Clinical Applications of Forced Oscillation Technique

resolution cr allows visualization of additional detail of the lung parenchyma that further helps to identify and characterize the pulmonary process. Are the cr appearances of pulmonary infections more specific than plain film findings? The purpose of the study by Barloon et al was to determine if Cf provided additional, clinically useful information regarding the presence of pulmonary infection. Although many of the cr findings of pulmonary diseases are nonspecific, the cr findings of invasive fungal disease, particularly invasive pulmonary aspergillosis in this clinical setting, are often distinguishable from other bacterial or viral infections. 10 As Barloon et al point out in their article, being able to identify cr findings of fungal infection in cases where the conventional radiographs remained nonspecific added confidence in the diagnosis of fungal infection and in the appropriateness of aggressive antifungal therapy. Finally, do early detection and characterization of lung infection by cr improve survival and ultimate outcome? Barloon et al concluded from their study that cr does provide significant information in the febrile bone marrow transplant patient with nonspecific chest film findings, but that the mortality is quite high in this group and that it may be difficult to prove definitively that Cf affects outcome. In order to test the ability of cr to affect outcome, one would need to screen all febrile aplastic patients and compare serial chest films to serial cr scans in their ability to detect and characterize early infection, at a time when intervention could affect outcome . Patients in the present study may have had fairly well-established pulmonary infection by the time the cr was performed, since those included in the study were symptomatic patients that the clinical staff suspected of having pulmonary disease, but whose serial chest radipgraphs failed to provide sufficient information to initiate or to continue treatment. It remains to be seen whether serial cr examinations, used as a primary screening modality, may indeed have a significant impact on mortality, particularly from fungal infection. Some clinical evidence does exist to suggest that early presumptive diagnosis of fungal infection with the aid of cr surveillance and rapid institution of aggressive antifungal therapy leads to improved survival.8 cr surveillance has also been used effectively to monitor disease activity, document progression or resolution of fungal infection in response to therapy, and detect reactivation offungal disease during subsequent treatment cycles.9 Whether an improvement in survival from bacterial and viral infections can be demonstrated with the aid of early cr detection has yet to be demonstrated. janet E. Kuhlman, M.D., F.C.C.P. Baltimore The RusseU H. Morgan Department of Radiology and Radiological Science, The Johns Hopkins Medical Institutions.

REFERENC ES 1 Hamilton PJ, Pearson ADJ. Bone marrow transplantation and the lung. Thorax 1986; 41:497-502 2 Paulin T, Ringden 0, Nilsson B, Lonnqvist B, Gahrton G . Variables predicting bacterial and fungal infections after allogeneic marrow engraftment. Transplantation 1987; 43:393-98 3 Meyers JD, Flournoy N, Thomas ED. Nonbacterial pneumonia after allogeneic marrow transplantation: a review of ten years' experience. Rev Infect Dis 1982; 4:1119-32 4 Watson JG. Problems of infection after bone marrow transplantation. J Clin Pathol 1983; 36:683-92 5 Winston DJ, Ho WG, Champlin RE , Gale RP. Infectious complications of bone marrow transplantation. Exp Hematol 1984; 12:205-15 6 Meyers RD, Young LS, Armstrong D , Yu D. Aspergillosis complicating neoplastic disease. Am J Med 1973; 54:&-15 7 Aisner J, Schimpf£ JC , Wiernik DH. Treatment of invas.ive aspergiUosis: relation of early diagnosis and treatment to response. Ann Intern Med 1977; 86:539-43 8 Burch PA, Karp JE, Men: WG, Dick JD, Kuhlman JE, Fishman EK. Favorable outcome of invasive aspergiUosis in patients with adult acute leukemia. JClin Oncol I987; 5:1985-93 9 Karp JE, Burch PA, Men: WG. An approach to intensive antileukemia therapy in patients with previous invasive aspergillosis. Am J Med 1988; 85:~ 10 Kuhlman JE, Fishman EK, Siegelman SS. Invasive pulmonary aspergillosis in acute leukemia: Characteristic 6ndings on CT, the CT halo sign, and the role of CT in early diagnosis. Radiology 1985; 157:611-14 11 Schaner EG, Chang AE, Doppman JL, Conkle DM, Flye MW, Rosenberg SA. Comparison ofcomputed and conventional whole lung tomography in detecting pulmonary nodules: a prospective radiologic-pathologic study. AJR 1978; 131:51-4 12 Zerhouni EA, Stitik FP. Siegelman SS, Naidich DP. Sagel SS, Proto AV. et al. CT of the pulmonary nodule: a cooperative study. Radiology 1986; 160:31~27 13 Muhm JR. Brown LR, Crowe JK, Sheedy PF, Hattery RR, Stephen DH. Comparison of whole lung tomography and computed tomography for detecting pulmonary nodules. AJR 1978; 181:981-84 14 McLoud TC, Wittenberg J, Ferrucci JT. Computed tomography of the thorax and standard radiographic evaluation of the chest: a comparative study. JComput Assist Tomogr 1979; 3:17(}.8() 15 Orr DP. Myerowitz RL, Dubois PJ. Pathoradiologic correlation of invasive pulmonary aspergillosis in the compromised host. Cancer 1978; 41:2028-39

