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Components of respiratory resistance monitored in mechanically ventilated patients

¹ Dept of Anaesthesiology and Intensive Therapy, ² Dept of Medical Informatics and Engineering, and ³ Division of Cardiac Surgery, University of Szeged, Szeged, Hungary

CORRESPONDENCE: Z. Hantos, Dept of Medical Informatics and Engineering, University of Szeged, Korányi fasor 9, Szeged, Hungary. Fax: 36 62544566. E-mail: hantos@dmi.u-szeged.hu

Keywords: airway resistance, forced oscillations, interrupter technique, respiratory monitoring, tissue resistance

Received: January 8, 2002
Accepted July 19, 2002

This study was supported by Ministry of Health grant No. 591/1996 06.

The interrupter technique is commonly adopted to monitor respiratory resistance (R_rs,int) during mechanical ventilation; however, R_rs,int is often interpreted as an index of airway resistance (R_aw).

This study compared the values of R_rs,int provided by a Siemens 940 Lung Mechanics Monitor with total respiratory impedance (Z_rs) parameters in 39 patients with normal spirometric parameters, who were undergoing elective coronary bypass surgery. Z_rs was determined at the airway opening with pseudorandom oscillations of 0.2–6 Hz at end inspiration. R_aw and tissue resistance (R_ti) were derived from the Z_rs data by model fitting; R_ti and total resistance (R_rs,osc=R_aw+R_ti) were calculated at the actual respirator frequencies. Lower airway resistance (R_awl) was estimated by measuring tracheal pressure.

Although good agreement was obtained between R_rs,osc and R_rs,int, with a ratio of 1.07±0.19 (mean±sd), they correlated poorly (r²=0.36). R_ti and the equipment component of R_aw accounted for most of R_rs,osc (39.8±11.9 and 43.0±6.9%, respectively), whereas only a small portion belonged to R_awl (17.2±6.3%).

It is concluded that respiratory resistance may become very insensitive to changes in lower airway resistance and therefore, inappropriate for following alterations in airway tone during mechanical ventilation, especially in patients with relatively normal respiratory mechanics, where the tissue and equipment resistances represent the vast majority of the total resistance.

This article has been cited by other articles:

				E. Oostveen, D. MacLeod, H. Lorino, R. Farre, Z. Hantos, K. Desager, and F. Marchal The forced oscillation technique in clinical practice: methodology, recommendations and future developments Eur. Respir. J., December 1, 2003; 22(6): 1026 - 1041. [Abstract] [Full Text] [PDF]