The forces applied to the lung in health and disease

Respiratory complications in the pediatric postanesthesia care unit

This article focuses on common respiratory complications in the postanesthesia care unit (PACU). Approximately 1 in 10 children present with respiratory complications in the PACU. The article highlights risk factors and at-risk populations. The physiologic and pathophysiologic background and causes for respiratory complications in the PACU are explained and suggestions given for an optimization of the anesthesia management in the perioperative period. Furthermore, the recognition, prevention, and treatment of these complications in the PACU are discussed.

Spontaneous effort causes occult pendelluft during mechanical ventilation

RATIONALE

In normal lungs, local changes in pleural pressure (P(pl)) are generalized over the whole pleural surface. However, in a patient with injured lungs, we observed (using electrical impedance tomography) a pendelluft phenomenon (movement of air within the lung from nondependent to dependent regions without change in tidal volume) that was caused by spontaneous breathing during mechanical ventilation.

OBJECTIVES

To test the hypotheses that in injured lungs negative P(pl) generated by diaphragm contraction has localized effects (in dependent regions) that are not uniformly transmitted, and that such localized changes in P(pl) cause pendelluft.

METHODS

We used electrical impedance tomography and dynamic computed tomography (CT) to analyze regional inflation in anesthetized pigs with lung injury. Changes in local P(pl) were measured in nondependent versus dependent regions using intrabronchial balloon catheters. The airway pressure needed to achieve comparable dependent lung inflation during paralysis versus spontaneous breathing was estimated.

MEASUREMENTS AND MAIN RESULTS

In all animals, spontaneous breathing caused pendelluft during early inflation, which was associated with more negative local P(pl) in dependent regions versus nondependent regions (-13.0 ± 4.0 vs. -6.4 ± 3.8 cm H2O; P < 0.05). Dynamic CT confirmed pendelluft, which occurred despite limitation of tidal volume to less than 6 ml/kg. Comparable inflation of dependent lung during paralysis required almost threefold greater driving pressure (and tidal volume) versus spontaneous breathing (28.0 ± 0.5 vs. 10.3 ± 0.6 cm H2O, P < 0.01; 14.8 ± 4.6 vs. 5.8 ± 1.6 ml/kg, P < 0.05).

CONCLUSIONS

Spontaneous breathing effort during mechanical ventilation causes unsuspected overstretch of dependent lung during early inflation (associated with reciprocal deflation of nondependent lung). Even when not increasing tidal volume, strong spontaneous effort may potentially enhance lung damage.

Measurement of vibration-induced volumetric strain in the human lung

Noninvasive image-based measurement of intrinsic tissue pressure is of great interest in the diagnosis and characterization of diseases. Therefore, we propose to exploit the capability of phase-contrast MRI to measure three-dimensional vector fields of tissue motion for deriving volumetric strain induced by external vibration. Volumetric strain as given by the divergence of mechanical displacement fields is related to tissue compressibility and is thus sensitive to the state of tissue pressure. This principle is demonstrated by the measurement of three-dimensional vector fields of 50-Hz oscillations in a compressible agarose phantom and in the lungs of nine healthy volunteers. In the phantom, the magnitude of the oscillating divergence increased by about 400% with 4.8 bar excess air pressure, corresponding to an effective-medium compression modulus of 230 MPa. In lungs, the averaged divergence magnitude increased in all volunteers (N = 9) between 7 and 78% from expiration to inspiration. Measuring volumetric strain by MRI provides a compression-sensitive parameter of tissue mechanics, which varies with the respiratory state in the lungs. In future clinical applications for diagnosis and characterization of lung emphysema, fibrosis, or cancer, divergence-sensitive MRI may serve as a noninvasive marker sensitive to disease-related alterations of regional elastic recoil pressure in the lungs.

Decrease in functional residual capacity and ventilation homogeneity after neuromuscular blockade in anesthetized preschool children in the lateral position

BACKGROUND

While functional residual capacity (FRC) is reduced in children undergoing general anesthesia, the lateral position leads to an increase in FRC compared with the supine position. The impact of neuromuscular blockade remains unknown. We tested the hypothesis that neuromuscular blockade leads to a decrease in FRC and increase in lung clearance index (LCI) while the application of positive endexpiratory pressure (PEEP) of 6 cmH(2)O leads to a restoration in both parameters.

METHODS

After approval of the local Ethics Committee, we studied 18 preschool children (2-6 years) without cardiopulmonary disease, who were scheduled for elective surgery. Anesthesia was standardized using propofol and fentanyl. FRC and LCI were calculated by a blinded observer using a SF6 multibreath washout technique with an ultrasonic transit-time airflow meter (Exhalyzer D). Measurements were taken in the left lateral position (PEEP 3 cmH2O) after 1. intubation with a cuffed tracheal tube, 2. neuromuscular blockade with rocuronium, and 3. the additional application of PEEP (6 cmH2O).

