FUNCTIONAL ELECTRICAL STIMULATION OF ABDOMINAL MUSCLES (FESAM) IN SPINAL CORD INJURY DURING PHRENIC NERVE STIMULATION

Franc Kandare¹, Uroš Stanič², Janez Jeraj², Gerhard Exner³, Jurij Šorli¹, Robert Jaeger², Antonio Pedotti⁴, Andrea Aliverti⁴, Raffaele Dellaca⁴

¹University Clinic of Respiratory and Allergic Diseases, Golnik, Slovenia; ²Institute Jožef Stefan, Ljubljana, Slovenia; ³Spinal Cord Injury Center, Hamburg, Germany; ⁴Centro di Bioingegneria, Milano, Italy

Abstract

The aim of this study was to determine the potential of functional electrical stimulation of abdominal muscles (FESAM) to provide ventilation assistance in individuals with spinal cord injury (SCI) who were dependent on diaphragmatic pacing.

Four individuals with complete SCI (levels C2 to C3) who had implanted phrenic nerve stimulators (PNS) were studied. FESAM was applied during either expiration or inspiration with a 4-channel custom-made electrical stimulator via surface self-adhesive electrodes placed on the abdominal wall. Flow and volume were measured with a pneumotachograph and 3-D optoelectronic plethysmography.

Tidal volume (V_T) and total ventilation (V_e) during PNS and FESAM during expiration were increased in three subjects (from 15 % to 45%) in comparison to PNS alone. FESAM applied during inspiration did not change V_T, but volume contribution of thorax to V_T increased.

These results might have potential for long-term clinical application, both for V_e increase and/or V_T redistribution. The timing and parameters of FESAM should be adjusted individually to the individuals needs. Thus FESAM could make PNS effects more physiological and reduce phrenic nerve stimulation demand.

Introduction/Background

There are more than 200,000 individuals with SCI in the United States and very likely a similar number in Europe.

In the second-half of the twentieth century, the mortality rate following SCI has continually declined, probably because of advances in trauma care, the advent of specialized SCI treatment centers, use of antibiotics, and other medical interventions. At the present time, the mean life expectancy in SCI after initial survival is approximately 85 to 90 percent of the general population [1]. Because of improved survival, the prevalence of individuals with SCI is expected to increase approximately 20 percent between 1994 and 2004 [2]. The estimated number of C1 and C2 injuries is about 1.7 % of total SCI population and these individuals are dependent on some for of artificial ventilation. Typically, these individuals utilize either mechanical ventilation (MV) or phrenic nerve stimulation (PNS). Both modes of artificial ventilation have known advantages and disadvantages. Research directed to improvements in each system attempt to approach as closely as possible the normal act of breathing. PNS represents a more physiological approach, but the use of only one respiratory muscle cannot substitute the complex play of numerous inspiratory and expiratory muscles which produce normal ventilation. Two previous studies showed that FESAM during expiration markedly increased ventilation both in normal subjects and individuals with SCI who were not dependent on artificial ventilation [3, 4].

The purpose of this study was to determine if FESAM during either the inspiratory or expiratory phase of respiratory cycle could make ventilation by PNS more physiological and sustainable.

Methods

Subjects

Four individuals permanently on PNS and/or partly on MV, provided informed consent and participated in this study. Anthropometric data of the subjects are shown in table 1. The experimental protocol was reviewed and approved by the hospital’s Institutional Review Board.

TABLE I

AntHropometric values

Subject/ gender	Age (year)	Height (cm)	Weight (kg)	Injury
MM/M	11	160	36	C2
KL/M	49	150	65	C2
SR/F	19	183	58	C2-C3*
SK/F	36	152	47	C2

* incomplete lesion

Protocol

Stimulation of abdominal muscles was performed with a custom-made electrical stimulator [5] in the sitting or supine position. Stimulation parameters during FESAM were: 1 second pulse train at 45 Hz, amplitude of stimulation 60-100 mA, and pulse width 25 ms triggered by the pneumotachograph at the beginning of inspiration or expiration. FESAM was delivered via surface self-adhesive electrodes (PALS Axelgaard, rectangular). These were placed on the abdominal wall so that mm. rectus abdominus (RA) and/or lateral abdominal muscles group (LA) (mm. transversi and mm. obliqui ext. et int.) were stimulated. For RA stimulation, electrodes were placed just below the costal margin and above symphysis bone near the median line. For LA stimulation, the electrodes were placed parallel to m. rectus abdominis muscle and in the lumbar region.

