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(Chest. 2002;122:473-478.)
© 2002 American College of Chest Physicians

Effect of Continuous Positive Airway Pressure on the Measurement of Thoracoabdominal Asynchrony and Minute Ventilation in Children Anesthetized With Sevoflurane and Nitrous Oxide*

Adrian Reber, MD, PhD; Jeremy M. Geiduschek, MD; Sandro A. Bobbià, MD; Heinz R. Bruppacher, MD and Franz J. Frei, MD

* From the Division of Pediatric Anesthesia (Drs. Reber, Bobbià, Bruppacher, and Frei), University Children’s Hospital of Basel, Basel, Switzerland; and the Department of Anesthesiology (Dr. Geiduschek), University of Washington School of Medicine and Children’s Hospital and Regional Medical Center, Seattle, WA.

Correspondence to: Adrian Reber, MD, PhD, Associate Professor, Department of Anesthesia, University of Basel/Kantonsspital, CH-4031 Basel, Switzerland; e-mail: Adrian.Reber{at}unibas.ch


   Abstract
TOP
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Study objectives: To quantify thoracoabdominal asynchrony (TAA) in children during anesthesia, and to measure the effect of continuous positive airway pressure (CPAP) on TAA, tidal volume (VT), and minute ventilation (E).

Design: Prospective, nonrandomized, controlled study.

Setting: Operating room of a university children’s hospital.

Participants: Ninety children aged 2 to 9 years scheduled for elective outpatient day surgery who were enrolled prospectively.

Methods: Each subject was anesthetized with sevoflurane 3% in equal parts O2 and N2O while breathing spontaneously through a facemask. Respiratory impedance plethysmography was used to calculate TAA indexes (phase angle [PA], phase relation in inspiration [PhRIB], phase relation in expiration, phase relation in total breath [PhRTB], and ratio of the inspiratory time to the total duration of the respiratory cycle [TI/TTOT]), VT, and E. Tidal gas flows were measured with a dual-hotwire anemometer with the sensor inserted between the facemask and the Y-piece of the anesthetic breathing circuit. This enabled the volume calibration of the respiratory impedance plethysmography equipment. The following conditions were compared: (1) no CPAP, (2) CPAP of 5 cm H2O, and (3) CPAP of 10 cm H2O.

Results: Eighty-one children completed the study protocol. All measurements of TAA with an inspiratory component (PA, PhRIB, PhRTB, and TI/TTOT) decreased significantly from baseline with the addition of CPAP to the circuit. Application of CPAP of 10 cm H2O decreased significantly mean VTs and Es compared with CPAP of 5 cm H2O and no CPAP. There were no differences in TAA for all conditions when comparing children scheduled for adenoidectomy with other surgical procedures.

Conclusions: With spontaneously breathing anesthetized children, TAA decreases with the application of CPAP. CPAP of 5 cm H2O was as effective as CPAP of 10 cm H2O in reducing PA, PhRIB, PhRTB, and TI/TTOT. However, CPAP of 10 cm H2O also caused a significant decrease in VT and E.

Key Words: continuous positive airway pressure • respiratory inductance plethysmography • thoracoabdominal asynchrony • upper airway obstruction


   Introduction
TOP
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
During a normal respiratory cycle, the circumferences of the rib cage and abdominal wall increase and decrease in synchrony. During general anesthesia with volatile agents and spontaneous ventilation, the rib cage and abdominal movements can be out of phase, and visual inspection of the thorax and abdomen will reveal asynchronous or paradoxical breathing patterns.1 In our experience, this is more common in children, and may be due to the greater chest wall compliance with younger age. Often, the occurrence of asynchronous or paradoxical breathing patterns are considered to be a manifestation of development of partial or complete upper airway obstruction. When this occurs, delivery of continuous positive airway pressure (CPAP) is sometimes used as the first in a series of attempted airway-opening maneuvers to reduce the obstruction. Rib cage and abdominal wall movement waveforms during the respiratory cycle can be recorded with respiratory inductance plethysmography. Analysis of the waveforms can yield a quantification of asynchronous or paradoxical respiratory patterns called thoracoabdominal asynchrony (TAA). Some of the specific measurements used to determine the degree of TAA include phase angle (PA), phase relation during inspiration (PhRIB), phase relation during expiration (PhREB), and phase relation during total breath (PhRTB). An increase in TAA has been demonstrated to correlate with increasing upper airway obstruction in an animal model.2 Also, TAA has been demonstrated to decrease in children with resolving upper airway obstruction due to laryngotracheobronchitis and during bronchodilator treatment in infants with bronchopulmonary dysplasia.3 4 To our knowledge, there have been no publications examining the changes of TAA that occur with manipulation of the upper airway during general anesthesia. The purpose of our study was to determine if the application of CPAP to spontaneously breathing unintubated anesthetized children has an effect on TAA and minute ventilation (E).


