Validity and reliability of cardiorespiratory measurements recorded by the LifeShirt during exercise tests

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Abstract

The LifeShirt is a novel ambulatory monitoring system that records cardiorespiratory measurements outside the laboratory. Validity and reliability of cardiorespiratory measurements recorded by the LifeShirt were assessed and two methods of calibrating the LifeShirt were compared. Participants performed an incremental treadmill test and a constant work rate test (65% peak oxygen uptake) on four occasions (>48 h apart) and wore the LifeShirt, COSMED system and Polar Sport Tester simultaneously. The LifeShirt was calibrated using two methods: comparison to a spirometer; and 800 ml fixed-volume bag. Ventilation, respiratory rate, expiratory time and heart rate recorded by the LifeShirt were compared to measurements recorded by laboratory equipment. Sixteen adults participated (6M:10F); mean (SD) age 23.1 (2.9) years. Agreement between the LifeShirt and laboratory equipment was acceptable. Agreement for ventilation was improved by calibrating the LifeShirt using a spirometer. Reliability was similar for the LifeShirt and the laboratory equipment. This study suggests that the LifeShirt provides a valid and reliable method of ambulatory monitoring.

Introduction

Exercise testing and physical activity monitoring are important in many areas such as health promotion in the general population or assessing patients with chronic disease. In terms of exercise testing the gold standard is cardiopulmonary exercise testing carried out in a laboratory. In-depth analysis of gas exchange, ventilation, electrocardiogram and blood pressure allows clinicians to elicit and interpret abnormalities in the cardiovascular and pulmonary systems (Porszasz et al., 2007). These measures have been shown to be important for diagnosis and prognosis, assessment of severity and progression of disease, understanding the physiological response to therapies and as an endpoint in clinical trials (Bradley and O’Neill, 2005, Bradley and Moran, 2008, McKone et al., 2002, Ram et al., 2005). Despite the extensive data gained, cardiopulmonary exercise testing is time and labour intensive and requires expensive equipment. Field tests such as shuttle walk/run tests provide a less expensive alternative which are easier to carry out in large groups of subjects or for repeated testing. Field tests are often used in large epidemiological studies (e.g. Northern Ireland Young Hearts Project, Boreham et al., 1993) however results are based on performance (e.g. distance run) and information on physiological response is limited.

Physical activity monitoring is becoming recognised as an important tool for both promoting physical activity and assessing the outcome of interventions. Methods used to date include self-reported recall questionnaires (e.g. Baecke Physical Activity Questionnaire (Baecke et al., 1982)), activity diaries (e.g. Bouchard Activity Diary (Bouchard et al., 1983)) and objective movement sensors such as pedometers and accelerometers. Although objective measures of activity may be more reliable they do not provide data on physiological response to activity.

The LifeShirt® (Vivometrics, Inc., Ventura, CA, U.S.A.) is a novel multi-function ambulatory device and consists of a Lycra® garment, data recorder, and computer-based analysis software (VivoLogic) (Grossman, 2004, Wilhelm et al., 2003). Respiratory inductive plethysmography is the core technology which has been shown to provide accurate non-invasive assessment of respiratory pattern (Carry et al., 1997). The basic system also incorporates a three-lead ECG and a tri-axial accelerometer. As the LifeShirt simultaneously monitors physiological measurements such as ventilation, respiratory rate and heart rate as well as activity, it provides an opportunity for monitoring response to exercise and physical activity outside of the laboratory.

The LifeShirt has been successfully used to measure key physiological measurements during activities of daily living in heart failure which were important in predicting mortality and tailoring therapy (Brack et al., 2007). To date three small studies have examined the validity of the LifeShirt and involved healthy volunteers (n = 45) and a small number of patients with cardiac (n = 5) and respiratory disease (n = 6) (Clarenbach et al., 2005, Witt et al., 2006, Heilman and Porges, 2007). However in these studies measurements were only examined at one time point. No study to date has examined validity and reliability of the LifeShirt over multiple time points. This is important as many areas of research involve repeated testing, including assessing treatment efficacy and training effects, monitoring physical activity, and collection of physiological data in epidemiological studies. Repeated testing is also important clinically in monitoring patients over the lifetime of their disease.

The hypotheses for this study were that cardiorespiratory measurements recorded by the LifeShirt during exercise would be comparable to measurements recorded by laboratory equipment and reliable over four occasions, and that calibration of the LifeShirt using a spirometer would improve the agreement in ventilation recorded by the LifeShirt and laboratory equipment. A further hypothesis was that activity measured by the LifeShirt would be reliable over four occasions.

Section snippets

Subjects

Sixteen individuals aged 18–30 were recruited via email advertisement within the University of Ulster. Participants were healthy (no known respiratory or cardiac disease) Caucasians. The protocol was approved by the University of Ulster ethics procedures and written informed consent obtained.

Procedure

Participants attended the Human Performance Laboratory (University of Ulster) on four occasions separated by at least 48 h. Visits took place at the same time of day with the same assessor and involved a

Results

16 young adults participated (6M:10F); mean (SD) age 23.1 (2.9) years, height 169.7 (9.1) cm and weight 67.3 (8.6) kg. Participants were moderately active, mean (SD) physical activity score 7.2 (1.1) units. In the incremental test all participants reached at least stage 11 (mean (SD): V˙O2kg1: 29 (3.2) ml min−1 kg−1, R: 1.02 (0.1)). The mean speed during the constant work rate test was 6.3 km h−1.

Discussion

This is the first study to assess validity and reliability of the LifeShirt in comparison to laboratory systems across multiple time points (four occasions). Validity and reliability over multiple time points is important as monitoring patients over the lifetime of their disease and assessing treatment efficacy involves repeated testing. Breath-by-breath analysis is fundamentally open to variation and exploratory analysis using individual breaths revealed a great amount of error variation. As

Acknowledgements

The authors would like to acknowledge Dr. John Brown and Roy Crowe in the Human Performance Laboratory, University of Ulster for assistance with exercise testing.

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