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PEDIATRIC ANESTHESIOLOGY

Continuous Monitoring of Dynamic Pulmonary Compliance Enables Detection of Endobronchial Intubation in Infants and Children

From the Department of Anesthesiology, David Geffen School of Medicine at UCLA, Los Angeles, California.

Address correspondence and reprint requests to Aman Mahajan, MD, PhD, Associate Professor, Department of Anesthesiology, David Geffen School of Medicine at UCLA, Box 951778, Los Angeles, CA 90095. Address e-mail to amahajan{at}mednet.ucla.edu.

Abstract

BACKGROUND: Auscultation of breath sounds is used routinely to confirm tracheal placement of endotracheal tubes (ETT). In infants and children, this method is limited by the conduction of breath sounds bilaterally, despite endobronchial intubation. Although several methods of detecting endobronchial intubation have been described, none is both simple and reliable. In this investigation, we determined whether changes in pulmonary compliance and airway pressures, measured using continuous side stream spirometry, can reliably detect endobronchial intubation in pediatric patients.

METHODS: Forty patients aged 1 month to 6 years were included. After endotracheal intubation the ETT was incrementally advanced as two observers monitored breath sounds and spirometry (Pressure-Volume Loops). Changes in pulmonary compliance, peak inspiratory pressure, or auscultation were reported, at which point ETT position was confirmed by fiberoptic bronchoscopy.

RESULTS: Endobronchial intubation decreased measured pulmonary compliance by 45 ± 11% (mean ± sd; P < 0.001, Range 26%–66%) and increased peak airway pressures by 26 ± 17% (mean ± sd; P < 0.001, Range 0–87). Changes in peak airway pressures were smaller and more variable when compared to changes in compliance. Breath-sound auscultation failed to detect endobronchial intubation in 7.5% of cases.

CONCLUSIONS: Pulmonary compliance changes are a sensitive and an accurate indicator of endobronchial intubation in infants and children. Both increased peak airway pressures and changes in breath sounds are less sensitive indicators of endobronchial intubation.

Unrecognized endobronchial intubation in children is common in clinical practice, and leads to preventable hypoxemia and atelectasis (1,2). Major pulmonary morbidity requiring intensive care unit (ICU) admission, although rare, has also been described (3,4). Despite the advances in monitoring, clinicians still lack a simple, reliable, and inexpensive method or device for detection of inadvertent endobronchial intubation. Chest radiography, the current "gold standard," incurs radiation exposure and consumes valuable operating room (OR) time. Fiberoptic bronchoscopy, a reasonable alternative to radiography, requires advanced expertise and training in pediatrics and relies on costly equipment that is often not readily available. Neither provides continuous patient monitoring for endobronchial intubation.

Clinical trials have evaluated the utility of more practical methods for detecting endobronchial intubation, including breath sound auscultation, pulse oximetry, visual chest rise, capnography, and peak inspiratory pressure (5–8). Novel devices utilizing acoustics and even magnetism have also been used (9–11). However, a simple and reliable method for detecting endobronchial intubation remains elusive.

Small children and infants are especially vulnerable to accidental endobronchial intubation since minimal changes in head position can lead to endobronchial migration of the endotracheal tube (ETT) (12). Published data in pediatric patients have demonstrated that by focusing attention to the problem and increasing awareness, practitioners could reduce the endobronchial intubation rate from 20% to 7%, but not eliminate it (13).

Continuous intraoperative spirometry using the D-Lite® all-in-one flow sensor (Datex Ohmeda, Madison WI) can immediately detect endotracheal tube kinking, air leaks, and even malposition (14–16). With this small, lightweight, and inexpensive sensor, one can accurately and reliably measure pulmonary compliance and airway pressures (17).

This study sought to define the changes in 1) dynamic pulmonary compliance, 2) peak inspiratory pressure, and 3) auscultated breath sounds that occur during endobronchial intubation. We hypothesize that decrease in dynamic pulmonary compliance using continuous spirometry accurately detects endobronchial intubation in pediatric patients, and that change in compliance is a more reliable monitor of endobronchial intubation compared to change in peak pressure or auscultated breath sounds.