RESULTS

Functional residual capacity mean (sd) decreased from 31.6 (4.4) ml.kg(-1) to 27.6 (4.2) ml.kg(-1) (P<0.001) following neuromuscular blockade while the LCI increased from 6.54 (0.6) to 7.0 (0.6) (P<or=0.001). After the application of PEEP (6 cmH2O), FRC increased to 32.4 (5.0) ml.kg(-1) whereas the LCI decreased to 6.58 (0.5) showing no significant changes from baseline measurements.

CONCLUSIONS

In the lateral position, neuromuscular blockade led to a significant decrease in FRC associated with a small increase in ventilation inhomogeneity. FRC and LCI were restored to baseline levels with the application of PEEP 3 cmH2O that is in addition to a background of PEEP 3 cmH2O giving a total of 6 cmH2O PEEP.

Impact of Trendelenburg positioning on functional residual capacity and ventilation homogeneity in anaesthetised children

Trendelenburg positioning, a head-down tilt, is routinely used in anaesthesia when inserting a central venous catheter to increase the calibre of the jugular or subclavian veins and to prevent an air embolism. We investigated the impact of Trendelenburg positioning on functional residual capacity and ventilation homogeneity as well as the potential reversibility of these changes by repositioning and/or a recruitment manoeuvre in children with congenital heart disease. Functional residual capacity and ventilation homogeneity were assessed in 20 anaesthetised children between the ages of 3 months and 8 years who required central venous catheterisation before undergoing cardiac surgery. Functional residual capacity was measured (1) in the supine position, (2) in the Trendelenburg position, (3) after repositioning supine and (4) after a recruitment manoeuvre to total lung capacity which was performed by manually elevating the airway pressure to 40 cmH(2)O for ten consecutive breaths. Adopting the Trendelenburg position led to a significant decrease in functional residual capacity (median [range]- 12 (6-21)%) and increase in lung clearance index (12 (2-19)%). Baseline values were not reached after repositioning supine in any patient until after a standardised recruitment manoeuvre was performed.

Impact of depth of propofol anaesthesia on functional residual capacity and ventilation distribution in healthy preschool children

BACKGROUND

Propofol is commonly used in children undergoing diagnostic interventions under anaesthesia or deep sedation. Because hypoxaemia is the most common cause of critical deterioration during anaesthesia and sedation, improved understanding of the effects of anaesthetics on pulmonary function is essential. The aim of this study was to determine the effect of different levels of propofol anaesthesia on functional residual capacity (FRC) and ventilation distribution.

METHODS

In 20 children without cardiopulmonary disease mean age (SD) 49.75 (13.3) months and mean weight (SD) 17.5 (3.9) kg, anaesthesia was induced by a bolus of i.v. propofol 2 mg kg(-1) followed by an infusion of propofol 120 microg kg(-1) min(-1) (level I). Then, a bolus of propofol 1 mg kg(-1) was given followed by a propofol infusion at 240 microg kg(-1) min(-1) (level II). FRC and lung clearance index (LCI) were calculated at each level of anaesthesia using multibreath analysis.

RESULTS

The FRC mean (SD) decreased from 20.7 (3.3) ml kg(-1) at anaesthesia level I to 17.7 (3.9) ml kg(-1) at level II (P < 0.0001). At the same time, mean (SD) LCI increased from 10.4 (1.1) to 11.9 (2.2) (P = 0.0038), whereas bispectral index score values decreased from mean (SD) 57.5 (7.2) to 35.5 (5.9) (P < 0.0001).

CONCLUSIONS

Propofol elicited a deeper level of anaesthesia that led to a significant decrease of the FRC whereas at the same time the LCI, an index for ventilation distribution, increased indicating an increased vulnerability to hypoxaemia.

Capsaicin exposure elicits complex airway defensive motor patterns in normal humans in a concentration-dependent manner

The airway defensive response to tussive agents, such as capsaicin, is frequently assessed by counting the number of cough sounds, or expulsive events. This method does not identify or differentiate important respiratory events that occur in the respiratory muscles and lungs, which are critical in assessing airway defensive responses. The purpose of this study was to characterize the airway defensive behaviours (cough and expiration reflex) to capsaicin exposure in humans. We observed complex motor behaviours in response to capsaicin exposure. These behaviours were defined as cough reacceleration (CRn) and expiration reflex (ERn), where n is the number of expulsive events with and without a preceding inspiratory phase, respectively. Airway defensive responses were defined in terms of frequency (number of expulsive events), strength (activation of abdominal muscles) and behaviour type (CRn vs. ERn). Thirty-six subjects (15 females, 24+/-4 yr) were instrumented with EMG electrodes placed over the rectus abdominis (RA), external abdominal oblique (EO) and the 8th intercostal space (IC8). A custom-designed mouth pneumotachograph was used to assess the airflow acceleration, plateau velocity and phase duration of the expulsive phase. Subjects inhaled seven concentrations of capsaicin (5-200 microM) in a randomized block order. The total number of expulsive events (frequency) and the sum of integrated EMG for the IC8, RA and EO (strength) increased in a curvilinear fashion. Differentiating the airway defense responses into type demonstrated predominately CR1 and CR2 (i.e. inspiration followed by one and two expulsive events, respectively) with very few ER's at <50 microM capsaicin. At higher concentrations (>50 microM) ER's with one or more expulsive events (ER1) appeared, and the number of CR's with three or more expulsive events (CR3) increased. The decrease in EMG activation and airflow measurements with each successive expulsive event suggests a decline in power and shear force as the number of expulsive events increased. Therefore, the airway defensive response to capsaicin is a complex motor pattern that functions to coordinate ER's and CR's with differing numbers of expulsive events possibly to prevent aspirations and keep air moving to promote clearance.

