INTRODUCTION

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RESPIRATORY INDUCTIVE PLETHYSMOGRAPHY (RIP) can be used to obtain a valid measure of tidal volume in humans (2, 6, 9, 12) and animals (1, 18). In the presence of airway obstruction, however, gas compression within the airways and thoracoabdominal asynchrony may contribute to inaccuracies in plethysmographic estimation of tidal volume (15, 21). Interestingly, the same factors that contribute to the inaccuracies of plethysmography have potential to be employed as measures of airway obstruction (7, 11). Specifically for RIP, there has been interest in measures of thoracoabdominal asynchrony (i.e., phase angle, phase relation) as noninvasive indicators of airway obstruction (3, 6, 12, 13, 23, 25, 27-29). What has not emerged from these studies, however, is a clear picture of how thoracoabdominal asynchrony quantitatively correlates with the severity of airway obstruction. A study performed by Hammer and co-workers (13) showed that phase angle significantly changed from baseline measurements in rhesus monkeys challenged with progressive (i.e., not randomly ordered) inspiratory (not inspiratory plus expiratory) loading, but the phase angle and inspiratory resistive load were not linearly related. Another study done in infants (22) indicated that RIP could detect a change in pulmonary mechanics during bronchoprovocation, but there was no association made between a change in pulmonary mechanics and a change in RIP variables. Hence, there is a need to study the quantitative relationship between RIP and classical mechanics in the setting of airway obstruction in humans and animals before these variables can be relied upon clinically.

Respiratory diseases are common in foals. To our knowledge, there is no objective, noninvasive monitoring system of lung mechanics in large animals. Impending respiratory failure must be assessed subjectively and by analysis of arterial blood gases. The most common etiologies of respiratory disease include bacterial pneumonia, premature birth with surfactant deficiency, chest trauma, or congenital heart disease (20, 26). As foals are prone to such respiratory diseases, yet highly ambulatory, we sought to find a noninvasive continuous monitoring system with potential accuracy for grading lung disease. Hence, foals were used as a model species to study the relationship between quantifiable RIP variables (i.e., phase angle and phase relation) and classical lung mechanics. We introduced graded resistive loads at the airway opening and, on a separate occasion, evoked bronchoconstriction by using histamine aerosol as provocative challenges.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

All procedures were approved by the Institutional Animal Care and Use Committee at Tufts University School of Veterinary Medicine.

Subjects. Eight healthy 2- to 4-mo-old mixed-breed foals (65-126 kg, mean 87.9 kg body wt) were used for this study. All foals were born at term and were obtained from a nurse mare farm within 2 wk after weaning, except for one foal (foal 8) that was kept with the mare throughout the length of the experiment. Foals were allowed to acclimate to their new environment for at least 2 days before any procedures were performed. Daily physical examinations were performed on each foal to ensure that it was free of clinical signs of disease (i.e., nasal discharge, abnormal lung sounds, cough, or fever). The orphan foals were stabled at night and were turned out each day into a grassy paddock for 6-8 h. They were fed a combination of mare's milk replacer, grain, and hay. No vaccines were administered other than those given routinely to their mares 1 mo prior to parturition (equine influenza, rhinopneumonitis, Eastern and Western encephalitis, tetanus toxoid, and rabies).

RIP. Foals were sedated with xylazine (1.25 mg/kg body wt), and their heads were maintained in a horizontal position during measurements. Respitrace bands (4 cm wide, adult-size Respibands; Ambulatory Monitoring Systems, Ardsley, NY) were placed on the foals, one at the 11th intercostal space and the other directly behind the 18th (last) rib. Measurements of thoracoabdominal asynchrony were obtained for all foals using calibrated RIP (oscillator unit, Ambulatory Monitoring Systems). The Respitrace bands were secured around the abdomen directly behind the eighteenth (i.e., last) rib, and the other around the rib cage at the eleventh intercostal space. The separate signals as well as the sum of the signals were digitized and recorded on a laptop computer. Phase angle and phase relation were determined by using a commercially available software package with a sampling rate of 50 Hz (RespiEvents version 4.2, Non-invasive Monitoring Systems, Miami Beach, FL). This system uses a qualitative diagnostic calibration (QDC) developed and described in detail by Sackner et al. (24). Basically, the QDC method is based on the isovolume maneuver equations but does not require subject cooperation or breathing through a pneumotachograph. It is carried out during a 5-min period of natural breathing during which time a baseline average is established such that the proportionality constant between the two compartments (abdomen and rib cage) is derived. The QDC has been shown to acceptably calibrate RIP for tidal volume in human adults (24) as well as newborns (2).