   Materials and Methods
TOP
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
The protocol was approved by the Ethics Committee of the University Children’s Hospital, Basel. Signed informed consent was obtained from one of the parents for each child in the study group. Ninety children aged 2 to 9 years scheduled for elective day surgery who were without lower respiratory tract disease, craniofacial dysmorphism, deformities of the chest or spine, or neuromuscular disorders were eligible.

Fifteen minutes prior to induction of anesthesia, subjects received midolazam, 0.3 mg/kg of body weight, either rectally or orally. Induction of anesthesia was with sevoflurane up to 8% volume mixed with equal parts O2 and N2O via facemask connected to a circle system anesthetic machine (Dameca 10890; Dameca a/s; Copenhagen, Denmark). ECG, noninvasive BP measurement, anesthetic gas analysis, and pulse oximetry were continuously monitored (Datex CardioCap II; Datex; Helsinki, Finland). After IV catheter placement, the delivered anesthetic concentration was changed to 3% sevoflurane with 50% O2 and N2O (total gas flow of 4 L/min). The subjects were in a supine position, with their heads resting on a prefabricated foam cushion to maintain 110° between the horizontal surface of the operating table and the line connecting the lateral corner of the eye and the tragus.5

Rib cage and abdominal wall motions were depicted from the output of an alternating current-coupled respiratory inductance plethysmograph (RespitracePlus; NonInvasive Monitoring Systems; Miami Beach, FL). The rib cage band was placed at the level of the nipples, and the abdominal wall band was placed at the umbilicus. Proper band placement was visually confirmed throughout each investigation. The plethysmographic output was recorded into software (RespiEvents 4.2e; NonInvasive Monitoring Systems) specifically designed to measure respiratory frequency, to calculate asynchrony indexes (see below), and to determine the ratio of the inspiratory time to the total duration of the respiratory cycle (TI/TTOT [the inspiratory duty cycle]).2 The operator was blinded to the recordings.

The PA was determined by using the RespiEvents software according to the method of Agostoni and Mognoni.6 This utilizes the XY plot of rib cage vs abdominal wall excursions (Lissajous figure) and a sine wave-dependent formula: sine PA = m/s, where m describes the distance between the intercepts of the rib cage-abdominal wall loop on a line drawn parallel to the x-axis, which is placed at half the distance between the maximal and minimal rib cage excursions, and s is the maximal abdominal wall excursion (Fig 1 ). For PA determination, only breaths for which the Lissajous figure was of acceptable morphology were included.7 Values for the PA are between 0° for complete synchrony and 180° for complete asynchrony.



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  		Figure 1.. Rib cage (RC) and abdominal wall (AB) motions are plotted against each other in a Lissajous figure. The ratio of m/s is an index of TAA. PA is calculated as the sine PA = m/s (see text). The counterclockwise direction indicates that abdominal wall expansion precedes rib cage expansion during inspiration.

 
PhRIB, PhREB, and PhRTB represent the percentage of time during one breath in which the rib cage and abdominal wall movements are in opposite directions (eg, rib cage inward and abdominal wall outward or vice versa). The PhRTB means the total time spent in asynchrony. PhRIB and PhREB indicate the lag of rib cage relative to abdominal wall motion at the start of inspiration and expiration, respectively.8 These thoracoabdominal coordination indexes are sine wave-independent values (in contrast to PA) between 0% and 100%, where 0% is complete synchrony and 100% is complete asynchrony.8

Tidal gas flows were measured with a dual-hotwire anemometer with the sensor inserted between the facemask and the Y-piece of the circle system (accuracy ± 8% from 0.09 to 32.7 L/m; dead space of 1 mL) [Florian; Acutronic Medical Systems; Hirzel, Switzerland]. Calibration was at zero flow with air. Airway pressure was measured just proximal to the facemask. Flow and pressure were displayed in real-time, and flow was integrated to provide tidal volume (VT) and E (Neonatal Respiration Monitor Florian; Acutronic Medical Systems). A correction factor of 0.717 was used to account for the presence of sevoflurane and nitrous oxide in the respiratory gasses. This enabled the volume calibration of the ratio of the respiratory inductance plethysmography equipment.