The effect of caudal block on functional residual capacity and ventilation homogeneity in healthy children*

Caudal block results in a motor blockade that can reduce abdominal wall tension. This could interact with the balance between chest wall and lung recoil pressure and tension of the diaphragm, which determines the static resting volume of the lung. On this rationale, we hypothesised that caudal block causes an increase in functional residual capacity and ventilation distribution in anaesthetised children. Fifty-two healthy children (15-30 kg, 3-8 years of age) undergoing elective surgery with general anaesthesia and caudal block were studied and randomly allocated to two groups: caudal block or control. Following induction of anaesthesia, the first measurement was obtained in the supine position (baseline). All children were then turned to the left lateral position and patients in the caudal block group received a caudal block with bupivacaine. No intervention took place in the control group. After 15 min in the supine position, the second assessment was performed. Functional residual capacity and parameters of ventilation distribution were calculated by a blinded reviewer. Functional residual capacity was similar at baseline in both groups. In the caudal block group, the capacity increased significantly (p < 0.0001) following caudal block, while in the control group, it remained unchanged. In both groups, parameters of ventilation distribution were consistent with the changes in functional residual capacity. Caudal block resulted in a significant increase in functional residual capacity and improvement in ventilation homogeneity in comparison with the control group. This indicates that caudal block might have a beneficial effect on gas exchange in anaesthetised, spontaneously breathing preschool-aged children with healthy lungs.

Postoperative pulmonary complications. When are preventive and therapeutic measures necessary?

Relevant clinical and physiologic data are available to help guide decision making in the management of patients who are at risk for or already have postoperative pulmonary complications. Prophylactic measures should be simple and focused. Inexpensive treatment methods should be tried before resorting to more individual-intensive and expensive measures. Proper treatment of postoperative atelectasis requires adequate patient assessment and knowledge of the therapeutic options. Costs, measured in dollars as well as in patient morbidity and mortality, are substantially greater when physicians fail to recognize and treat reversible disease before elective surgery and when effective postoperative therapy is not provided in accordance with individual needs.

Influence of lung and chest wall compliances on transmission of airway pressure to the pleural space in critically ill patients

Nineteen patients with acute respiratory failure were divided into three groups according to their total compliance (CT). Transmission of airway pressure to the pleural space was then evaluated by measurement of esophageal pressure at both end-expiration and end-inspiration, and at three levels of PEEP. Chest wall (CW) and lung complicance (CL) were also calculated from simultaneous measurements of lung volume changes induced by tidal delivery. In group 1 (CT greater than 45 ml/cmH2O), 37 percent of airway pressure was transmitted to pleural space. In group 2 (CT between 45 and 30 ml/cmH2O), 32 percent of airway pressure was transmitted to the pleural space. In group 3 (CT less than 30 ml/cmH2O), only 24 percent of airway pressure was transmitted to the pleural space. These differences are statistically significant (p less than 0.001) and illustrate the influence of a progressive increase in lung stiffness (CL = 100.3 +/- 17.2 ml/cmH2O in group 1, CL = 45.0 +/- 6.3 ml/cmH2O in group 2, and CL = 28.6 +/- 8.9 ml/cmH2O in group 3) on transmission of airway pressure to the pleural space. Despite lesser transmission of airway pressure to the pleural space in the most damaged lungs, no significant difference was found between groups with regard to transmural venous pressure changes throughout the study.

Does abdominal breathing affect regional gas exchange?

Does the use of the abdomen during inspiration alter gas exchange to the bases of the lungs? Most previous investigators have concluded that this technique does not affect regional ventilation. However, if gas concentrations are used as an index of gas exchange, if the subject has normal lungs, is seated and performs the maneuver with the assistance of a physiotherapist, the answer is an unequivocal yes. This study does not pinpoint the exact mechanism by which this alteration is accomplished, but suggests that changes in thoracoabdominal shape influence regional blood flow. It does not indicate whether this effect is beneficial or detrimental. Whether the large number of patients with obstructive lung disease who attempt this maneuver in an unsupervised manner alter gas exchange in any way is uncertain. However, the data do suggest that some patients might be affecting their lung function, and we should learn more about the precise mechanisms.