METHODS

After obtaining IRB approval and parental informed consent, we prospectively enrolled 40 ASA I-III pediatric patients into the study. This study group included children scheduled to undergo general, orthopedic, neurological, urologic, head and neck, and vascular surgery. Patients with known pulmonary, major abdominal, neuromuscular disease, chest wall deformities, obesity, or abnormal body habitus were excluded from enrollment. After a standard IV or inhaled general anesthetic induction and institution of adequate neuromuscular blockade (train-of-four monitoring), an uncuffed ETT without a Murphy eye, was orotracheally inserted just beyond the vocal cords. After audibly confirming an ETT gas leak at 16 cm H₂O, the pediatric D-Lite® sensor was connected between the ETT and the Y-piece of the breathing system while the Datex Capnomac Ultima gas monitor (Datex Ohmeda) was programmed to display pressure-volume loops (Fig. 1A). The D-Lite sensor consists of a combination of small pitot tube-type pressure sensing ports, which allow continuous measurement of pressure and flow during a respiratory cycle (For details, see Ref. 17). The Datex Capnomac calculates the compliance (C, units = mL/cm H₂O) of the respiratory system using the following equation: C = (Tidal volume_expiratory) (Pressure_{End inspiratory} – Pressure_{End expiratory})^–1.

View larger version (37K): [in this window] [in a new window]   Figure 1. Patient spirometry. (A) The D-Lite Sensor with three sampling ports. During inspiration as the gas flows from the ventilator to the patient, A measures total pressure, B measures static pressure. Difference between the two gives dynamic pressure, which is proportional to velocity of gas flow. During expiration the process is reversed. C measures CO2, O2, and anesthetic gas concentration. (B) Pressure-volume loop during an endobronchial intubation. Left: Pressure-volume loop with the endotracheal tube (ETT) in trachea showing a compliance of 7.1 mL/cm H2O. The slope of the loop reflects the pulmonary compliance. Right: Pressure-volume loop after endobronchial intubation shows a decrease in dynamic compliance to 3.6 mL/cm H2O and an increase in peak pressure.

Mechanical volume-controlled ventilation with a tidal volume of 10 mL/kg was then initiated, leaving respiratory rate adjustment based upon the age of the child and to achieve an end tidal CO₂ 36 mm Hg. All patients' lungs were ventilated using the Datex Ohmeda Aestiva anesthesia system that was set on volume control mode ventilation with an inspiratory pause. Two additional anesthesiologists, not caring for the patient, were randomly assigned to monitor either 1) spirometric variables (pulmonary compliance and peak inspiratory pressure (Fig. 1B), Observer #1; or 2) the quality of the auscultated bilateral breath sounds, Observer #2. After the observers established a baseline set of these variables, the primary attending anesthesiologist was then asked to advance the ETT in 5–6 mm increments, pausing between advances for several respiratory cycles. During the pauses, the two observers would independently evaluate for changes in either spirometric variables (compliance, peak pressure) or bilateral breath sounds, respectively. Each observer was blinded to the findings of the other, but not to the actions of the primary attending anesthesiologist managing the ETT. This cycle was repeated until Observer #1 noted a change in lung compliance or peak pressure that was more than 20% from baseline, Observer #2 noted loss of bilateral breath sounds, or the ETT met resistance to further advancement. We felt that a 20% margin would be easily and quickly recognized, and would also represent a clinically significant difference. A small change, even if statistically significant, may be clinically insignificant and dramatically increase the rate of false positives. Upon reaching one of these endpoints, the attending anesthesiologist temporarily secured the ETT and then performed a fiberoptic examination to determine its exact location within the tracheobronchial tree. Once this position was recorded, the ETT was appropriately positioned in the trachea under fiberoptic guidance and secured for surgery.

Statistical analysis was performed using the SPSS statistical analysis software (SPSS Inc., Chicago, IL). We considered absolute changes in peak pressure and dynamic compliance along with percentage changes in peak pressure and dynamic compliance as possible candidate variables for predicting endobronchial intubation. Percent change in pulmonary compliance and peak pressure was defined as the difference between the bronchial and tracheal values divided by the tracheal value ([bronchial – tracheal/tracheal] x 100%). The "best" variable was defined as the one with the lowest or no correlation with age or weight (age or weight independent), and with the largest Z score. The Z score is a measure of the average difference in the respective variable between the trachea and bronchus in standard deviation units or Z = (mean_variable in bronchus – mean_variable in trachea)/sd_d, where sd_d is the standard deviation of the individual bronchial-tracheal differences. Linear regression analysis was performed to compare the relationship between the peak pressure or dynamic compliance variables and age or weight (correlation coefficient = r). Analysis of variance was used to define statistical significance (P < 0.05). Results are reported as mean ± standard deviation or as a range.

