INTRODUCTION

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THE INVESTIGATION of respiratory sensation has been primarily concerned with determining the minimum level of added mechanical load to breathing, usually resistive or elastic, that subjects can detect and their ability to estimate the magnitude of detected loads (3, 4, 6, 7, 29). Scaling methods have been used to study the magnitude estimation (ME) of respiratory-load sensations (3, 6, 29). ME requires subjects to provide an estimate of the sensory magnitude of a suprathreshold load using a numerical scale or cross-modality matching. The perception of suprathreshold respiratory mechanical loads is described by Steven's Psychophysical Law: the magnitude of the sensation is a power function related to the stimulus intensity and type of stimulus.

The determination of the mechanical parameters that are transduced by the respiratory mechanoreceptors responsible for inspiratory load perception has been, however, difficult to study. One report suggested that diaphragm afferents monitoring the changes in diaphragmatic tension, as represented by the transdiaphragmatic pressure (Pdi), mediated the response (30). It reported a correlation between the load detection latency and the Pdi.

Inspiratory occlusion in humans elicits a respiratory-related evoked potential (RREP) recorded over the somatosensory region of the cerebral cortex (9). These evoked potentials are similar to the evoked responses recorded with mechanical stimulation of afferents in the hand and ankle (9). It was previously reported that the P₁ amplitude of the RREP was related to the ME of the load (19). Mouth pressure (Pm) has been used to trigger the collection of the electroencephalographic (EEG) activity, which was then averaged to obtain the RREP (9, 19, 23, 25). The RREP is an event-related potential that can only be observed if the parameter used to trigger signal averaging is related to the activation of the afferents eliciting the dipole recorded from the scalp. This means that a population of respiratory mechanoreceptors is activated by a mechanical change produced by an inspiratory load and is eliciting the P₁ peak of the RREP.

Inspiratory occlusion produces a large negative pressure change in Pm as the inspiratory muscles contract against the closed airway, resulting in decompression of the gas within the lung and tracheobronchial tree. During quiet breathing, the diaphragm is the primary muscle generating the inspiratory pumping force. Pdi is one measure of the diaphragmatic force generated during an inspiratory effort and is directly correlated to the Pm. Pdi is recorded as the difference between the esophageal pressure (Pes) and gastric pressure (Pga). The change in Pm during an occlusion is the result of the continued inspiratory effort against the closed airway and has been used as a measure of inspiratory drive (28). This increased negative Pm during the occlusion or inspiratory loading reflects a more negative pressure throughout the airways, a more negative transthoracic pressure (as measured by Pes), and an increased Pdi. It suggests that the relationship between the P₁ amplitude of the RREP and resistive-load magnitude (19) is positively correlated with the mechanical changes in the airway and respiratory muscles induced by inspiration against the load. Thus the greater negative Pm and Pes and increased Pdi are measures of mechanical forces produced by inspiratory loads that activate mechanoreceptors that project to higher brain centers and elicit the RREP. However, the correlations among the P₁ peak of the RREP, load perception, and inspiratory pressures are unknown. The purpose of this study was to determine the relationship among the resistive load magnitude, inspiratory pressures (Pm, Pes, and Pdi), and the RREP P₁ peak amplitude.

	METHODS

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All subjects were adults. They were informed of the nature of the study before starting the experiment, and consent was obtained. The Institutional Review Board, University of Florida, reviewed and approved this project. Thirteen men and two women, average age 36 yr, with no history of pulmonary or neurological disease participated in this study. At the beginning of the experiment, a standard set of instructions was presented to the subjects, informing them of their task. The subjects were told to respire as normally as possible. When they saw the light illuminated, they were to provide an estimate of the difficulty of breathing in on their next inspiration. After estimating that breath, they were told to continue respiring as normally as possible.

The subjects were seated comfortably in a reclining lounge chair with their back, neck, and head supported. Surface cup gold electrodes were placed at scalp positions C_Z, C₃, and C₄ according to the International 10/20 system. The ground electrode was attached to the right earlobe. All the surface electrode impedances were checked and adjusted until the impedances were <3 k. Bipolar EEG activity was recorded with C₃ and C₄ referenced to C_Z: C_Z-C₃ and C_Z-C₄. The scalp electrodes were connected to EEG amplifiers, band-pass filtered (0.3 Hz-1 kHz), and amplified (Neurodata Acquisition System, Grass Instruments, Quincy, MA).

