Open Access

Spatiotemporal regulation of the cough motor pattern

  • Cheng Wang1,
  • Sourish Saha2,
  • Melanie J Rose1,
  • Paul W Davenport1 and
  • Donald C Bolser1Email author
Cough20095:12

DOI: 10.1186/1745-9974-5-12

Received: 3 March 2009

Accepted: 22 December 2009

Published: 22 December 2009

Abstract

The purpose of this study was to identify the spatiotemporal determinants of the cough motor pattern. We speculated that the spatial and temporal characteristics of the cough motor pattern would be regulated separately. Electromyograms (EMG) of abdominal muscles (ABD, rectus abdominis or transversus abdominis), and parasternal muscles (PS) were recorded in anesthetized cats. Repetitive coughing was produced by mechanical stimulation of the lumen of the intrathoracic trachea. Cough inspiratory (CTI) and expiratory (CTE) durations were obtained from the PS EMG. The ABD EMG burst was confined to the early part of CTE and was followed by a quiescent period of varying duration. As such, CTE was divided into two segments with CTE1 defined as the duration of the ABD EMG burst and CTE2 defined as the period of little or no EMG activity in the ABD EMG. Total cough cycle duration (CTTOT) was strongly correlated with CTE2 (r2>0.8), weakly correlated with CTI (r2<0.3), and not correlated with CTE1 (r2<0.2). There was no significant relationship between CTI and CTE1 or CTE2. The magnitudes of inspiratory and expiratory motor drive during cough were only weakly correlated with each other (r2<0.36) and were not correlated with the duration of any phase of cough. The results support: a) separate regulation of CTI and CTE, b) two distinct subphases of CTE (CTE1 and CTE2), c) the duration of CTE2 is a primary determinant of CTTOT, and d) separate regulation of the magnitude and temporal features of the cough motor pattern.

Background

Cough is an important airway defensive behavior. It is characterized by coordinated ballistic-like bursts of activity in inspiratory and expiratory muscles. Airflows during intensive coughs can reach 12 L/s in humans [1]. Although it has been proposed that cough and breathing share a common neurogenic control system [2], significant regulatory differences exist between the two behaviors. For example, during eupnea, there are well-known relationships between inspiratory volume (VI) and inspiratory time (TI) and between expiratory volume (VE) and expiratory time (TE). Smaller VI or VE are associated with longer TI or TE durations during breathing [3]. This volume timing behavior is mediated by slowly adapting pulmonary stretch receptors (PSR) However, Romaniuk et al [4] suggested that phasic PSR afferent feedback does not play an important role in the development of cough. This suggestion was supported by our previous study in which we found that there was no relationship between volume and phase durations during repetitive tracheobronchial coughing in spontaneously breathing cats [5]. These observations indicate that the regulation of cough motor pattern is fundamentally different than that of breathing. It follows that presumptions of how the cough motor pattern is controlled that are based on our knowledge of the control of the pattern of breathing may be subject to significant error.

In preliminary experiments, we observed that a period of expiratory motor quiescence existed between the end of the expiratory motor burst and the onset of the next inspiration during repetitive cough, consistent with the existence of two subphases within the cough expiratory period [4, 6], as first proposed by Romaniuk et al [4]. The presence of two subphases within the expiratory interval of cough is consistent with the control of the expiratory interval during breathing, and if substantiated, would be consistent with the synaptic network model of Shannon and coworkers for cough [2] which accounts for expiratory motor discharge that occurs largely restricted in the early portion of the expiratory phase. However, the extent to which this network model can fully account for spatiotemporal features of the cough motor pattern is not well understood. A significant limiting factor in testing this model is the relative lack of experimental information regarding the control of cough phase durations and intensity. In this study, we investigated the spatiotemporal features of the cough motor pattern during repetitive coughs. We hypothesized that the expiratory period during cough is composed of two subphases each of which is regulated separately. Furthermore, we speculated that the duration of the expiratory interval is a primary determinant of the total cough cycle time.