Data were collected during each of the following conditions: (1) single-hand chin lift with upper and lower molars contacting, with lips open (baseline); (2) chin lift plus CPAP of 5 cm H2O; and (3) chin lift plus CPAP of 10 cm H2O. CPAP was introduced into the anesthetic circuit by adjusting the pressure relief valve and measured from the graphic display of the Florian equipment. Each condition was applied in all children, in the order described above, for 2 min by the same investigator to eliminate interinvestigator variability. The experimental protocol was discontinued in any subject with complete airway obstruction based on clinical impression (no breathing sounds detected by the investigator) and lack of recorded tidal gas flow. In these cases, artificial respiration with neuromuscular paralysis and consecutive endotracheal intubation was performed. Data collection was completed before the beginning of the operation, and anesthesia was continued according to the practice of the anesthetist of record.

Variables and statistical data obtained from the plethysmograph were calculated from the mean and SD of 20 consecutive respiratory cycles in the steady state.5 Since asymmetrical data distribution was expected and found to be present, data were presented uniformly as median and interquartile range (the value of the 75th percentile minus the value of the 25th percentile). This decision was made because it was unlikely that age distribution would fit the normal curve, and TAA variables from a previous study of ours9 also showed asymmetrical data distribution. The three airway situations (CPAP of 0 cm H2O, 5 cm H2O, and 10 cm H2O) were compared by analysis of variance for repeated measures. For post hoc comparisons, Tukey test was applied if appropriate and probability values were calculated. Mann-Whitney U test was applied to compare data for children scheduled for adenoidectomy vs other surgical procedures. A p value < 0.05 was considered significant. For all calculations, Statistica/W 5.0 software (StatSoft; Tulsa, OK) was used.


   Results
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
Of the 90 children who were enrolled in the study protocol, data from 9 children were excluded: data for 8 children were excluded due to complete airway obstruction at baseline (adenoidecotomy [n = 2], and for other surgical procedures [n = 6]; age range, 2 to 6 years), and data for 1 child were excluded due to technical difficulties with the airway pressure relief valve and inability to maintain a constant level of CPAP. The 81 remaining subjects had a median age of 5.0 years (interquartile range, 3.5 years) and weight of 19.0 kg (interquartile range, 7.0 kg). The types of surgical procedures performed were adenoidectomy (n = 21), inguinal herniorrhaphy (n = 8), circumcision (n = 40), and other (n = 12). There were no subjects scheduled for tonsillectomy or other head and neck surgical procedures.

Sixty-three children had Lissajous figures with acceptable morphology to allow calculation of a PA. All measurements of TAA with an inspiratory component decreased significantly from baseline with an addition of CPAP to the circuit (Table 1 ). Because TAA data were skewed, all data are presented uniformly as median (25 to 75% interquartile range). Additional CPAP significantly reduced PA (from 122° [interquartile range, 39°] at baseline, to 120° [interquartile range, 79°] with CPAP of 5 cm H2O, and 116° [interquartile range, 85°] with CPAP of 10 cm H2O). PhRIB was also significantly reduced (from 67% [interquartile range, 38°], to 54% [interquartile range, 51°], and 50% [interquartile range, 48°] respectively). TI/TTOT and PhRTB are shown in Table 1 . There were no significant changes in PhREB. Respiratory rate significantly increased with the application of CPAP of 10 cm H2O (from 36 breaths/min [interquartile range, 9 breaths/min] at baseline, to 39 breaths/min [interquartile range, 9 breaths/min]). VT and E significantly decreased with an addition of CPAP of 10 cm H2O to the circuit (Table 1) . From baseline, there was an average reduction of 31% for VT values and 23% for E values. Pulse oximetry values remained >= 95% throughout the study protocol.


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  		Table 1.. Effect of CPAP on TAA and E (All Patients)*

 
Representative results from within-group measurements for the study subjects scheduled for adenoidectomy and all other procedures are shown in Figures 2 , 3 , respectively. For comparison between groups, the subjects scheduled for adenoidectomy had a significantly higher age 6.0 years [interquartile range, 2.0 years] vs 5.0 years [interquartile range, 3.3 years] for other surgical procedure groups) and lower E at baseline (103 mL/kg/min [interquartile range, 40 mL/kg/min], vs 123 mL/kg/min [interquartile range, 67 mL/kg/min] for other surgical procedure groups). In spite of this age difference, all other comparisons between groups for all TAA measurements, TI/TTOT, respiratory rate, VT, and E at baseline and with CPAP of 5 cm H2O and CPAP of 10 cm H2O were not significantly different.



