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

Authors


  • *

    The work should be attributed to the Division of Anaesthesia, University Children's Hospital, Basel, Switzerland.

Dr B. S. von Ungern-Sternberg
E-mail: bvonungern@uhbs.ch

Summary

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.

Various anaesthetic agents, airway instrumentations and ventilation strategies are used in infants and children undergoing anaesthesia. Each of these factors may affect functional residual capacity (FRC), which is one of the most important respiratory parameters of gas exchange. There is very little information available on the magnitude and the relative contribution of these different factors (e.g. neuroaxial blockade with local anaesthetics) on functional residual capacity in anaesthetised children [1–4].

Muscle relaxation induced by caudal epidural anaesthesia with bupivacaine decreases abdominal muscle tone while sparing the muscles of the respiratory system (diaphragm, intercostal muscles). 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. This alteration of the equilibrium between the thoracal and abdominal compartments might affect the functional residual capacity and ventilation homogeneity of the lungs. This study evaluated the impact of caudal block with bupivacaine on FRC and ventilation homogeneity in healthy preschool-aged children.

Methods

Following approval by the local Ethics Committee, Basel, Switzerland, and after obtaining parental written informed consent, 52 children, aged 3–8 years and weighing 15–30 kg undergoing elective surgery under general anaesthesia and caudal block were studied. Exclusion criteria included clinical evidence of cardiopulmonary diseases (including respiratory tract infection during the previous 2 weeks prior to surgery) and thoracic deformities. Blocked randomisation was generated using a computer-generated random number, and randomisation was concealed until used. Patients were randomly allocated to receive caudal block (caudal block group) or no intervention (control group).

One hour before induction of anaesthesia, patches of an eutectic mixture of local anaesthetics (EMLA) were applied to the dorsal sides of each child's hands. Premedication consisted of midazolam 0.3 mg.kg−1 administered orally or rectally 15 min prior to induction of general anaesthesia. In uncooperative patients, nitrous oxide 70% in oxygen was administered for the insertion of an intravenous cannula. Nitrous oxide was immediately stopped when venous access was achieved. Until correct positioning of the laryngeal mask airway was confirmed, all children received an Fio2 of 1.0 and thereafter an Fio2 of 0.5 for the rest of the study. General anaesthesia was induced with a propofol target controlled infusion system for children (Alaris Asena PK, Alaris Medical Systems, Inc., San Diego, CA) [5, 6]. For induction of anaesthesia, a dose calculated to obtain a plasma concentration of 7 μg.ml−1 propofol was administered until a laryngeal mask airway (LMA-ProSeal, LMA North America, Inc., San Diego, CA) was inserted (size 2 for children 15–20 kg and size 2.5 for children 20–30 kg). As soon as the correct position of the laryngeal mask airway was confirmed (auscultation and absence of gas leakage confirmed by equal inspiratory and expiratory volumes measured by spirometry), the target controlled infusion was decreased to a calculated plasma concentration of 2.5 μg.ml−1 and kept constant for the remainder of the study.

Five minutes after positioning of the laryngeal mask airway, the first measurements were made with all children breathing spontaneously. Thereafter, all children were placed in the left lateral position for 5 min. In the caudal block group, all children received a test dose of 0.2 ml.kg−1 bupivacaine 0.25% + epinephrine 1 : 200 000 followed by 0.8 ml.kg−1 bupivacaine 0.175%, whereas in the control group, no intervention took place. Finally, all children were turned back to the supine position and after 15 min the second set of measurements was performed; the effectiveness of the caudal block was assessed by pinching the skin at the level L1 and absence of movement was judged to be an effective block [7].

For the FRC measurements, an ultrasonic transit-time airflow meter (Exhalyzer D with ICU insert, Eco Medics, Duernten, Switzerland), which simultaneously measures flow and molar mass of the breathing gas, was placed between the fresh gas supply and the laryngeal mask airway. The technical setup of the measurement equipment has been described previously [8]. Briefly, this airflow meter combines accurate flow measurements with instantaneous mainstream gas analysis of molecular mass in a single sensor. This analysis is based on an ultrasonic transit time detection measured at a high sampling frequency (400 Hz) with piezoelectric sensors that demonstrate a high linearity over a wide amplitude range.