Conventional lung mechanics. The foals were fitted with a solid plastic facemask sealed 5-10 cm behind the external nares with a latex shroud, with ~80 ml of dead space. The nosepiece of the mask was affixed to a pneumotachograph (Fleisch 2, OEM Medical, Lenoir, NC) connected to a differential pressure transducer (DP45-28, Validyne Engineering, Northridge, CA). The pneumotachograph was calibrated by using the electronic integration of flow introduced through the pneumotachograph with a precision syringe (3L syringe, Hans Rudolph, Kansas City, MO). Transpulmonary pressure (esophageal-mask pressure) was measured with an esophageal balloon catheter (length 10 cm, perimeter 3.8 cm, wall thickness 1 mm) sealed over the distal end of a polypropylene catheter (4 mm ID, 5 mm OD, length 100 cm). The esophageal balloon catheter was passed within the thoracic portion of the esophagus, the balloon was inflated with 2 ml of air, and when maximal negative pressure excursions were observed, the proximal end was exited through an airtight latex diaphragm in the mask, and the catheter was taped in place. End-expiratory pleural pressures ranged between 2 and 5 cmH₂0. The esophageal balloon catheter was connected to a second differential pressure transducer (DP45-14, Validyne Engineering). The pressure transducer used for esophageal pressure measurements was calibrated statically with a water U-manometer. The signal derived was amplified, sampled at 30 Hz, and digitized for processing using pulmonary mechanics analyzer software (Buxco XA Biosystems, Buxco Electronics, Sharon, CT). Flow, tidal volume, and pleural pressure were recorded continuously and displayed by the computer on a breath-by-breath basis. These measurements were then used by the computer to yield a computation of total pulmonary resistance (RT) by using the isovolume method of Amdur and Mead (5). Dynamic compliance (Cdyn) was computed as the change in volume divided by the change in transpulmonary pressure at two points of zero flow during the inspiratory portion of the breath. Five to ten breaths taken from each measurement period were averaged for each data point. No phase delay or signal attenuation of the pressure and flow sensors was observed up to 5 Hz.

Histamine bronchoprovocation. Histamine bronchoprovocation was performed after methods described for adult horses by Derksen et al. (10) and Klein and Deegen (17) and in foals by Hoffman et al. (14). In those studies, it was found that Cdyn reflected bronchoconstriction in a dose-dependent fashion more reliably than did RT. Histamine aerosol was delivered to foals by use of a jet nebulizer (Pari LC JET, Pari, Paris, France), which produced fine particles (mass median diameter of 1.6 µm) at a flow rate of 0.35 ml/min using a high-pressure (30 psi) compressor (Compare, model NE-C08, Omron HealthCare, Vernon Hills, IL) with an output of 9 l/min. The order of challenges was as follows: normal saline solution (control) followed by histamine diphosphate (Sigma Chemical, St. Louis, MO) in saline solution at doubling concentrations (0.5, 1.0, 2.0, 4.0, 8.0, 16.0, and 32.0 mg/ml). Each solution was nebulized to the foal for a total of 2 min. Lung function measurements were resumed 20 s after the end of each nebulization period throughout the peak response and continued until there was a return of Cdyn to the postsaline baseline. Each dose of histamine or saline was separated by at least 5 min from the previous dose, which in humans has been shown to avoid a cumulative effect of histamine (16). Aerosol challenges were discontinued when Cdyn decreased to at least 35% of the postsaline challenge values, a dose of 32 mg/ml of histamine was used, or the foals showed signs of respiratory difficulty (i.e., accentuated abdominal lift, coughing, nostril flaring, or greater than a doubling of respiratory rate).

Upper airway resistive loads. This portion of the experiment was performed on a different day from the bronchoprovocation. The lung function testing systems were arranged as previously described, with the exception that the negative end of the pressure transducer used to measure pleural pressure was left open to atmosphere rather than connected to the facemask. A nasotracheal (NT) tube (Bivona, 10 mm OD, length 55 cm) was passed into the trachea of the foal. Lidocaine (0.3%) was administered into the trachea for local anesthesia, and the cuff was inflated. A tracheal catheter (polypropylene, 4 mm OD, length 60 cm) containing numerous distal side holes was passed so that the end of the catheter was distal to the NT tube for measurement of tracheal pressure. The proximal end of the NT tube was attached to a PVC ball valve ( in. maximum ID, in. OD, length 4 cm) that served as a controllable resistor. The valve was attached to the pneumotachograph. The loaded resistances induced were ~150, 200, and 250% of baseline resistance (R150, R200, and R250, respectively) as determined by breath-by-breath monitoring of total respiratory system plus resistor resistance. Post hoc analysis of total resistance revealed that values at steady state did not correspond to exactly 150, 200, and 250% of baseline, so actual values were used for comparison with phase angle and phase relation during the same time segments. The order of the resistive loads was randomized after an initial baseline reading was obtained with the ball valve maximally opened in place. Next, lower airway lung resistance (RL) was measured by using tracheal and pleural pressures to discern whether resistive loading was affecting lower airway resistance as well and to better characterize our model. Each loading lasted 2 min and was followed by a capnographic reading (Multinex Datascope, Paramus, NJ).