Methods

Fifteen cats (3.6 ± 0.3 kg) were anesthetized with pentobarbital sodium (35 mg/kg iv). Supplemental anesthetic was administered when necessary (5 mg/kg, iv). Atropine sulfate (0.1 mg/kg, iv) was administered to block reflex airway secretions. The trachea, femoral artery, and femoral vein were cannulated in all animals. The animals were allowed to spontaneously breathe room air. Blood pressure (mean 139 ± 5 mm Hg) and body temperature were continuously monitored. End-tidal PCO2 was continuously monitored all animals but only recorded (36 ± 1 mm Hg) in 11/15 animals. Body temperature was controlled by a heating pad and maintained at 37.5 ± 0.5°C.

Electromyograms (EMG) of respiratory muscles were recorded with bipolar insulated fine wire electrodes by the technique of Basmajian and Stecko [7]. EMGs were recorded from the transversus abdominis or rectus abdominis (ABD, expiratory) muscles and parasternal (PS, inspiratory) muscles. The PS electrodes were placed at T3 proximal to the sternum after exposing the ventral surface of the muscle. Transversus abdominis electrodes were placed 3-4 cm lateral to the linea alba. Rectus abdominis electrodes were placed about 1 cm lateral to the linea alba. Proper placement of each set of electrodes was confirmed by the appropriate inspiratory or expiratory phased activity during breathing and/or cough.

Repetitive tracheobronchial (TB) coughs were elicited by mechanical stimulation of the intrathoracic trachea with a thin flexible polyethylene cannula [8, 9]. For TB stimulation, the cannula was introduced into the extrathoracic trachea and advanced so that its tip was at the approximate location of the carina. The cannula was rotated at 1-2 Hz and retracted and advanced repeatedly across a distance of approximately 2 inches during the stimulus trial. However, movement of the trachea during coughing may have resulted in significant variations in how the cannula contacted the airway mucosa during the stimulus trials. Each cough stimulus lasted for 10 seconds. One to three minutes elapsed between stimulus trials.

Cough was defined as a sequence of a large burst in PS muscle EMG followed by a burst in ABD muscle EMG [8]. These criteria distinguished cough from other airway defensive behaviors such as expiration reflex [10, 11], augmented breath [12], and aspiration reflex [13, 14].

All EMGs were amplified, rectified, filtered (300-5000 Hz), and integrated (time constant 100 ms). The amplitude of the ABD muscle EMG, amplitude of the PS muscle EMG, cough inspiratory (CTI) and expiratory (CTE) durations were obtained from the moving averages of the EMGs. The PS and ABD muscle amplitudes were normalized to their peak amplitudes during cough in each animal. The phases of cough are illustrated in Figure 1. CTI is the duration from the onset to the peak of PS EMG burst. CTE was defined as the duration from the peak of PS EMG burst to the onset of the next PS EMG burst. CTE was further subdivided into two subphases CTE1 defined as the period of the expiratory muscle motor burst during cough and CTE2, a period of motor quiescence flowing the expiratory muscle motor burst. CTTOT is the duration from the onset of one PS EMG burst to the onset of the next PS EMG burst.
Figure 1

An example of individual phase duration relationships during a repetitive series of TB coughs. CTI - cough TI , CTE - cough TE (the sum of CTE1 and CTE2), CTTOT - total cough cycle time (the sum of CTI CTE1 and CTE2), CTE1 - cough expiratory subphase E1, CTE2 - cough expiratory subphase E2. Note CTE2 and CTTOT vary by over 100% in the selected cough cycles while CTI and CTE1 change very little. RA EMG- rectus abdominis (expiratory) muscle electromyogram, PS EMG-parasternal (inspiratory) muscle electromyogram.