The application of sulphur hexafluoride (SF6, molecular mass 146 g.mol−1) as a tracer gas into the inspiratory part of the breathing system increases the total molecular mass of the breathing gas until a steady state is reached. Following the discontinuation of sulphur hexafluoride, the molecular mass decreases breath by breath until a steady state is reached again as soon as the sulphur hexafluoride has been completely washed out of the lungs (multibreath washout technique). Analysis of the washout curve allows for the calculation of the FRC, physiological dead space volume, lung clearance index and mean dilution number. The lung clearance index and mean dilution number are commonly used to measure the degree of ventilation homogeneity [9–14]. The lung clearance index is calculated as the cumulative expired volume needed to lower the end-tidal tracer gas (SF6) concentration to 1/40 of the starting concentration divided by the FRC, i.e. the number of lung volume turnovers needed to clear the lungs of the marker gas [15, 16]. The mean dilution number is the ratio between the first and the zeroth moments of the washout curve. The number of volume turnovers was calculated using the cumulative expired alveolar volume [12, 14]. An increase in lung clearance index or mean dilution number reflects a decrease in ventilation homogeneity. A blinded reviewer performed FRC, dead space, mean dilution number and lung clearance index calculations using Spiroware software (Version 1.5.2, ndd Medizintechnik AG, Zürich, Switzerland).

Statistics

Sample size calculation was performed using the nQuery Advisor 4.0 software programme (Statistical Solutions Ltd, Boston, MA), and was based on pilot data and data from previous studies (unpublished data). A sample size of 23 patients per group had an 80% power to detect a difference of at least 8% between the functional residual capacity before and after effective caudal block, assuming a standard deviation of differences of 8.4%, using an anova for repeated measures with a 0.025 two-sided significance level to adjust for multiple comparisons. To take into account non-adherence to the protocol (failure to insert laryngeal mask airway or failure of regional blockade), three additional patients per group were included.

Demographic and procedural data were analysed for normal distribution by the Shapiro-Wilk test, and data are reported as mean (SD) or median (interquartile range). Repeated measures were analysed with regression techniques using PROC MIXED procedures in SAS software version 9.1 (SAS Institute, Cary, NC). The regression model used the patient's group assignment (G), the repeated measures factor (I, indicates the pre-intervention and postintervention measurements), and the interaction between the two (G*I) as independent variables [Y = b0 + b1(G) + b2(I) + b3(G*I)]. Here, the interaction parameter b3 is of interest, because a statistically significant non-zero value for b3 indicates that the two patient groups reacted differently to the intervention. If a significant interaction existed, pairwise t-tests were used within groups to determine the extent of changes within groups. Since two tests were performed for each variable, a p-value < 0.025 was considered statistically significant to control for Type I error.

Results

All 52 children were successfully studied and all regional blocks performed in the caudal block group were effective. Demographic data are shown in Table 1; the characteristics of the patients in both groups were similar but with a predominance of male subjects.

Table 1.   Characteristics of patients. Values are median [interquartile range] or mean (SD). M I = Measurement I, M II = Measurement II.
 Caudal block
(n = 26)
Control
(n = 26)
Male : Female 20 : 6 24 : 2
Age; months 62 [47–71] 62 [48–83]
Height; cm109.5 [106–119]115 [105–121]
Weight; kg 18.75 [17–23] 20.1 [16–23]
Respiratory rate M I; breath.min−1 27.5 (4.08) 27.9 (3.44)
Respiratory rate M II; breath.min−1 27.1 (4.03) 28.1 (3.45)
Tidal volume M I; ml.kg−1  6.28 (1.75)  5.91 (1.62)
Tidal volume M II; ml.kg−1  6.56 (2.38)  5.99 (1.64)
Dead space M I; ml.kg−1  2.29 (0.71)  2.13 (0.62)
Dead space M II; ml.kg−1  2.32 (0.76)  2.1 (0.56)

The respiratory parameters tidal volume, respiratory rate and dead space volume were also similar between the groups at baseline (Measurement I) and did not change throughout the study period (Measurement II) (Table 1).