RESULTS

The mean age of the patients in our group was 2.4 years (range 0.2–6 years) and their mean weight was 13.6 kg (range 2.8–36 kg). The study group included 23 males and 17 females. Continuous spirometry and fiberoptic bronchoscopy were successfully performed in all the subjects. No prolonged systemic oxygen desaturations (Sao₂ <94% for >60 s) during fiberoptic bronchoscopy confirmation of ETT placement were noted in our study.

Of the four variables considered (absolute change in bronchial and tracheal-peak pressures and compliance respectively, and the percentage change between bronchial and tracheal-peak pressures and compliance respectively), percentage change in dynamic compliance had the largest Z score (–4.3) and had no significant correlation with age (r = 0.11, P = 0.5), making it the best variable predicting endobronchial intubation. Percentage change in peak pressures although not significantly correlated (r = 0.15, P = 0.3) with age had much lower Z score (–1.5). Absolute changes in peak pressures and compliance had lower Z scores and were significantly correlated with age (P < 0.05). Figure 2A shows that percentage change in compliance (and peak pressure) is independent of age even though compliance itself increases almost linearly with age (r = 0.93). A similar relationship was observed when these variables were correlated with weight in our patient group (percentage change in compliance and weight, r = 0.25; compliance and weight, r = 0.85). This likely reflects a normal coupling between age and weight of children in our study group who were of normal body habitus.

View larger version (12K): [in this window] [in a new window]   Figure 2. Relationship between age and percent change in compliance or peak pressure during endobronchial intubation. (A) Linear regression showing that both percent change in compliance (compl %) and peak inspiratory pressures (peak inspiratory pressure %) are age independent (r = 0.11 and r = 0.15, respectively) in 40 subjects; (B) Linear increase in dynamic compliance (mL/cm H2O) with age (r = 0.93).

Observer #1 assigned to monitor the spirometric data (pulmonary compliance and peak inspiratory pressures) (Fig. 1B) identified every case of endobronchial intubation. Observer #2 assigned to auscultate breath sounds successfully identified 37 of 40 cases of endobronchial intubation, but failed to detect three cases (7.5% failure rate). The ETT migrated into the right mainstem bronchus rather than the left in each patient. The percent decrease in pulmonary compliance attributed to endobronchial intubation was 45 ± 11 (Mean ± sd; P < 0.001, Range 26%–66%), while the reciprocal percent increase in peak inspiratory pressure was 26 ± 17 (Mean ± sd; P < 0.001, Range 0%–87%) (Fig. 3). Fourteen of 40 (35%) patients had <20% change in peak pressures (compliance change in this group was 41 ± 8%). Twelve (30%) of these patients had only minimal changes in peak pressure during endobronchial intubation (<15% change from baseline), and of these three patients had essentially no change from baseline (peak pressure 1 cm H₂O). Two of these three patients had persistent bilateral breath sounds although the compliance changed (27% and 44% respectively) and fiberoptic bronchoscopy confirmed endobronchial intubation. The stated results remained consistent across all patient age groups. Infants aged <1 year demonstrated similar percent changes in dynamic compliance and peak airway pressure when compared to children in age groups 1–3 years and 3–6 years. Figures 2 and 3 summarize these results graphically.

View larger version (24K): [in this window] [in a new window]   Figure 3. A comparison of percent change in compliance versus percent change in peak pressure during endobronchial intubation. (A) Patients of all ages (n = 40): % change in compliance 45 ± 11, % change in peak pressure 26 ± 17; (B) Patients <1 year of age (n = 11):% change in compliance 41 ± 11, % change in peak pressure 19 ± 10; (C) Patients 1–2 years of age (n = 14): % change in compliance 50 ± 9, % change in peak pressure 27 ± 16; (D) Patients 3 years of age (n=15): % change in compliance 43 ± 10, % change in peak pressure 29 ± 20. TC and BC = Value of Tracheal and Bronchial compliance respectively (mL/cm H2O) in each pediatric age group; TP and BP = Tracheal and Bronchial pressures respectively (cm H2O) in each group. Values reported are mean ± sd; P < 0.001 for both percent change in compliance and peak pressure during endobronchial intubation.

Confirmation of ETT placement using continuous spirometry could be performed with only a few additional minutes. As reported by Campos et al., (5) we also found that endobronchial intubation caused an immediate and a discrete change not only in peak airway pressure but also in dynamic compliance.