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  		Figure 2.. The effect of CPAP on measurements of TAA with an inspiratory component (PA, top; PhRIB, middle; and TI/TTOT,bottom). Results from the study subjects scheduled for adenoidectomy and all other procedures ("others") are presented. Data were skewed, and all data are therefore presented uniformly as median and 25 to 75% interquartile range. • = no CPAP; {circ} = median value at CPAP of 5 cm H2O; {square} = median value at CPAP of 10 cm H2O; * = Significantly different from no CPAP (p < 0.05).

 


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  		Figure 3.. The effect of CPAP on measurements of VT (top) and respiratory rate (f) [bottom]. Results from the study subjects scheduled for adenoidectomy and all other procedures (Others) are presented. Data were skewed, and all data are therefore presented uniformly as median and 25 to 75% interquartile range. • = no CPAP; {circ} = median value at CPAP of 5 cm H2O; {square} = median value at CPAP of 10 cm H2O; *Significantly different from no CPAP (p < 0.05). {dagger}Significantly different from CPAP of 5 cm H2O.

 

   Discussion
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
Our major finding was that children breathing spontaneously 3% sevoflurane in equal parts O2 and N2O had asynchronous inspiratory breathing patterns determined by TAA indexes. With spontaneously breathing anesthetized children, TAA decreases with the application of CPAP. CPAP of 5 cm H2O was as effective as CPAP of 10 cm H2O in reducing PA, PhRIB, PhRTB, and TI/TTOT. However, CPAP of 10 cm H2O also caused a significant decrease in VT and E and significantly increased respiratory rate.

From animal studies, it is known that an increase in upper airway obstruction correlates with increasing TAA.2 Any narrowing at the tonsillar level may have a major impact on the distal pharynx, which can be "sucked in" or even obstructed. The lateral walls of the pharynx have a complex architecture, with a number of muscles that have different biomechanical relationships with each other and with other pharyngeal structures.10 Thus, upper airway obstruction leads to an increase of the TI/TTOT in combination with an increase of the PA. With an addition of CPAP, upper airway narrowing improved (decreased), as it was demonstrated with decreased TI/TTOT and PA in the present study. Lifting the chin could increase pharyngeal compliance so that the tonsils are sucked in without counterbalance from muscle activity.11 Thus, chin lift may even worsen TAA compared to baseline conditions with the chin unsupported.9 When the lateral pharyngeal walls are altered by hypertrophic lymphatic tissue, thickness, or fat in obese patients with obstructive sleep apnea,11 12 chin lift without CPAP should be avoided.

We did not perform a baseline study while the patients were awake because we did not want to provoke any possible agitation to the patients resulting from the use of a mouthpiece used to measure the spirometric parameters, and any discomfort to the patient may produce secretions and concomitant changes in laryngeal characteristics. Also, during anesthesia, a baseline control measurement while an oropharyngeal airway is inserted can eliminate or minimize upper airway obstruction and decrease TAA. Therefore, we did not perform the baseline control measurement, as introducing an oropharyngeal airway without fiberoptic control of positioning can affect the position of the tip of the epiglottis, which makes analysis of thoracoabdominal movements suspect to bias.13 An endotracheal tube would eliminate these concerns by bypassing the entire pharyngeal airway, including the glottic structures. However, the inner diameter of an endotracheal tube is limited in size because of the anatomic limitation of the glottic opening. It might be argued that this tube will increase resistance and cause changes in thoracoabdominal movements.

The effect of CPAP has been suggested to be multifactorial, including dilating and splinting the upper airway, changing chest wall stability, pulmonary mechanics, lung volume, and respiratory muscle dynamics.14 15 16 17 18 19 CPAP also enhances end-expiratory lung volume, which prevents alveolar collapse and, thus, assists the next inspiration.14 17 Our study subjects were presumably all without lower respiratory tract disease, and therefore the likely dominant effect of CPAP on our study subjects was a change in upper airway geometry. Depending on the amount of chest wall compliance, with partial airway obstruction, the rib cage can move inward during inspiration.20 To compensate, the diaphragm may have to undergo greater contraction, leading to a greater outward movement of the abdominal wall. When these movements are measured by respiratory inductance plethysmography, asynchrony is recorded. If CPAP decreases partial upper airway obstruction, then there will be either less inward excursion or an outward excursion of the rib cage during inspiration. The result will be greater synchrony of rib cage and abdominal wall motion and a decrease in the measures of TAA. This was confirmed by our results. TAA decreased during inspiration, and there was no significant change during expiration.

