Wei
authors.initials: W
authors.lastname: Gao
authors.affiliation: NIHR Respiratory Biomedical Research Unit, Royal Brompton and Harefield NHS Foundation Trust and Imperial College, London, UK. Electronic address: m.polkey@rbht.nhs.uk.
authors.forename: Michael I
authors.initials: MI
authors.lastname: Polkey
authors.affiliation: NIHR Respiratory Biomedical Research Unit, Royal Brompton and Harefield NHS Foundation Trust and Imperial College, London, UK. Electronic address: research@williamman.co.uk.
authors.forename: William D-C
authors.initials: WD
authors.lastname: Man
mesh_terms: D000368:Aged; D001823:Body Composition; D015992:Body Mass Index; D003430:Cross-Sectional Studies; D017097:Electric Impedance; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D015999:Multivariate Analysis; D011379:Prognosis; D029424:Pulmonary Disease, Chronic Obstructive / Q000175:diagnosis*; D011788:Quality of Life; D016138:Walking
keywords: Bioelectrical impedance analysis; Body composition; Chronic obstructive pulmonary disease; Fat-free mass; Phase angle