INTRODUCTION

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All the well-recognized muscles of expiration such as those in the anterolateral wall of the abdomen, the expiratory intercostals, and the triangularis sterni are paralyzed following traumatic transection of the lower segments of the cervical cord. Patients with traumatic tetraplegia, however, are still able to empty their lungs actively by contracting the clavicular portion of the pectoralis major (1). This muscle bundle contracts during forced expiration and cough (2, 3), and we have recently demonstrated that in many patients, the rise in intrathoracic pressure resulting from this contraction may produce dynamic compression of the intrathoracic airways (4).

Because the abdominal muscles in these patients are paralyzed, however, the rise in intrathoracic pressure is transmitted through the flaccid diaphragm to the abdominal cavity to produce a paradoxical expansion of the abdomen (2, 3). One would expect that this paradoxical movement would, in turn, reduce the rise in intrathoracic pressure and, with it, the degree of dynamic airway collapse. The present studies were undertaken to evaluate the magnitude of this effect. Isovolume-pressure flow (IVPF) curves were thus constructed from forced expiratory vital capacity maneuvers in a group of tetraplegic patients; a nonelastic strap was then applied around the abdomen to prevent abdominal expansion.

	METHODS

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Patients

Eight male patients who had participated in our previous study (4) were studied. Descriptions of the patients are given in Table 1. They had suffered accidental fracture-dislocation of the cervical spine between the fifth and eighth cervical vertebra and were studied 6 to 200 mo after injury. At the time of the studies, they were all confined to wheelchairs and were in a clinical stable state with no respiratory symptoms, and their chest radiographs were within normal limits; only one patient (Patient 4) was an active smoker. All patients were informed of the nature and extent of the investigation, and all gave verbal consent to the procedures as approved by the Human Studies Committee of our institution.

Measurements

The procedures were carried out with the patient comfortably seated in his wheelchair. All restrictive garments were removed, and functional residual capacity (FRC), vital capacity (VC), total lung capacity (TLC), and residual volume (RV) were determined in duplicate by the closed circuit helium dilution technique (model 2400; Sensormedics, Anaheim, CA). An esophageal latex balloon (length 10 cm) was then positioned in the lower third of the esophagus and connected to a differential pressure transducer (Validyne, Northridge, CA) via a 100 cm long catheter. The balloon was filled with 1 ml of air. The patient then breathed through a heated Lilly-type pneumotachograph connected to another differential pressure transducer, and he performed a series of 15-30 expiratory VC maneuvers of varying intensity. Two successive efforts were always separated by a 2 to 3 min period of rest. Volume was obtained by digital integration of the flow signal.

The signals of esophageal pressure (Pes), expiratory flow (exp), and volume were recorded on a videotape recorder and an Olivetti PC operated at a sample rate of 200 Hz. After each expiratory VC maneuver, Pes and exp were plotted as a function of lung volume on the computer screen, and the patient was told how strong the next expiratory effort should be. IVPF curves (4) were displayed at regular intervals, and expiratory maneuvers were repeated until enough data points were obtained.

After completion of the control measurements, two or three nonelastic straps were placed around the abdomen from the pubis to the rib cage margin with the subject relaxing at end-expiration, and measurements of static lung volumes and IVPF curves were repeated. The straps were fastened so as to provide a tight abdominal support.

Data Analysis

Because abdominal strapping significantly increases VC in tetraplegic subjects (7), we compared not only the peak values of Pes and exp obtained before versus after strapping but also the values measured at similar absolute lung volumes. Thus, in each patient, absolute lung volumes in the two conditions were calculated, and values of Pes and exp were compared at 10% intervals between 80 and 20% of the largest strapped VC. Instantaneous values of exp achieved at these absolute lung volumes were also plotted against the corresponding values of Pes to construct IVPF curves.

Statistical assessments were made using paired t tests; the level of significance was taken as p < 0.05.

	RESULTS

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All patients had a restrictive ventilatory impairment (Table 2). In the control condition, the VC and TLC were decreased on average to 61.6 and 82.2% of the predicted normal value (8), whereas RV was elevated in most patients. After strapping the abdomen, there was a significant increase in VC, and a significant reduction in FRC and RV (Table 2). The reduction in FRC ranged from 5.7 to 28.8% and averaged 15.5% of the initial, control value. These effects of abdominal strapping were similar to those found in our previous study (7).

Table 3 summarizes the effect of abdominal strapping on the peak values of Pes and exp obtained during maximal efforts in the eight patients studied, and Figure 1 shows the average values of maximal Pes and exp obtained at all 10% intervals between 80 and 20% of the strapped VC before and after strapping. The changes produced by strapping were small and inconsistent.