Flow and Volume Measurements

Tidal volume (V_T) and total ventilation (V_e) were measured with pneumotachograph. Data was collected with BIOPAC acquisition system. In subject KL, MM and SR ventilation was measured also by 3-D optoelectronic plethysmography, which has been shown to correlate well with volumes measured with the pneumotachograph [6].

Results

Mean values of V_T and Ve during PNS alone and PNS with FESAM during either inspiration or expiration in supine position are shown in Table II. The PNS breathing rate was individually adjusted varying from 8 to 14 breaths per minute.

TABLE II

Values of V_T and Ve during PNS alone and PNS with FESAM applied during inspiration (i) or expiration (e). Mean percentage changes in parenthesis

	PNS	PNS + FESAMe	PNS	PNS + FESAMi
VT (ml)	455±23	567±78 (+25%)	462±24	484±53 (+5%)
Ve (l/min)	4.7±0.26	5.8±0.75 (+23%)	5.05±0.21	5.1±0.5 (+1%)

In figure 1 the mean percent of partial volume changes of the trunk (UT = upper thorax, LT = lower thorax, AB = abdomen) during PNS and PNS with FESAM during expiration in sitting position in subjects MM, and SL are shown. In figure 2 the same parameters are given for subjects MM and KL in supine position. In figure 3 the volume changes of different parts of trunk during PNS and PNS with FESAM during inspiration and expiration in supine position are shown (subject MM). In figure 4 the same parameters are given for subjects MM in sitting position.

Discussion/Conclusions

The effects of FESAM on augmentation of pulmonary ventilation in neurologically intact subjects and individuals with SCI who can breathe spontaneously have been demonstrated in previous studies [3, 4]. The effects of FESAM in individuals with SCI has not been previously reported. In neurologically intact subjects the increase of V_T with FESAM was due to proportional increase of all trunk compartments, whereas in individuals with SCI who could breathe spontaneously FESAM induced increases in V_T were mainly due to volume changes of abdominal compartment, but significant increase of lower thorax compartment were also observed.

The situation in individuals with injury level above C3 is different, because all respiratory muscles including diaphragm are denervated and mechanical properties of the chest are changed. Normal movement of the rib cage is dependent on normal function of intercostal and other rib cage muscles. The diaphragm interacts with the lower rib cage via its muscular insertions and the zone of apposition, but these actions are dependent upon increases in abdominal pressure during inspiration. Paralysis of the abdominal musculature and high abdominal compliance [7,8] leads to a small zone of apposition and to small abdominal pressure changes during inspiration. To compensate for these mechanical changes in individuals with SCI who have intact diaphragm function or PNS, ventilation can be maintained by compensatory increases in neural output to the diaphragm. Practical immobility of the rib cage represents higher demands for diaphragm and high risk for respiratory complications (e.g., pneumonia, atelectasis).

The results of present study show that FESAM during expiration can significantly increase V_T and V_e in the supine position, but volume distribution is not changed in desired direction, which is more physiological during PNS (Fig. 1). In supine position (Fig. 2) during FESAM we observed an increase in volume changes of LT, which is promising, because these individuals spend most of their time in a supine position. Volume distributions varied between individuals, but all of them showed small contribution of rib cage to V_T, some compartments even moved paradoxically.

Changes of V_T with FESAM applied during inspiration were not significant. We observed small, but non-significant increases of V_T, but, more importantly, distribution of volume changes were in favor to increase the chest contribution to V_T (or reducing the paradoxical movement of the rib cage) and reciprocal lowering of abdominal contribution to V_T (Fig. 3 and 4).

Our observations indicate that FESAM applied in addition to PNS during inspiration or expiration can modify the effects of PNS. According to an individual’s demands FESAM can be applied during expiration in favor to reduce PNS parameters, or, it can be applied during inspiration if changes in compartmental volume distribution is desired. According to high risk of respiratory complications and PNS complications is clinical use of FESAM as an adjunct to PNS ventilatory support may be justified. The use of compartmental analysis by 3D optoelectronic plethysmography is optional for optimization of FESAM application.

References

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Acknowledgments:

The work on this project was partly funded by 4 FP EU BIOMED 2 project BREATH, No: PL 962363.

We thank the staff at The Spinal Cord Injury Center, Hamburg, with for assistance with performing these experiments, and the individuals at that center who participated in the experiments.