DISCUSSION

Our study demonstrates that endobronchial intubation elicits a substantial decrease in dynamic pulmonary compliance. Continuous spirometry, while measuring changes in dynamic pulmonary compliance, is thus ideally suited to detect endobronchial intubation in infants in children. The consistently large percent change in compliance, coupled with a narrow standard deviation, makes this parameter highly clinically significant. Since every patient experienced a decrease in pulmonary compliance of at least 26%, a change large enough to be easily identified, measuring percent change in pulmonary compliance proved to be a very sensitive and reliable test for endobronchial intubation. Used as a clinical test, a >25% decrease in pulmonary compliance was 100% sensitive and 100% specific for detection of endobronchial intubation (no false negatives or positives, respectively).

The percent change in dynamic pulmonary compliance elicited by endobronchial intubation also remained constant across all age groups studied, although tracheal compliance increased almost linearly with age (0–6 years), suggesting that percent change in compliance is a reliable clinical test for diagnosing endobronchial intubation in pediatric patients of varying ages. Incidentally, our results confirm previously published data that show a linear increase in pulmonary compliance with increasing age in infants and children (18).

In comparison, percent change in peak inspiratory pressure proved a far less reliable clinical marker of endobronchial intubation. The smaller mean percent change, coupled with a much wider standard deviation, reduced the clinical significance of even statistically significant findings. In a substantial percentage of the study population, endobronchial intubation triggered only a small, clinically less significant, change in peak pressures (<15% increase in peak inspiratory pressure). Measuring percent change in peak inspiratory pressure was not a sensitive or a reliable test for detecting endobronchial intubation when compared to changes in dynamic compliance. This discrepancy between compliance and inspiratory pressure changes is surprising, and suggests that varying amounts of leak around the ETT (a function of the ETT versus airway diameter) could have contributed to a difference between the dialed and actual tidal volume. Changes in peak inspiratory pressure are dependent upon the actual tidal volumes delivered to (and seen by) the single lung, which might be different than the tidal volumes preset on the volume-cycled ventilator due to the presence of varying amounts of leak around the ETT and partial ventilation of the left lung as the ETT approaches the carina and the right bronchus. This might have been a mechanism in two patients in our study who demonstrated minimal change in peak inspiratory pressure or auscultation of breath sounds despite an endobronchial intubation. Changes in dynamic compliance (and fiberoptic bronchoscopy), however, were able to confirm ETT position. Compliance, which reflects Volume/Pressure and is calculated as (Tidal volume_expiratory) (Pressure_{End inspiratory} – Pressure_{End expiratory})^–1, is likely a more reliable measure of pressure-volume relationship in this setting. Other contributing mechanisms cannot be excluded. Future studies with methodologies that include studying the leak and actual volumes delivered to the lung will clarify this intriguing observation.

Rolf and Cote found changes in peak inspiratory pressure equally unreliable in detecting endobronchial intubation in their trial of 196 children undergoing general endotracheal anesthesia (6). An increase in peak inspiratory pressure was recorded in only 3 of the 14 episodes of endobronchial intubation. However, all patients were hand-ventilated during inadvertent endobronchial intubation and tidal volumes were not recorded. Thus, they could not exclude the possibility that practitioners were automatically adjusting tidal volume to maintain peak pressures deemed appropriate.

In contrast, Campos et al. demonstrated in 14 children that endobronchial intubation causes an immediate increase in peak inspiratory pressure (mean increase 2431 cm H₂O, P < 0.0001) (5). If we instead report their results as a percent change from baseline, the percent increase in peak pressure of 34 ± 16 cm H₂O (mean ± sd) would be slightly more than our reported result (26 ± 17 cm H₂O). Furthermore, only 7% of their population (n = 1) did not experience a >15% increase in peak pressure during endobronchial intubation, as compared to 30% (n = 12) noted in our study. A difference in study design and methodology with respect to number of patients, ventilation parameters, leaks, and final position of ETT in the bronchus may have contributed to some of the differences.