Clinical Applications of Forced Oscillation Technique Dubois et al• introduced the forced oscillation technique (FOT) in 1956 as a method to characterize the mechanical properties of the respiratory system over a wide range of frequencies. However, only in the 1970s, when microprocessor techniques became available, allowing the analysis of complex signals by means of the Fourier transform, were more in-depth investigations possible of frequency characteristics of the impedance of the respiratory system and of its two components-the real part or resistance (Rrs), and the imaginary part or reactance (Xrs, which CHEST I 99 I 4 I APRIL, 1991

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is influenced by both the inertance and capacitance of the system). 2-s Also, the clinical potential of the method became apparent: the FOT is rapid and demands only passive cooperation which makes it especially appealing for children, for epidemiologic surveys and for conditions in which quiet breathing instead offorced expiratory maneuvers are preferred. Nowadays, the FOT has proved its usefulness in many conditions. 6 The absolute value of Rrs at low measuring frequencies (up to 4-8 Hz) is very similar to that of plethysmographic airway resistance. In healthy subjects, Rrs is almost frequency independent or even increases slightly with frequency (currently applied frequencies are 2 to 25-50 Hz), except in children, in which it decreases with increasing frequency.7 Xrs is negative at low frequencies (capacitance prevails), it becomes zero at about 8 Hz (the resonant frequency), and it is positive at higher frequencies (inertance prevails). The pattern of impedance values (Rrs and Xrs) in the various states associated with abnormalities of pulmonary function consists of an increase in Rrs especially at low frequencies, and a decrease in Xrs with a shift of the resonant frequency to higher frequencies. These changes have been shown to be well correlated with those of routine lung function tests in different pathologic conditions such as COPD (after challenge tests or bronchodilation), upper airways obstruction, interstitial lung disease, and chest wall abnormalities. 8·•• Yet, the relative changes of impedance values with respect to other routine lung function values (FEV., PEF, Raw) may vary depending on the type of pathology; this may be of diagnostic interest. The FOT is more sensitive to upper or lower airway obstruction than to interstitial lung disease and chest wall pathology yet, even in the latter, FOT indices may be affected. The F0T appears also to be especially suitable for challenge tests 10 and conversely, for reversibility tests ofairflow obstruction. Since it is sensitive to incipient airway obstruction12 it might become an interesting screening technique for epidemiologic surveys and for detecting people at risk, such as those exposed to occupational hazards. 12• 13 The limitations of the FOT should be recognized similarly as for other functional techniques. Although model studies have been successfully applied to the FOT, the physiologic correlates for the changes in the different indices remain hypothetical.4·6 Furthermore, the shunt impedance of the cheeks has a major effect on the results. A comparison of the conventional technique, 2 in which the forced oscillations are applied at the mouth, and the head generator technique minimizing the influence of the cheekS suggests, however, that there is mainly a proportionality difference in the results: Rrs tends to be higher with the head generator in pathologic conditions, especially at higher frequencies, resulting in a decrease and even a