The subject wore a nose clip and breathed through a mouthpiece connected to a nonrebreathing valve (Fig. 1). Care was taken to suspend the valve to eliminate the need for the subject to bite the mouthpiece yet maintain an airtight seal. The subject was instructed to relax all facial and mouth muscles. The manifold was hidden from the subject's view. Pm was recorded from a port in the center of the valve. Pm was displayed on an oscilloscope in front of the experimenter and used for timing the inspiratory interruption. The occlusion was presented after the onset of inspiration, usually during the first one-half of the inspiration, and timed by visual observation of the Pm pattern by the experimenter.

View larger version (34K): [in this window] [in a new window]   Fig. 1.   Schematic representation of experimental preparation. See text for explanation. Pdi, Pm, Pb, Pga, Pes: transdiaphragmatic, mouth, balloon, gastric, esophageal pressure, respectively; R0, no-load port.

In addition to EEG activity, Pm, Pes, Pga, and Pdi were recorded. A thin-walled latex balloon (length 10 cm, diameter 3.5 cm) was placed over a polyethylene catheter (inner diameter = 0.14 cm). Two balloon catheters were connected to two matched, calibrated differential pressure transducers (Micro Switch, 14PC). A topical anesthetic (Citacaine 2%) was applied to the oropharynx before each experiment to reduce the gag reflex, and each balloon was lubricated with 2% viscous xylocaine. Pga was measured by advancing one balloon catheter transnasally down the esophagus until there was a sharp rise in pressure during a sniff, indicating that the balloon entered the stomach (27). Pes was measured with a second, identical latex balloon catheter advanced transnasally until the balloon was in the middle one-third of the esophagus. During calibration, Pga and Pes sensitivities were adjusted to zero, and Pdi was determined electronically as the difference between Pga and Pes (2, 20). The subjects listened to music of their choice, which masked experimental noises and reduced any artifacts that might be related to auditory cues. The subjects were also asked to close their eyes throughout the experiment to prevent eye blinks and reduce visual distractions.

The subjects initially inspired through the minimum-resistance port of the loading manifold (Fig. 1). The balloon occlusion valve was connected to the opening of this port. The inspiratory load was presented by silently inflating the balloon during an inspiration, which closed the minimum-resistance port, interrupting the inspiratory effort, and channeled the inspired air through one of four manifold resistance ports. The occlusion-valve balloon pressure was recorded with a differential pressure transducer, amplified, and used to trigger the computer for data sample collection. Each load was separated by two to six unloaded breaths. The load was applied for a duration of ~500 ms. Three suprathreshold resistive loads were used: R₁ = 2.0, R₂ = 9.0, and R₃ = 21.0 cmH₂O · l¹ · s. Inflating the balloon with the no-load (R₀) port open served as the control.

EEG activity was monitored with an oscilloscope (Tektronix 5111A). The EEG activity, Pm, Pes, Pga, Pdi, and occlusion balloon pressure were led into an on-line signal-averaging computer system and digitized at 3 kHz (Cambridge Electronics Design). Each digitized sample was stored on computer disk for subsequent analysis.

For each subject, four inspiratory load levels were presented in an order to minimize temporal, order, and sequence effects as described previously (19). Subjects received four sets of 80 trials each, with a rest period taken between each of the sets. Within each set of 80, load trials were tested by using 16 blocks of five trials each, with the same load presented within a block to minimize manifold manipulations. Ordering of the different blocks was randomly determined with the restriction that each successive set of four blocks contained only one block of each load level. The subject provided a ME of the fifth load presentation in each load block. A light was illuminated to cue the subject to provide an estimate of the load on the next inspiration. Subjective rating of the perceived intensity of the resistive load was provided by using a modified Borg category scale, a 0-10 continuous scale used for measuring respiratory sensation (5). Thus, over the set of 80 trials, each block of five trials of a given load was presented four times (20 total trials). The randomization was independently drawn for each set of 80 trials and for each subject. There were four sets, making 80 presentations of each load (and no-load) magnitude available for signal averaging and 16 ME of each load (and no-load) magnitude. The time required for all four trials was ~2 h. Including subject preparation time, the entire study session lasted <3 h.

Data analysis. For each load presentation, a 500-ms epoch of EEG activity, Pm, Pes, Pga, and Pdi was stored on disk for subsequent computer signal averaging (SIGAVG, Cambridge Electronic Design). Each load magnitude was averaged separately. The computer stored the individual load trials in the order of presentation. The averaged response for an individual load magnitude was obtained as described previously (19). An individual presentation was included in the average if 1) there was a stable prestimulus EEG activity baseline, 2) no EEG transient activity exceeded ±50 µV, and 3) the onset of the Pm change related to the load aligned with previous presentations. This analysis was repeated for each load magnitude and the no-load control. There were no peaks in the no-load control average that corresponded to the peaks observed with the load-elicited RREP. However, to further control for contamination of the RREP by artifacts, the no-load control-averaged signals were subtracted from each R average.