Results are expressed as mean values ± SD. Data were analyzed by linear regression to determine the relationships between cough phase durations and amplitudes. The runs test was used to evaluate linearity of the data. We suggested based on our findings in the cat [15] that the anterolateral abdominal muscles acted as a unit during cough. As such, the normalized data from both abdominal muscles were pooled for the correlation analysis. Multiple regression analysis (stepwise regression) was performed to identify primary determinants of the cough cycle time, in which CTTOT was applied as the dependent variable and CTI, CTE1, CTE2, inspiratory EMG amplitude, and expiratory EMG amplitude were the independent variables. For clarity, the squares of linear regression correlation coefficients were designated as r2, and multiple regression coefficients of determination were designated as R2. Multiple collinearity analysis identified these variables as unrelated to one another. CTE was not included in the multiple regression model because multiple collinearity analysis identified this variable as strongly related to CTE2. To identify the relative contributions of each independent variable to the variance in CTTOT, we conducted a stepwise exclusion protocol in which each of these factors were removed from the dataset and the R2 recalculated [16]. Thus, the contribution of each variable to the variability in CTTOT could be inferred.

Results

A total of 1093 tracheobronchial coughs were elicited in 15 animals. Repetitive tracheobronchial coughing was characterized by sequential inspiratory and expiratory bursting separated during the expiratory phase of each cough cycle by intervals of relative motor quiescence (Fig. 1). These motor quiescent intervals had highly variant durations, even during an ongoing series of repetitive coughing (Fig. 1). Based on these observations, we have separated the cough cycle into three phases: cough inspiratory (CTI), cough expiratory phase 1 (CTE1), and cough expiratory phase 2 (CTE2). CTI is defined by the duration of the inspiratory phase (Fig. 1). CTE1 is the period of ballistic-like expiratory motor discharge (Fig. 1) and CTE2 is the period of relative motor quiescence between the end of CTE1 and the onset of the next CTI (Fig. 1). In some cases, tonic activity in ABD EMGs could be observed during CTE2, but it was clearly distinguished from the ballistic-like expiratory motor bursting during CTE1. Furthermore, tonic activity could sometimes be observed in the ABD EMGs during CTI, but this activity was much smaller in amplitude than the ABD burst during CTE1. We have observed this expiratory co-activation with inspiratory muscles before and have termed it pre-expulsive activity [15].

For all coughs the mean total cough cycle time was 1.76 ± 0.81 s. Phase durations for cough were: CTI = 0.49 ± 0.25 s. CTE1 = 0.31 ± 0.16 s, and CTE2 ± 0.96 ± 0.67 s. The average cough inspiratory amplitude was 49 ± 24%% of maximum. The average ABD EMG amplitude was 51 ± 23% of maximum.

Transient increases in the frequency of coughing within a bout were associated with a larger relative decrease in CTE2 (Fig. 2). Regression analysis revealed strong linear correlations between CTTOT and CTE2 (r2 = 0.89 ± 0.04). A weak correlation existed between CTTOT and CTI (r2 = 0.24 ± 0.05). There were no significant relationships between CTTOT and CTE1 (r2 = 0.09 ± 0.03), inspiratory (r2 = 0.07 ± 0.02), or expiratory amplitudes (r2 = 0.11 ± 0.03) and CTTOT (Table 1). There was only a weak correlation between inspiratory and expiratory amplitudes during cough (r2 = 0.29 ± 0.05, Table 2). Values for r2 for relationships between the other variables were all less than 0.13 (Table 2).
Table 1

Correlation coefficients (r2) from regression relationships between CTTOT and phase durations and EMG amplitudes during repetitive TB coughs in individual animals.