disappearance of the frequency dependence. The resonant frequency is also decreased.•• Investigations on future developments of the FOT are mainly directed at expanding the measuring range to higher frequencies, 1 ~ 16 to measuring instantaneous impedance values, thus separating inspiratory and expiratory values, to relating impedance values to lung volumes (FRC), and to effectively bypassing the oropharynx. M. Demedts, M.D., F.C .C.P.;]. A. Van Noord, M.D.; K. P. Van De Woestijne, M.D. Leuven, Belgium Laboratorium fOr Pneumology, Department of Patbopbysiology, Catbolic University of. Leuveo, Belgium. Reprint~: Prof Demedtr, Uniwrrity Ho.pUGI, ~eroeld 1, 8 3041 IWlenberg, lklgMn

REFERENCES 1 Dubois AB, Brody AW, Lewis DH, Burgess BF Jr. Oscillation mechanics oflungs and chest in man. J Appl Physiol1956; 8:587-

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2 Landser FJ, Nagels J, Demedts M, Billiet L, Van De Woestijne K.P. A new method to determine frequency characteristics of the respiratory system. J Appl Physiol1976; 41:101-06 3 Michaelson ED, Grassman ED, Peters WR. Pulmonary mechanics by spectral analysis offorced random noise. J Clin Invest 1975; 56:121~. 4 Peslin R, Fredberg JJ. Oscillation mechanics of the respiratory system. In: Macklem PT. Mead J, eds. Handbook of physiology, Section 3: The respiratory system, Vol III. Mechanics of breathing. Bethesda: American Physiological Society 1986; 14566 5 Peslin R. M~thodes de mesure de l'im~dance respiratoire totale par oscillations forcees. Bull Eur Physiopathol Respir 1986; 22:621-31 6 Van Noord JA. Oscillation mechanics of the respiratory system: clinical applications and modelling (PhD Thesis). Leuven: University of Leuven, 1990 7 Duiverman EJ, Clement J, Van De Woestijne KP, Neyens HJ, Van Den Bergh ACM, Kerre bijn KF. Forced oscillation technique; reference values for resistance and reactance over a frequency spectrum of 2-26 H:z: in healthy children aged 2, 3121/2 years. Bull Eur Physiopathol Respir 1985; 21:171-78 8 Van Noord JA, Clement J, Cauberghs M, Mertens I, Van De Woestijne KP. Demedts M. Total respiratory resistance and reactance in patients with diffuse interstitial lung disease. Eur Respir J 1989; 2:846-52 9 Van Noord JA, Wellens W, Clarysse I, Cauberghs M, Van De Woestijne KP, Demedts M. Total respiratory resistance and reactance in patients with upper airway obstruction. Chest 1987; 92:475-80 10 Van Noord JA, Clement J, Van De Woestijne KP, Demedts M. Total respiratory resistance and reactance as a measurement of response to bronchial challenge with histamine. Am Rev Respir Dis 1989; 139:921-26. 11 Van Noord JA , Clement J, Van De Woestijne KP. Demedts M. Total respiratory resistance and reactance in patients with asthma, chronic bronchitis and e mphysema. Am Rev Respir Dis (in press) 12 Clement J, Landser FJ, Van De Woestijne KP. Total resistance and reactance in patients with respiratory complaints with and without airways obstruction. Chest 1983; 83:215-20

13 Peslin R, Pham QT, Teculescu D, Gallina C, Divivier C. Comparative value of respiratory input and transfer impedances in field studies. Bull Eur Physiopathol Respir 1987; 23:37-42 14 Cauberghs M, Van De Woestijne KP. Effect of upper airway shunt and series properties on respiratory impeda.JM:e measurements. J Appl Physiol1989; 66:2274-79

15 FarreR, Peslin R, Oostveen E, Sulci B, Divivier C, Navajas D. Human respiratory impedance from 8 to 256 Hz corrected for upper airway shunt. J Appl Physiol1989; 67:1973-81 16 Dorlcin L, Lutchen KR, Jackson AC. Human respiratory input impedance for 4 to 200 hz: physiological and modeling consideration. J Appl Physiol 1988; 64:823-31

Plan to AHend ACCP's

57th Annual Scientific Assembly f1rd San Francisco ~ November 4-8, 1991

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