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, 537 (Pt 3), 999-1008

Postural Activity of the Diaphragm Is Reduced in Humans When Respiratory Demand Increases

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Postural Activity of the Diaphragm Is Reduced in Humans When Respiratory Demand Increases

P W Hodges et al. J Physiol.

Abstract

1. Respiratory activity of the diaphragm and other respiratory muscles is normally co-ordinated with their other functions, such as for postural control of the trunk when the limbs move. The integration may occur by summation of two inputs at the respiratory motoneurons. The present study investigated whether postural activity of the diaphragm changed when respiratory drive increased with hypercapnoea. 2. Electromyographic (EMG) recordings of the diaphragm and other trunk muscles were made with intramuscular electrodes in 13 healthy volunteers. Under control conditions and while breathing through increased dead-space, subjects made rapid repetitive arm movements to disturb the stability of the spine for four periods each lasting 10 s, separated by 50 s. 3. End-tidal CO(2) and ventilation increased for the first 60-120 s of the trial then reached a plateau. During rapid arm movement at the start of dead-space breathing, diaphragm EMG became tonic with superimposed modulation at the frequencies of respiration and arm movement. However, when the arm was moved after 60 s of hypercapnoea, the tonic diaphragm EMG during expiration and the phasic activity with arm movement were reduced or absent. Similar changes occurred for the expiratory muscle transversus abdominis, but not for the erector spinae. The mean amplitude of intra-abdominal pressure and the phasic changes with arm movement were reduced after 60 s of hypercapnoea. 4. The present data suggest that increased central respiratory drive may attenuate the postural commands reaching motoneurons. This attenuation can affect the key inspiratory and expiratory muscles and is likely to be co-ordinated at a pre-motoneuronal site.

Figures

Figure 1
Figure 1. Experimental set-up
The subjects stood with their left arm attached to a potentiometer. Dead-space was increased by breathing through a tube. Visual feedback of the amplitude of arm movement was provided.
Figure 2
Figure 2. Change in respiratory variables with quiet breathing and breathing with increased dead-space
Mean data for each group are shown for (A) minute ventilation, (B) end-tidal CO2 and (C) proportion of rib cage relative to abdominal motion during inspiration. Measurements made during periods of arm movement (^); and those made during periods of breathing without arm movement (•). Asterisks between the ^ and • indicate a significant difference between the two conditions, i.e. with and without arm movement, and asterisks above or below the pairs of data points indicate a difference between measurement epochs. Note the increase in minute ventilation, end-tidal CO2 and proportion of rib cage motion during hypercapnoea. Data are presented as means ±s.e.m.
Figure 3
Figure 3. Electromyographic data during quiet breathing and hypercapnoea
Electromyographic (EMG) and rib-cage movement are shown for a representative subject. Note the increase in respiratory rate, diaphragm EMG during inspiration and the respiratory modulation of transversus abdominis EMG during dead-space breathing.
Figure 4
Figure 4. Raw data (upper panels) and power spectra (lower panels) for three periods of arm movement
Data from a representative subject are shown for the first, second and fourth arm movement period. The frequencies of breathing and arm movement are indicated by the vertical dashed lines in the power spectra. The respiratory rate increased with dead-space breathing. Ventilation was increased (see increased rib-cage movement, and diaphragm and transversus abdominis electromyography (EMG)) during the second and fourth arm movement periods. The third movement period (not shown) did not differ from the second and fourth periods. By the second period of arm movement, there was no peak in the power spectrum of diaphragm EMG at the frequency of arm movement.
Figure 5
Figure 5. Electromyographic (EMG) power for three muscles at the frequencies of respiration and arm movement
The amplitude of power in the EMG spectra at the frequency of respiration (upper panels) and frequency of arm movement (lower panels) are shown. Note the reduction in power of diaphragm EMG at the frequency of arm movement in the second period of arm movement (60-70 s) and the corresponding increase in power at the frequency of respiration. A similar trend, although not significant, can be seen for transversus abdominis. * Significant difference between periods of arm movement.
Figure 6
Figure 6. Variation in the amplitude of respiratory and trunk muscle EMG with respiration during arm movements
The EMG amplitude during inspiration (•) and that during expiration (^) are shown. All amplitudes are normalised to the peak EMG recorded during the 4 min period of dead-space breathing. The EMG amplitudes during respiration at the end of the dead-space breathing are shown for comparison. Note the reduction in amplitude of diaphragm EMG during expiration in the second and subsequent arm movement periods. Erector spinae EMG increased during limb movement, but it was not modulated with respiration. * Significant differences in EMG amplitude between periods of arm movement for a specific respiratory phase.
Figure 7
Figure 7. Pga modulation with arm movement
A, raw data from a representative subject. B, group data (n = 3) for the mean Pga and peak-to-peak amplitude of Pga with arm movement (m) and respiration (r). In A, a short period before and during arm movement is shown. The horizontal dashed line indicates the mean during the arm movement period. The mean Pga and the amplitude of the peak-to-peak changes in Pga with arm movement and respiration, expressed as a percentage of the peak Pga amplitude and averaged across the subjects, is shown in B. Note the reduction in mean Pga and peak-to-peak Pga amplitude for arm movements, and the increase in peak-to-peak Pga amplitude with respiration as respiratory demand is increased. * Significant differences between periods of arm movement.

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