Animal

CTTOT Simple Linear Regression Coefficients (r2)

 

CTI

CTE1

CTE2

CTE

I Amp

E Amp

1

0.48

0.01

0.93

0.93

0.02

0.04

2

0.48

0.06

0.87

0.87

0.00

0.04

3

0.20

0.04

0.86

0.86

0.04

0.16

4

0.07

0.46

0.98

0.98

0.02

0.16

5

0.57

0.07

0.90

0.90

0.003

0.05

6

0.24

0.16

0.93

0.95

0.00

0.15

7

0.49

0.02

0.35

0.35

0.05

0.0009

8

0.32

0.0007

0.92

0.96

0.24

0.32

9

0.17

0.10

0.98

0.99

0.19

0.29

10

0.26

0.003

0.88

0.95

0.05

0.02

11

0.05

0.10

0.98

0.99

0.08

0.13

12

0.18

0.17

0.92

0.94

0.04

0.01

13

0.008

0.14

0.94

0.87

0.27

0.30

14

0.001

0.07

0.98

0.97

0.02

0.03

15

0.08

0.017

0.96

0.84

0.06

0.01

 

0.24 ± 0.05

0.09 ± 0.03

0.89 ± 0.04

0.89 ± 0.04

0.07 ± 0.02

0.11 ± 0.03

The only high r2 value is for the relationship between CTTOT and CTE2.

Table 2

Average correlation coefficients (r2) from regression relationships between cough phase durations and EMG amplitudes during repetitive TB coughs.

 

Simple linear regression coefficients for cough phase or EMG magnitude (r2 ± SE)

 

CTI

CTE1

CTE2

I Amp

E Amp

CTI

X

0.03 ± 0.01

0.09 ± 0.02

0.08 ± 0.03

0.05 ± 0.01

CTE1

X

X

0.09 ± 0.03

0.04 ± 0.01

0.1 ± 0.03

CTE2

X

X

X

0.07 ± 0.02

0.12 ± 0.02

I Amp

X

X

X

X

0.29 ± 0.05

There were only weak correlations between individual phase durations and a moderate relationship between inspiratory and expiratory EMG amplitudes during coughing.

Figure 2

Regression relationships between cough phase durations and amplitudes during TB coughs from one animal. Strong linear relationships exist between CTTOT and CTE and CTE2 but CTI and CTE1 appear to be relatively constant in spite of a 300% variation on CTTOT. I amp-inspiratory muscle EMG amplitude, E amp-expiratory muscle EMG amplitude.

Multiple regression analysis of CTTOT to CTI, CTE1, and CTE2 showed that R2 only decreased by 0.08 when CTI was excluded from the equation, and 0.034 when CTE1 was excluded. This suggested the exclusion of CTI had a minimal effect on CTTOT. The R2 value decreased by 0.67 when CTE2 was excluded from the analysis, suggesting CTE2 was the most important contributor to CTTOT.

Discussion

The first major finding of this study was that cough expiratory phase can be divided into two subphases, CTE1 and CTE2. The second finding of this study was that CTE, mainly CTE2, is the primary determinant of CTTOT. Fluctuations in the duration of CTTOT are primarily the result of increases or decreases in CTE2. Given that EMG burst amplitudes were not correlated with phase durations during cough, our results also suggest separate regulatory mechanisms for the intensity and cycle durations of cough.

This is the first report to quantify that the expiratory phase during coughing, like that of breathing, can be composed of two phases. This concept was first proposed by Romaniuk and coworkers [4], but some of the temporal relationships that we illustrate in Figure 1 can be seen in figures in studies published by other groups [17, 18]. In fact, Korpas and Tomori [18] show figures that suggest that periods of motor quiescence in the expiratory period during repetitive coughing exist in cats (Fig 32, p. 76), rabbits (Fig 42, p. 107), and in a separate study, dogs [19] (Fig 1A). During breathing, the activity patterns of spinal respiratory motoneurons have been used to subdivide the expiratory phase into two phases, the postinspiratory phase (E1) and active expiration phase (E2) [2025]. The E1 phase of breathing represents the "passive" stage of expiration in which chest wall and abdominal muscles are relatively quiescent. The E2 phase can be associated with "active" expiration in which chest wall and abdominal muscles can exhibit an augmenting discharge [22, 26]. Our evidence for the existence of two phases of the expiratory interval during cough is primarily based on observations related to the expulsive motor burst and the existence of a variable duration of the subsequent motor quiescence. The E1 and E2 phases during cough differ significantly from those of breathing. For example, CTE1 is demarked by ballistic expiratory motor activation, whereas this phase during breathing represents a period of relative quiescence of expiratory pump muscles [4, 26]. During CTE2, there is a period of motor quiescence, and during breathing E2 expiratory pump muscles can be very active [4, 22].