Auscultation of breath sounds failed to detect endobronchial intubation in 7.5% of our study cases (n = 3). This failure occurred despite the best possible circumstances for detection, namely the auscultator waiting to detect impending endobronchial intubation. In clinical practice, we would expect the failure rate to be significantly higher, as the index of suspicion would likely be lower. Our results are remarkably similar to those described by Verghese et al. in their study of 153 children undergoing general endotracheal anesthesia in the pediatric catheterization laboratory (13). After the anesthesiologist's confirmation of bilateral breath sounds, they used fluoroscopy to determine actual ETT position. The incidence of unrecognized endobronchial intubation of 20% reported during their interim analysis (n = 55) was reduced to 7.1% in the remaining 98 patients, for a final incidence of 11.8%. Despite heightened awareness and attention to the frequent incidence of endobronchial intubation during the interim analysis, they were unable to reduce the auscultation failure rate below 7% during the remainder of the trial. We hypothesize that enhanced breath sound transmission from the ventilated to the nonventilated hemithorax contributed to this failure rate. The small size of the pediatric thorax, coupled with the inherent air leak of uncuffed ETTs, could enhance this phenomenon in certain patients.

We did not evaluate the impact of endobronchial intubation on arterial oxygen saturation. Barker et al. demonstrated, in four anesthetized dogs, that the pulse oximeter recorded little change in oxygen saturation during intentional endobronchial intubation if the fraction of inspired oxygen exceeded 0.5 (8). Rolf and Cote's study (6) in 196 children did describe eight episodes of minor desaturation (Spo₂ 95% for 60 s) and four episodes of major desaturation (Spo₂ 85% for 60 s) during endobronchial intubation. They concluded that the pulse oximeter can be used to detect endobronchial intubation in most cases. However, they do not report the fraction of inspired oxygen used in their study. Furthermore, because they used auscultation to confirm ETT position rather than a definitive modality (radiography, fiberoptic examination), the true incidence of endobronchial intubation is unknown. Their published incidence of unrecognized endobronchial intubation of 5.1% likely underestimates the true incidence, as patients who did not desaturate would have likely evaded detection. Finally, we feel that an ideal monitor should alert the practitioner to the possibility of endobronchial intubation before a critical incident such as a major arterial desaturation occurs.

Since all of our patients were deeply anesthetized and paralyzed during data collection, one cannot generalize our results to all clinical settings in the OR or in the ICU. Changes in anesthetic depth, neuromuscular blockade, lung volumes, bronchomotor tone, and patient position during the course of a surgical procedure or in the ICU can alter dynamic pulmonary compliance; therefore, an increase in pulmonary compliance does not always indicate endobronchial intubation. Nonetheless, given the high sensitivity and specificity of compliance changes this can be an additional important monitor for early identification of respiratory abnormalities in mechanically ventilated patients. A sudden decrease in compliance should prompt the clinicians in the OR or ICU to identify clinical scenarios (such as endobronchial intubation, kinked tubes, mucus plugs, pneumothorax, etc.) that can affect ventilation acutely. Future investigations will hopefully determine the sensitivity and specificity of the test under varying clinical conditions.

We used uncuffed ETTs in all patients, and thus were unable to standardize the leak pressure. This could have led to an overestimation of dynamic pulmonary compliance in some patients (19). However, since we were measuring percent change in pulmonary compliance and not absolute values, the significance of this error should be minimal. Furthermore, since no patient had a leak below 16 cm H₂O, and since the use of uncuffed ETTs in young children is considered standard of care, we feel that our data were collected under realistic clinical conditions. Pressure controlled mode is often being used to ventilate pediatric patients. Although we studied only volume control ventilation, we would expect that, in theory, our results should also apply to pressure control ventilation, since the compliance changes will now relate to the changes in expired tidal volume (with the pressure remaining constant). However, these results will need to be validated with pressure control ventilation in clinical settings.

In summary, a decrease in dynamic pulmonary compliance >25% is a sensitive and specific marker for endobronchial intubation in infants and children. It is superior to both 1) change in peak inspiratory pressure and 2) breath sound auscultation for detection of endobronchial intubation. We propose that changes in dynamic compliance can be used as a reliable test for endobronchial intubation in infants and children.

ACKNOWLEDGMENTS

We thank Jefferey Gornbein, Dr.PH, from UCLA Department of Biomathematics and Statistical Consulting for providing the statistical support for this study. We acknowledge valuable discussions with Konstantin Yastrebov, MD, PhD, Director of Tasmanian institute of Critical Care, Australia.

Footnotes

Accepted for publication March 21, 2007.

Supported by grants from NIH/NHLBI P01 HL078931 (Dr. Mahajan).

Conflict of interest: None.

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This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a colleague Similar articles in this journal Similar articles in Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Request Permissions Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Mahajan, A. Articles by Wald, S. Search for Related Content PubMed PubMed Citation Articles by Mahajan, A. Articles by Wald, S. Related Collections Airway Ventilation Monitoring (Non-cardiac) Patient Safety Pediatrics