Since the degree of TAA during inspiration is thought to be a marker for upper airway obstruction, it would be expected that children with some degree of upper airway obstruction due to adenoid hypertrophy would have higher measures of asynchrony indexes. However, we did not find this in our study. It is possible that the children in the nonadenoidectomy group may also have had enlarged adenoids. Unfortunately, screening for this through history or physical examination was not part of our protocol. It is also possible that all children in the age range studied may have had upper airway anatomy that predisposed to some degree of airway obstruction during sevoflurane with N2O anesthesia in the presence or absence of adenoidal hypertrophy. If so, the most clinically significant manifestation would be complete airway obstruction with paradoxical rib cage and abdominal wall motion. This situation occurred in eight study subjects but was not allowed to persist in order to obtain recordings from enough breath cycles to reliably measure TAA. However, this group did not contain a disproportionate number of study subjects scheduled to undergo adenoidectomy.

Our results showed that CPAP of 5 cm H2O decreased TAA. Another study21 found an improvement in ventilation of sedated infants undergoing interventions that compromise upper airway patency. In the present study, CPAP of 10 cm H2O caused a significant decrease in VT and E and significantly increased respiratory rate. From baseline, there was an average reduction of 31% for VT values and 23% for E values. Any CPAP application > 10 cm H2O may theoretically further decrease VT and E, and may cause hyperinflation of healthy lungs and splinting of a healthy chest wall affecting inward distortion during inspiration. Moreover, the positive pressure threshold to overcome the esophageal sphincter tone is known to be 20 cm H2O and should be avoided. It may be argued that measured VT and E values were low in the present study compared with the literature.1 22 However, these investigators performed ventilatory measurements after endotracheal intubation, which would eliminate these concerns by bypassing the entire pharyngeal airway including the glottic structures. In our anesthetized patients, partial upper airway obstruction occurred influencing chest wall motion and ventilatory parameters.

In infants, Allen and coworkers4 measured synchronous chest wall motion during sleep (PA ranged 0 to 15°). It is possible that the quantity of TAA measured at baseline in our study was not related to upper airway obstruction, but was primarily due to an anesthetic effect on the intercostal muscles, which was "corrected" with the addition of CPAP. Brown and coworkers23 measured PhRTB as part of a protocol comparing the respiratory effects of sevoflurane and halothane. In patients aged 2 to 24 months breathing spontaneously with a laryngeal mask airway, both sevoflurane (2.5%) or halothane (1%) in 66% N2O caused an asynchronous breathing pattern with a greater effect in the halothane group.23 In another study,1 children > 12 months old showed a systematic increase in inspiratory neural duty cycle (TI/TTOT) with increasing halothane concentration. Although, interestingly, infants < 12 months old manifested a constant duty cycle with increasing halothane concentration.1 The determination of a quantitative relationship between sevoflurane concentration and TAA will require further study.

In conclusion, our results have shown that asynchronous and paradoxical thoracoabdominal motions often occur during inspiration in spontaneously breathing children during sevoflurane anesthesia. In these children, the application of CPAP reduced moderate-to-severe asynchronous breathing movements, which may be a consequence of decreasing partial upper airway obstruction. A CPAP of 10 cm H2O compared with 5 cm H2O was equivalent in reducing TAA. However, in contrast to the latter, CPAP of 10 cm H2O also decreased VT and E.


   Acknowledgements
 
We thank Joan Etlinger for editorial work, and Professor Göran Hedenstierna, Department of Clinical Physiology, University Hospital, Uppsala, Sweden, for reviewing this article.


   Footnotes
 
Abbreviations: CPAP = continuous positive airway pressure; PA = phase angle; PhREB = phase relation in expiration; PhRIB = phase relation in inspiration; PhRTB = phase relation in total breath; TAA = thoracoabdominal asynchrony; TI/TTOT = ratio of the inspiratory time to the total duration of the respiratory cycle; E = minute volume; VT = tidal volume

Dr. Geiduschek was on paid sabbatical leave by the University of Washington School of Medicine, Department of Anesthesiology.

All work was performed at the University Children’s Hospital, Basel, Switzerland.

This study was supported by grant 3200-056034.98 of the Swiss National Science Foundation.


   References
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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