Our study showed that the duration of the CTE1 phase during repetitive coughing is relatively fixed and that the duration of CTE2 is variable. Romaniuk reported CTE was prolonged during obstructed cough in which the trachea was occluded at the end-inspiration and maintained throughout the subsequent expiration [4]. Our results are consistent with the idea that the enhanced vagal afferent stimulation resulting from airway occlusion has a preferential effect to prolong the duration of CTE2.

Poliacek et al. reported [27] that CTI during laryngeal coughs was 50% longer than during TB coughs, and the two types of coughing had similar CTE1 durations in the present study. In our protocol, bouts of repetitive TB coughs were elicited, whereas Poliacek et al. [27] elicited mostly single coughs. Furthermore, the results of our previous study, showed that CTI during single TB coughs or first coughs of a bout is significantly longer than during repetitive coughs [5]. These observations indicate that some features of the motor pattern of coughing can exhibit a high degree of variation depending upon the region of the airway from which it is elicited and whether single or repetitive behaviors are produced. In essence, all coughs are not the same, even within a series of repetitive coughing. However, selected components of the cough motor pattern are fixed, such as the duration of CTE1.

The lack of relationship between inspiratory and expiratory motor burst amplitudes differs from that reported previously for the fictive cough model in the cat by our group [28]. In that study, we showed that there was a linear relationship between inspiratory and expiratory neurogram amplitudes during fictive cough that was disrupted by codeine. The effect of codeine was manifest at doses that did not significantly suppress either inspiratory or expiratory amplitudes, but were sufficient to reduce cough number. The results of that study were consistent with the existence of a neurogenic mechanism for coordinating inspiratory and expiratory motor drive during coughing that was separate from simple inhibition of excitatory motor drive to one or both of the motoneuron pools. In the fictive model, cough is produced in the absence of active or passive muscle movement in decerebrated, paralyzed animals [2, 9, 13]. Therefore, the contribution of sensory feedback from active muscle movement to the cough motor pattern generator is eliminated. The rate of lung inflation during cough in the fictive cough model is typically similar to that during fictive breathing and peak lung volume is likely to be less than that produced in spontaneously breathing animals, presumably resulting in altered pulmonary afferent feedback. It is conceivable that these differences in somatic and pulmonary afferent feedback this may cause changes in the cough motor pattern in the fictive model relative to the spontaneously breathing preparation. However, we believe that the absence of a coordinating mechanism between inspiratory and expiratory motor drive in spontaneously breathing animals is most likely related to the presence of anesthesia. Sodium pentobarbital was used in the present experiments and this anesthetic has been successfully employed in cough studies for many years [13, 18, 29]. Cats are capable of producing intense coughing while anesthetized with sodium pentobarbital.

Our results are consistent with the concept that the synaptic model of Shannon and coworkers can account for expiratory phase durations during cough. In Shannon's model, the expiratory augmenting (E-Aug) neurons in the Botzinger complex consist of at least two subpopulations based on their discharge patterns during cough [2]. As such, these synaptic relationships governing the discharge patterns of rostral ventral respiratory column expiratory neurons could account for a cough expiratory interval composed of two subphases. Our results are significant in that they show that the expiratory interval during cough is, in fact, controlled in this fashion. Furthermore, our findings extend our understanding of the regulation of the motor pattern of respiratory muscles by the respiratory pattern generator.

