HABITUATION OF VENTILATORY RESPONSE DURING
F Kandare1, U Stanič2, J Jeraj2, J Šorli1, and R J Jaeger3
1University Clinic of Respiratory and Allergic
Diseases, 4204
2lnstitute Jožef Stefan, Jamova 39, 1000
3Research Service, Edward Hines Jr. VA Hospital,
ABSTRACT
In
five normal subjects the effects of prolonged functional electrical stimulation
(
After an
initial increase of pulmonary ventilation up to 90 % of the quiet breathing
value in some subjects, it declined exponentially to a plateau value of about
30 % over the initial value. The same
behavior was observed with tidal volume, while the breathing rate after initial
increase remained constant during stimulation. The effects of stimulation are
similar between the groups of stimulated muscles and protocols used.
Irrespective of the underlying mechanism(s) for these observations, the
alternative mode of stimulation of different abdominal muscles (RA and LA
muscles groups) could possibly offer better results during prolonged clinical
applications.
Keywords:
INTRODUCTION
Pulmonary
ventilation is continuous cyclic process enabling adequate respiratory gas
exchange from birth to death. If significant disturbances in this process
occur, lifespan can be reduced. Both
invasive and noninvasive methods using different devices to overcome the
periods of insufficient ventilation have been described [1,2]. All devices and
systems are prepared for long-term use enabling adequate ventilation according
to metabolic demands without significant harm to respiratory system. Previous
experiments [3] with functional electrical stimulation of abdominal muscles showed,
that significant augmentation of pulmonary ventilation was achieved in
short-term experiments. This method
might be clinically applicable if the effects could be maintained for
sufficiently long periods to fulfill clinical demands.
The
purpose of this study was to assess the long-tem effects of functional
electrical stimulation of abdominal muscles on pulmonary ventilation in
neurologically intact subjects with average physical fitness.
METHODS
Subjects:
Five neurologically intact subjects (one female and four males, avg. age 27
years) were tested. The pulmonary
function tests of all five subjects were within normal limits, the subjects had
no current pulmonary complaints. The study was approved by National Ethic
Committee of Republic of Slovenia and written informed consent was
obtained from each subject.
Evaluation
of pulmonary function: Pulmonary ventilation and other metabolic parameters
were measured breath by breath with an Oxycon Beta System (TripleV system for
flow measurements, paramagnetic O2 analyzer, fast infrared CO2
analyzer) and average minute values were calculated. Maximal inspiratory and
expiratory (MIP, MEP) pressures were measured with Pmax fi. Morgan.
Electrical
stimulation: Stimulation of abdominal muscles were performed with a custom-made
electrical stimulator [4] through surface self adhesive stimulating electrodes
(PALS Axelgard, rectangular 5x 9 cm) placed on the abdominal wall. For rectus
abdominis muscles stimulation, electrodes were placed just below costal margin
and above syphysis bone near the median line. For stimulation lateral abdominal
group of muscles (external and internal oblique and transversus abdominis
muscles), the electrodes were placed in vertical direction parallel to m.
rectus abdominis muscle and in the lumbar region. Stimulation parameters were:
1 second pulse train at 45 Hz, amplitude
of stimulation 50-70 mA, and pulse width 150 ms. For each subject
the initially selected amplitude was constant over all protocols.
Protocol:
Experiments were performed in supine position using three different protocols.
1. Ten minutes of quiet breathing, three
hours of RA stimulation, ten minutes of quiet breathing
2. Ten minutes of quiet breathing, followed
with ten minutes of RA stimulation with progressive periods of quiet breathing
from one to 5 minutes and corresponding shortening of stimulation time
3. Same protocol as 2, with alternating
stimulation of RA and LA to the level of 5 minute alternating RA/LA stimulation
Each
protocol was performed on separate days in the same period of day from 1 to
5pm. Time between experiments was 1 week or more.
RESULTS
The mean values of ventilatory and
other parameters during quiet breathing and period of 3 hours of FES are shown in table 1. In Figure 1. time
course of breathing rate and total
ventilation
obtained during first protocol is shown. The breathing rate with an average
increase of 3 breaths per minute, is nearly constant during FES, whereas
ventilation, after peaking in the first 10 minutes, decline to a constant value
due to tidal volume lowering. Maximal inspiratory and expiratory pressures
before and after 3 hours of stimulation are not significantly different (MIP:
99±26 mm Hg / 104±38 mm Hg and MEP: 128±30 mm Hg /134±34 mm Hg respectively). The results of
ventilation measurements versus cumulative time of stimulation for RA
constructed from values obtained from all three protocols and LA from third
protocol are shown on Figure 2. The values of total ventilation after the
initial increase, decline in an exponential manner reaching a new basal level
after approximately 20 minutes, irrespective of the various stimulating and
non-stimulating periods in the different protocols. CO2 elimination
showed the same pattern as ventilation reaching new basal value at the same
time as ventilation did.
DISCUSSION
In normal
subjects during quiet breathing, the abdominal muscles are usually not
significantly involved in expiration although some observations have showed
activity of these muscles during quiet breathing [5]. In the state of increased metabolic activity
or increased respiratory load, as in COPD, these muscles do become activated
during quiet breathing [6]. For maintaining adequate pulmonary ventilation
stimulated muscles must have ability to repeatedly contract and relax over long
periods of time. In short-term
experiments it has been proved that FES of abdominal muscles augments pulmonary
ventilation due to tidal volume increase.
As is
shown in the present experiments of three hours duration, after the initial
increase, ventilation declined and in about 10 to 15 minutes reached a plateau
which lasted until the end of the stimulation period. A similar pattern of
decline in ventilation was observed when the stimulation was interrupted with
periods of quiet breathing. There are more possible explanations of these
observations. Rhythmic contractions as are performed during FES are not
normally observed and are therefore unusual, producing the signs of stimulated
muscular fatigue sooner as observed during physiologic activities and
low-frequency fatigue of the muscle can be responsible for lowering of initial
tidal volume increase. Similar observations were found after voluntary
hyperventilation [7]. The unchanged maximal expiratory pressures after
prolonged stimulation indicated that high frequency fatigue was not
significantly involved.
The other
possible explanation is the habituation to constant stimulation, but this is
difficult to prove it. The subjects' report that their sensations of
stimulation during the initial period are changed and they feel decreased
intensity of stimulation lasted during residual part of experiment, although
the amplitude of stimulation was constant. Another plausible explanation of the
observations is lowered end-expiratory level (FRC) as a effect of FES of
abdominal muscles and possibly consequently lowered activity of inspiratory
muscles (diaphragm) or intervention of other regulatory mechanisms of
ventilation which were responsible for lowering tidal volume to near basal
values. All of these possible mechanisms
will be studied in future experiments.
Irrespective
of the mechanisms involved for prolonged FES pulmonary ventilation, data of the
present study support the alternative mode of stimulation of different
abdominal muscles (RA and LA muscles groups) as possibly offer better results
during prolonged application.
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