	RESULTS

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The P₁ amplitudes, Pm, Pes, Pdi, Pdi slope, and ME significantly increased with increased inspiratory resistive load magnitude. The resultant changes in the P₁ peak of the RREP, Pdi, Pes, and Pm for one subject are presented in Fig. 2.

View larger version (24K): [in this window] [in a new window]   Fig. 2.   Respiratory-related evoked potential, Pdi, Pes, and Pm response for 1 subject are elicited by inspiratory interruption with the 21 cmH2O · l1 · s resistance. CZ-C3 and CZ-C4, electrode pairs.

There were significant differences (P < 0.05) among the reported ME for each R (see Fig. 5). There were no significant changes in Pga for the different resistive loads. The log-log plot of the group-averaged Pdi, Pes, and Pm against R demonstrated linear (Table 1) relationships (Fig. 3). There was no significant difference in the power function between Pdi and Pdi slope to R (Table 1). The relationship of the group-averaged P₁ amplitudes to R is presented in a log-log plot (Fig. 4).

View larger version (14K): [in this window] [in a new window]   Fig. 3.   Log-log relationship between Pdi, Pes, and Pm (±SE) and inspiratory resistive load. Pressure recorded was pressure measured at time of peak P1 amplitude. , Group mean Pdi; , group mean Pes; , group mean Pm.

View larger version (13K): [in this window] [in a new window]   Fig. 4.   Log-log relationship between amplitude of P1 peak and resistive load magnitude. , Group mean 0-peak P1 amplitudes (±SE) measured from CZ-C3 electrode pairs; , group mean 0-peak P1 amplitudes (±SE) measured from CZ-C4 electrode pairs.

The log-log plot of the group-averaged ME showed a linear (Table 1) relationship to resistance (Fig. 5). This study demonstrated a high correlation between all the response variables and R.

View larger version (11K): [in this window] [in a new window]   Fig. 5.   Log-log relationship between magnitude estimation (±SE) and resistive loads.

The slope of the linear regression equation of the log-transformed data relates the psychological magnitude to the physical magnitude. ME has a linear log-log relationship with the Pm, Pes, Pdi, Pdi slope, and the P₁ amplitudes. The relationships of ME as the response variable for Pm, Pes, and Pdi (Table 1) is represented in Fig. 6. There was no significant difference in the coefficient obtained for Pdi slope and Pdi as they are related to the ME of the load (Table 1). The ME of the load is linearly related to the P₁ amplitudes (Fig. 7).

View larger version (15K): [in this window] [in a new window]   Fig. 6.   Log-log relationship between magnitude estimation (±SE) and Pdi, Pes, and Pm (±SE). Pressure recorded was pressure measured at time of peak P1 amplitude. Symbols are as defined in Fig. 3 legend.

View larger version (14K): [in this window] [in a new window]   Fig. 7.   Log-log relationship between amplitude of P1 peak and magnitude estimation of resistive loads. Pressure recorded was pressure measured at time of peak P1 amplitude. Symbols are as defined in Fig. 4 legend. Group mean magnitude estimation (±SE) of same resistive loads presented as an interruption of inspiration are plotted on ordinate.

With the use of the P₁ amplitude as the response variable to pressure, direct linear relationships (Table 1) were found (Fig. 8). The relationship of Pm to the P₁ amplitude is the result of the relationship of Pm to the driving pressure of the respiratory apparatus. There was a linear relationship between Pm and Pdi for the log-transformed results recorded in this study: Pm = 0.7731 + 1.1827 Pdi (r² = 0.99).

View larger version (12K): [in this window] [in a new window]   Fig. 8.   Log-log relationship between amplitude of P1 peak and Pm. Pressure recorded was pressure measured at time of peak P1 amplitude. Symbols are as defined in Fig. 4 legend. Group mean peak Pm (±SE) of same resistive loads presented as an interruption of inspiration are plotted on abscissa.

Pdi is linearly related to the P₁ amplitude and is illustrated in the log-log graph of Pdi vs. the P₁ amplitudes (Fig. 9). There were no significant differences between the coefficients for Pdi and Pdi slope (Table 1).

View larger version (14K): [in this window] [in a new window]   Fig. 9.   Log-log relationship between amplitude of P1 peak and Pdi. Pressure recorded was pressure measured at time of peak P1 amplitude. Symbols are as defined in Fig. 4 legend. Group mean peak Pdi (±SE) of same resistive loads presented as an interruption of inspiration are plotted on abscissa.