Statistical analyses. ANOVA was used to compare the baseline value of phase angle and phase relation with the value at each histamine dose or resistive load. A Spearman rank correlation (r_s) test was performed to compare percent change in Cdyn to percent change in phase angle or phase relation after each dose of histamine or to test for a correlation between the percent change in RT to the percent change in phase angle or phase relation (Statistix version 4.1, Analytical Software, Tallahassee, FL). A P value of 0.05 or less was considered significant.

	RESULTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

Histamine bronchoprovocation. Coefficients of variation for respiratory frequency, tidal volume, maximum change in transpulmonary pressure, Cdyn, RL, and phase angle during the baseline period of data collection were 5.9, 5.5, 2.9, 2.5, 4.4, and 13% respectively. Representative Lissajous figures are shown for foal 2 at baseline and after maximal bronchoconstriction (Fig. 1). There was no significant change in phase angle for foals as a group at maximal bronchoconstriction (range 6-62°, mean = 27.1, SD = 14.2, P = 0.35). Individual values for lung mechanics, phase angle, and phase relation are shown in Table 1. When comparing the percent change in Cdyn to the concomitant percent change in phase angle for each histamine dose administered (Fig. 2), no correlation was found (r_s = 0.02). Two of the eight foals showed a consistent decrease in phase angle with increasing bronchoconstriction. The other six foals showed phase angles that changed in both a positive and negative direction with increasing bronchoconstriction. The absolute percent change in Cdyn did not correlate with the absolute percent change in phase angle (r_s = 0.34).

View larger version (37K): [in this window] [in a new window]   Fig. 1.   Lissajous figures from foal 2 at baseline (left) and during maximal bronchoconstriction induced with nebulized histamine diphosphate (16 mg/ml) (right).

View larger version (10K): [in this window] [in a new window]   Fig. 2.   Percent change in dynamic compliance (Cdyn) compared with concomitant percent change in phase angle in response to histamine aerosol challenge in foals (n = 8).

View larger version (11K): [in this window] [in a new window]   Fig. 3.   Percent change in Cdyn compared with concomitant percent change in phase relation (n = 8 foals).

Upper airway resistive loading. There was no change in end-tidal CO₂ during resistive loading. Individual values for the resistances measured during the experiment and the actual values determined after post hoc analysis are shown in Table 2. Although RL increased slightly during resistive loading, this was not a significant change (baseline RL mean = 2.58, SD = 2.8; RL at 150% baseline RT mean = 3.3, SD = 3.4, P = 0.23; RL at 200% baseline RT mean = 4.0, SD = 4.4, P = 0.12; RL at 250% baseline RT mean = 5.2, SD = 6.4, P = 0.12). The individual responses are shown in Table 2. Phase angle changed significantly at resistive loads of 150% and 250% RT value (P < 0.05), and there was a strong trend (P = 0.09) toward a change at 200% RT value. However, the correlation between percent change in resistance compared with percent change in phase angle was not significant (Fig. 4). Five of the eight foals showed an increase in phase angle with increasing RT. Two foals showed decreasing phase angle with increasing RT, and one foal changed in both a positive and negative direction relative to the induced resistance. The absolute percent change in RT vs. the absolute percent change in phase angle showed a significant correlation (r_s = 0.67, P < 0.05), which indicated that there was a pattern to the change in thoracoabdominal synchrony, but it differed between foals. There was also a significant change in phase relation between baseline RT and 250% RT value (P < 0.05). However, between the percent change in RT and the percent change in phase relation, there was no significant correlation found (Fig. 5). Five of the eight foals showed an increase in phase relation with an increase in RT. Two of the foals showed a decrease in phase relation with an increase in RT, and one foal changed in both a positive and negative direction with increasing resistive load. There was a significant correlation with the absolute percent change in RT to the absolute percent change in phase relation (r_s = 0.66, P < 0.05), indicating that resistance affected phase angle, but not in the same direction in all foals.

View larger version (11K): [in this window] [in a new window]   Fig. 4.   Percent change in total respiratory resistance (RT) compared with the concurrent percent change in phase angle during fixed upper airway resistive loading in foals (n = 8).