View larger version (19K): [in this window] [in a new window]   Figure 1.   Mean ± SE values of maximal esophageal pressure (Pes) and expiratory flow (exp) obtained at 10% intervals between 80 and 20% of the strapped VC before ( closed symbols) and after (open symbols) abdominal strapping in the eight patients studied. Values obtained before and after strapping were not significantly different except for exp at 80% VC (p < 0.05).

Figure 2 displays the IVPF curves obtained at 70, 50, and 30% of the strapped VC before and after abdominal strapping in three representative patients, and Table 3 summarizes the data in all patients. The patient shown in the left panels (Patient 1) of Figure 2 had no plateau of flow in the control condition but showed some flow plateaus at all lung volumes after strapping. The curves after strapping were also displaced to the left, such that flow was increased at any given pressure. Three other patients of the study (Patients 4, 6, and 8) showed qualitatively similar alterations. Indeed, although these patients already had flow plateaus at one or two lung volumes before strapping, after strapping Patient 8 showed plateaus at all lung volumes between 80 and 40% VC, and Patients 4 and 6 showed plateaus at all lung volumes below 70-80% VC. On the other hand, abdominal strapping did not alter the shape or the position of the IVPF curves in four patients. Patients 2 (Figure 2, middle panels) and 3 never had any flow plateaus, and Patients 5 (Figure 2, right panels) and 7 showed clear-cut plateaus at all lung volumes both before and after strapping.

View larger version (24K): [in this window] [in a new window]   Figure 2.   Isovolume pressure-flow curves computed at 70, 50, and 30% of the strapped VC before (closed symbols) and after (open symbols) abdominal strapping in three representative patients. Each symbol corresponds to one expiratory effort. Patient 1 had no plateau of flow in the control condition but did show flow plateaus after strapping; Patient 5 had flow plateaus in the two conditions of the study and Patient 2 did not show flow plateaus either before or after strapping.

Differences in the tightness of the strapping, as assessed by the decrease in FRC, could not account for the intersubject variability in the effects of strapping on exp, Pes, and IVPF curves. These variable effects were not related either to the values of pressure or flow measured in the control condition.

	DISCUSSION

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The abdominal muscles in tetraplegic subjects are paralyzed, causing the abdomen to move paradoxically outward during expiratory maneuvers (1). By eliminating or reducing abdominal expansion, abdominal strapping should reduce the pressure dissipation and therefore allow these patients to generate higher intrathoracic pressures during forced expiration. In agreement with this hypothesis, four patients of the current study did show an increase in maximal Pes after strapping (Table 3). Strapping also resulted in the development of flow plateaus in four patients. However, these changes were small. The increases in Pes amounted to only 3-8 cm H₂O and the flow plateaus thus achieved remained limited compared to those seen in healthy subjects (5, 6). This suggests that in tetraplegic patients, the pressure dissipation related to paradoxical expansion of the abdomen plays a minor role in the low intrathoracic pressures generated during forced expiration. Alternatively, it has been suggested that in healthy subjects abdominal strapping may have an inhibitory effect on the diaphragm during voluntary sniffs (9), and it is possible that strapping also elicits an inhibition of the expiratory muscles during strenuous expiratory efforts. Such an inhibitory effect might account for the present finding that the maximal pressures in three patients were reduced, rather than increased, after strapping (Table 3). Indeed, these patients were highly motivated and produced reproducible data, as illustrated by the IVPF curves shown in Figure 2 (right panels), and it is unlikely that these reductions in pressure resulted from suboptimal motivation. There are, however, no data of which we are aware to support the idea that strapping may cause such an expiratory muscle inhibition.

Abdominal strapping also resulted in greater exp and caused a displacement of the IVPF curves toward higher flow values in four patients. Previous studies in normal subjects have established that after chest wall strapping, the static pressure-volume (PV) curve of the lung is shifted to the right and that this increase in lung recoil pressure (Pst[l]) causes an increase in expiratory flows (10). We have previously observed that chest wall strapping produces an increase in Pst(l) in tetraplegic subjects as well (A. De Troyer, unpublished observations), and four patients of the current study displayed a displacement of the zero flow intercept on the IVPF curves toward more negative values of Pes (Figure 2, left panels). Such a displacement is consistent with an increase in Pst(l). As for the increases in Pes, however, the increases in exp observed in our patients, when present, were small; these increases did not exceed 1 L/s, even at the highest lung volumes.

In conclusion, strapping the abdomen has only small and inconsistent effects on forced expiration in tetraplegic subjects. As a corollary, this procedure is unlikely to produce any clinically relevant improvement in the efficiency of cough. Alternative strategies, such as electrical stimulation of the abdominal muscles (13), should therefore be investigated.