It is not clear how the model of Shannon and coworkers can account for a fixed CTE1, while CTE2 is highly variant. Our data showed that the CTE1 was independent of ABD burst intensity, CTTOT, CTE, and the previous CTI. Our data also indicate that the duration of CTE2 determines CTTOT length. Based on these observations and inspection of the model of Shannon and coworkers, when the frequency of repetitive cough is increased (i.e. CTE2 and thus CTTOT decreased), inspiratory decrementing neurons should have a stronger inhibition on the activity of the E-Aug late neurons, an action which would shorten CTE2. But the model cannot answer the question why CTE1 duration is not also reduced when CTE2 decreases by 50% or more (Fig 1). Our observation that CTE1 is relatively invariant indicates that this phase also has an upper limit in duration.

Correlation analysis showed that there was no relationship between cough expiratory amplitude and CTE1 duration. Similarly, there was no correlation between the inspiratory amplitude and CTI. These results are consistent with a previous study, showing there was no relationship between expiratory volume and CTE, or between inspiratory volume and CTI [5]. These observations are not consistent with what is predicted from Shannon's model. According to this model, input from rapidly adapting receptor relay neurons excites neurons that regulate both inspiratory and expiratory phase durations as well as E-Aug early neurons, expiratory premotor neurons, and inspiratory augmenting premotor neurons that that provide excitatory motor drive to spinal expiratory and inspiratory motor pathways. This feature of the model suggests that the magnitude of expiratory motor activation during cough should be related to expiratory phase duration, and the magnitude of inspiratory motor activation should be related to inspiratory phase duration.

It should be noted that the cats in our preparation were spontaneously breathing whereas Shannon's experiments were based on a fictive cough model. In the fictive model, cough was produced in the absence of active or passive muscle movement in decerebrated, paralyzed animals [28, 30, 31]. Therefore, the contribution of sensory feedback from active muscle movement to the cough motor pattern generator was eliminated. The rate of lung inflation during cough in the fictive cough model is typically similar to that during fictive breathing and peak lung volume is likely to be less than that produced in spontaneously breathing animals, presumably resulting in altered pulmonary afferent feedback. It is conceivable that these differences in somatic and pulmonary afferent feedback may cause changes in the cough motor pattern in the fictive model relative to the spontaneously breathing preaparation. Furthermore, we stimulated repetitive cough whereas Shannon used single cough stimulation. It has been reported that the first cough in a series or a single cough compared to repetitive coughs has different cough motor patterns [5].

Conclusions

Our findings provide information regarding the functional organization of the central neurogenic mechanism for cough. Reconfiguration of the respiratory pattern generator to produce coughing not only changes the arrangement of the respiratory neural network but it also changes fundamental features that govern how the motor pattern is controlled. Cough and breathing differ in that: a) motor drive and phase durations are controlled separately for cough, and b) the E2 subphase is the dominant regulator of total cycle duration for cough.

Abbreviations

ABD: 

abdominal

CTI

cough inspiratory time

CTE

cough expiratory time

CTE1

first segment of cough expiratory phase

CTE2

second segment of cough expiratory phase

CTTOT

total cough cycle time

E1: 

postinspiratory phase of breathing

E2: 

active expiratory phase of breathing

E-Aug: 

expiratory augmenting neuron

EMG: 

electromyogram

E-amp: 

expiratory amplitude

I-amp: 

inspiratory amplitude

PC02: 

partial pressure of exhaled carbon dioxide

PSR: 

pulmonary stretch receptor

PS: 

parasternal muscle

RA: 

rectus abdominis

SD: 

standard deviation

TB: 

tracheobronchial

TE

breathing expiratory time

TI

breathing inspiratory time

VE

expired volume during breathing

VI

inspired volume during breathing.

Declarations

Acknowledgements

Supported by HL 70125, HL 89104, James and Esther King Biomedical Research Program BM-040.

Authors’ Affiliations

(1)
Department of Physiological Sciences, College of Veterinary Medicine, University of Florida
(2)
Department of Statistics, College of Liberal Arts and Sciences, University of Florida

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© Wang et al; licensee BioMed Central Ltd. 2009

This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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