	DISCUSSION

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This study demonstrated a relationship between the P₁ peak of the RREP and the inspiratory driving pressure. Although the specific afferents stimulated by a resistive load interruption of inspiration are unknown, the amplitude of P₁ peak of the RREP is correlated with the magnitude of the R-related change in driving pressure.

Increases in inspiratory extrinsic load change the pattern of airflow and pressure in the respiratory system and may be transduced by the afferents from the mouth, airways, and the respiratory muscles, including the diaphragm (4, 6-8, 11, 13, 16, 22, 26). It is particularly difficult to determine the specific mechanical parameter(s) mediating the perception of inspiratory mechanical loads because load-dependent changes in pressure, flow, and volume are transmitted throughout most of the respiratory tract and pump. Mechanoreceptors in the mouth and pharynx may be activated by rapid changes in Pm and may contribute to the RREP. However, large increases in Pm during hypercapnia did not change the amplitude of the P₁ peak (10). This suggests that negative Pm values <4 cmH₂O have minimal contribution to the RREP.

It has been previously hypothesized that inspiratory mechanical load sensation is mediated in part by inspiratory pump mechanoreceptors that transduce the load-related changes in muscle length and tension. During quiet breathing, Pdi is the sum of the decrease in Pes (which reflects pleural pressure) and any increase in Pga that may have occurred. Grimby et al. (15) suggested that the diaphragm works in concert with the rib cage muscles such that any change in Pdi is equal to the changes in the chest wall pressures and pleural pressures. Hence, Pdi reflects the driving pressure of the chest wall and is the best measure available, in human subjects, of the force produced by the diaphragm's contraction (21). Because there is a close relationship between Pdi and Pes, a proportion of Pdi attributable to Pes can vary, depending on the breathing pattern of the individual (14). This study found similar responses of Pdi and Pes to the resistive loads because there were no significant changes in Pga.

Several factors can affect the measurement of Pdi. The position of the relaxed diaphragm, and thus its position on its length-tension curve, depends on the subject's lung volume (1, 24). This factor did not influence the measurements in this experiment because all the subjects were at rest and breathing at their functional residual capacity. There can be a strong recruitment of accessory muscles during forceful inspiratory maneuvers, resulting in a low Pdi (12). However, the resistive loads were presented momentarily, as an interruption of inspiration, and it is doubtful that the Pdi measurement was significantly affected by accessory muscle activation. The pressure-related mechanosensory signal elicited by the loads occurs for a constant but unknown time (central conduction and neural processing time) before the P₁ peak, making the pressures recorded coincident with the P₁ peak a quantitative overestimate of these pressures. Yet, because the pressure is changing at a constant rate, the correlation of the P₁ amplitude response appears and remains closely associated with all measures of inspiratory pump force (Pm, Pes, Pdi, Pdi slope). This suggests that the forces generated by the respiratory muscle pump (and the associated mechanoreceptors) are correlated with the mechanical stimulus involved in eliciting the P₁ peak of the RREP.

The perceived magnitude of added loads is directly related to the inspiratory muscle force and indirectly related to the added load (18). Specifically, the rate and temporal pattern of Pdi augmentation precedes load detection and increases in response to graded elastic or resistive inspiratory loads (30). This experiment documented an increase in Pdi and its rate of change in response to a resistive load. Thus changes in inspiratory muscle pumping forces may play a role in inspiratory load perception. The similarities in the power function for Pdi and Pes are not surprising as these parameters are directly related to each other. In addition, Pm is related to Pdi because, during inspiration against an extrinsic load, Pm is a function of the respiratory system's driving pressure.

Previous studies strongly suggested that changes in diaphragmatic tension, as reflected by pressure changes within the respiratory system, contribute to respiratory sensation (3, 30). This study found that the psychophysical and neural events are described by the same power function, suggesting a basic relationship among Pm, Pes, Pdi, the RREP P₁ amplitude, and the ME of the resistive load. Although the principal afferents responsible for the RREP are not known, they probably are associated with the airways, diaphragm, and chest wall, which are mechanically linked to the driving pressure of the respiratory apparatus during normal breathing. Further studies will be necessary to delineate the role of the specific afferents that combine to elicit the P₁ peak of the RREP and resistive load perception.

In summary, humans can perceive and estimate the magnitude of ventilatory loads by mechanisms not clearly understood. A RREP can be recorded over the somatosensory cortex, and its P₁ peak amplitude increases with increases in a resistive load. The observed log-log relationship between Pm, Pes, and Pdi and the P₁ peak amplitude is in turn directly related to the ME of the load. It is, therefore, concluded that ME of inspiratory loads is related to the inspiratory pump mechanical forces and the P₁ amplitude of the RREP.