View larger version (12K): [in this window] [in a new window]   Fig. 5.   Comparison of percent change in RT to concomitant percent change in phase relation in foals during fixed resistive loading (n = 8).

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

Phase angle and relation were examined in this study because previous data demonstrated their relevance to detection of acute airway obstruction (13, 29). For phase angle or relation to have merit for gauging airway obstruction, they would need to correlate with classical measures of obstruction, but the data reported here do not support that conclusion. This is consistent with previous clinical data that have shown that there is wide variation in the degree of phase angle changes during upper airway obstruction (25).

We hypothesized that as the lower airways constrict during histamine challenge, the decrease in Cdyn would be associated with the recruitment of abdominal and other accessory muscles and this that might alter thoracoabdominal synchrony (22). This study, however, suggests that histamine-induced bronchoconstriction, to an extent that was clinically obvious, did not alter phase angle in a consistent fashion. Similarly, it has been shown in human infants that induced bronchoconstriction did not correlate linearly with a change in phase angle (22). This finding suggests several possibilities. The measurement of phase angle impinges on which compartment (i.e., thorax or abdomen) initiates inspiration and expiration. A change in the leading compartment during bronchoconstriction can obscure the measurement phase angle, despite an obvious change in breathing pattern (27). Moreover, Koterba et al. (18) demonstrated that adult horses breathe with a biphasic pattern (i.e., two separate inspiratory and expiratory efforts associated with each respiratory effort). This type of pattern may lead to a distorted Lissajous figure in equids, which would cause inaccurate phase angle measurements. We were concerned that phase angle may not have correlated with Cdyn or RT because of these confounding effects. To investigate this possibility, we examined the effects of these experimental obstructions on phase relation. Phase relation takes into account the dynamic relationship between abdominal and thoracic movements by looking at the absolute difference between the movements of thoracic and abdominal compartments, independent of which compartment initiates inspiration or expiration. The phase relation, however, did not change significantly during histamine bronchoprovocation, indicating that a change in the leading compartment was not a confounding variable to the measurement of phase angle. Tobin et al. (27) and Sackner et al. (23) found similar results when studying humans with chronic obstructive disease, and therefore the pattern of thoracoabdominal asynchrony was unpredictable during lower airway obstruction.

Although the phase angle did not change during bronchoconstriction, there was a significant effect of the resistive loads applied at the airway opening on phase angle in individual foals. Four foals showed an increase in phase angle, two showed a decrease in phase angle, and one changed in both directions with an increasing resistive load. As a group, no correlation was found between the percent change in RT and the percent change in phase angle. Similar to phase angle, there was a significant change in phase relation during upper airway respiratory loading. However, phase relation failed to correlate with the percent change in RT as for phase angle. It is clear, therefore, that a change in the lead compartment did not obfuscate the correlation between resistive load and thoracoabdominal asynchrony.

The breathing strategy in response to bronchoconstriction and resistive loading appeared to vary considerably. The variation in breathing strategy may be based on physical factors such as chest wall compliance, physical maturity of breathing muscles, elastic recoil pressures, or dynamic factors such as dynamic hyperinflation and an increase in functional residual capacity or development of intrinsic positive end-expiratory pressure. Further studies are needed to investigate these factors. Other studies done in primates have suggested similar variation in phase angle responses during resistive loading. Hammer et al. (13) found in nonhuman primates that a fixed inspiratory load evoked a significant change in phase angle but that the degree of change did not correlate linearly with the severity of upper airway loading. Although the resistive loads employed here were clinically evident, they were significantly lower than the loads employed in the study by Hammer et al. (13), which may explain the differences in results as well. Clark and co-workers (8) recently reported that the shape of the flow curve derived from RIP (sum) lacked sensitivity and specificity for detection of upper airway obstructions during sleep in humans. They postulated that individuals with a high degree of fixed upper airway resistance in which pressure and flow are nearly linearly related cannot be distinguished from normal individuals by using phase angle. On that line, Newth et al. (19) found that phase angle was logarithmically correlated with the product of esophageal pressure and rate in anesthetized rhesus monkeys with graded inspiratory loading. We speculate that the fixed inspiratory and expiratory type of resistive loading in this study might have affected the outcome, and RIP may fare differently in the setting of dynamic (e.g., inspiratory greater than expiratory) resistive load.

This study does suggest that phase angle and phase relation are more sensitive to upper airway loading than to that during lower airway constriction, within ranges that could be observed in clinical patients for each perturbation. The lack of correlation between phase angle or phase relation and conventional lung mechanics indicates that these variables are not linearly correlated with traditional measures of fixed upper or lower airway obstructions in foals. Further studies are needed to determine the impact of dynamic changes in airway caliber during measurements of thoracoabdominal asynchrony.