Distribution of ventilation in young and elderly adults determined by electrical impedance tomography
Introduction
The mechanical properties of the chest–lung system influence the distribution of gas within the lung. Stiffening of the chest wall, reduction of lung elastic recoil and diminished respiratory muscle strength are known to affect respiratory mechanics in the elderly (Gibson et al., 1976, Rossi et al., 1996, Babb and Rodarte, 2000). The effect of aging on global static and dynamic ventilatory parameters is well established (Campbell and Lefrak, 1978, Rossi et al., 1996, Zeleznik, 2003), however, regional lung ventilation in the elderly has only seldom been studied. The few studies performed to date have used radioactive tracer gases to follow the topographical distribution of ventilation along the vertical axis only during slow, quasi-static maneuvers (Holland et al., 1968, Kronenberg et al., 1973). These techniques do not permit continuous assessment of regional lung ventilation during undisturbed spontaneous breathing.
Recently, a radiation-free imaging technique, electrical impedance tomography (EIT), has become available, allowing non-invasive study of regional lung ventilation in a cross-section of the chest. Various validation studies using established techniques (e.g., computed tomography and ventilation scintigraphy) have proved the capacity of EIT to trace local air volume changes in lungs (Frerichs et al., 2002, Serrano et al., 2002, Hinz et al., 2003a, Hinz et al., 2003b). On this basis EIT has been proposed as a possible future tool for monitoring regional lung ventilation in a clinical setting (Frerichs et al., 2001b, Rimensberger, 2002, Caples and Hubmayr, 2003, Stenqvist, 2003).
The majority of patients, including those who may benefit from the possible clinical use of EIT in the future, are of older age. However, the specific changes in regional lung function associated with aging are not known in detail. Therefore, the aim of our study was to use modern EIT technology to study the distribution of ventilation in a group of elderly subjects, and to establish how the distribution of gas in the lungs is modified by the change in body position during several ventilatory maneuvers. Additionally, a comparison was made with a group of young subjects studied using the same protocol.
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Subjects
Eight young (26 ± 1 years, mean age ± S.D.) and eight old (73 ± 5 years) healthy men were studied. The study was approved by the local ethical committee and written informed consent was obtained from all participants. All subjects were nonsmokers with no clinical evidence of cardiorespiratory disease. Basic physical characteristics and results of lung function testing are given in Table 1. Lung function was assessed by spirometry and body plethysmography (Masterlab, Jaeger, Wuerzburg, FRG) and
Resting tidal breathing
The regional tidal volumes in the left and right lungs determined in all postures and both groups of subjects during resting tidal breathing revealed a small difference between the young and elderly: The regional tidal volume in the left lung in the right lateral posture was significantly lower in the young subjects. The corresponding EIT data, i.e. the sums of end-inspiratory-to-end-expiratory relative impedance changes in the left and right lung regions, are given in Fig. 2. (The four
Discussion
In our study, two groups of healthy young and old subjects were studied to determine the effect of age on the distribution of gas in lungs during various ventilatory maneuvers. The subjects were followed in four postures whereby the effect of the changing orientation of chest in the gravity field on regional ventilation distribution could be assessed. The findings of the present study are in keeping with the current state of knowledge of the physiological effects of age and gravity on
Conclusion
The present study revealed several differences between regional ventilation distribution in healthy young and old subjects in various breathing conditions. The results manifest how changes in respiratory mechanics associated with the physiological aging process affect the distribution of gas in the lungs. We expect that our results obtained in healthy subjects will serve as a basis for future EIT studies on older patients suffering from pulmonary diseases and undergoing ventilator therapy.
Acknowledgement
This study was supported by the German Aerospace Center (DLR) and the German Ministry for Education and Research (BMBF) grant no. 50 TK 9804.
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