The parameters of primary interest in this study were functional residual capacity, lung clearance index and mean dilution number. The statistically significant interaction term of the regression analysis for all these parameters indicated that the change between the measurement levels was different between the two groups (functional residual capacity: F = 57.16, p < 0.0001; lung clearance index: F = 26.8, p < 0.0001; mean dilution number: F = 19.3, p < 0.0001). In the caudal block group, each of the variables changed by about 20%, whereas in the Control group they hardly changed at all (Table 2, Fig. 1).

Table 2.   Parameters.
ParameterMeasurementWithin group test (MI vs MII)*
p-value
Between group
test (MI)*
p-value
III
  • Values are mean (SD).

  • *

    Measurement I = first measurement 5 min after placement of the laryngeal mask airway. Measurement II = second measurement 15 min after application of the caudal block.

FRC; ml.kg−1
 Caudal block17.0 (4.34)20.5 (5.11)< 0.0010.91
 Control17.2 (4.88)17.1 (4.76)0.72 
Lung clearance index
 Caudal block12.0 (2.46) 9.37 (1.65)< 0.0010.24
 Control10.8 (2.69)10.7 (2.57)0.98 
Mean dilution number
 Caudal block 2.97 (0.83) 2.44 (0.48)< 0.0010.67
 Control 2.73 (0.83) 2.72 (0.84)0.99 
Figure 1.

 Functional residual capacity (FRC) (ml.kg−1) after induction of anaesthesia (Measurement I = baseline) and after ± caudal block (Measurement II) in the caudal block group and in the control group.

Discussion

This study examined the effects of caudal block with bupivacaine, the most commonly performed neuroaxial block in paediatric anaesthesia, on FRC and ventilation homogeneity in anaesthetised, spontaneously breathing, preschool-aged children with healthy lungs. The caudal block increased FRC and improved ventilation homogeneity significantly, whereas no changes were observed in the control group.

This is the first study in children examining the impact of a neuroaxial blockade on FRC. Only studies performed in adults are available to compare our findings on the effect of epidural anaesthesia on FRC [17–21]. While some of the adult studies found no effect of lumbar or thoracic epidural anaesthesia [18–20], others showed a decrease [21] or an increase in FRC [17]. These conflicting results are probably the result of the various anaesthetic agents and dosages used and/or their different muscle relaxant properties. In line with our results, Warner et al. [17] demonstrated an increase in FRC following epidural anaesthesia with an upper sensory level at T3 that was associated with a decrease in the intrathoracic blood volume and a caudal displacement of the diaphragm at end expiration. These anatomical changes are the consequence of the abdominal motor blockade induced by the epidural administration of a local anaesthetic [17].

In general, the level of the blockade is influenced by the level of the epidural puncture (e.g. lumbar, thoracic), the type of local anaesthetic used, its concentration and its spread. Using a similar volume of bupivacaine 0.25% (1 ml.kg−1), to that used in our study, Blanco et al. [22] observed a median level of T8 in children aged 1–6 years. Thus, for anatomical reasons, motor blockade following caudal block is more likely to occur in the muscles of the abdominal wall (T5-T12) than in the diaphragm (C3–C5) or the intercostal muscles (T1–T10). The subsequent reduction in abdominal wall tension results in caudal displacement of the diaphragm at end-expiration and might explain the observed increase in functional residual capacity [17].

The balance between the chest wall and lung recoil pressure and the tension of the diaphragm determines the static resting volume of the lungs. Any substance or intervention influencing inhomogeneously the muscle tone of the thoracal and abdominal compartments has the potential to interfere with this equilibrium and to reset it at a new level [23]. Muscular activity, especially of the diaphragm, is required to maintain functional residual capacity to counteract the muscular and gravitational forces of the chest and abdominal walls [24–26]. The high chest wall compliance of the infant results in relatively lower trans-pulmonary pressures at end-expiration that leads to an increased tendency for collapse of the small peripheral airways even during normal tidal breathing [27]. This makes an infant more susceptible to changes in muscle tone such as those caused by lumbar epidural anaesthesia administered by the caudal route [28–30]. It might therefore be that caudal block has even more pronounced effects on FRC in infants and younger children.

Ventilation distribution can be assessed by analysing the washout curve of an insoluble tracer gas and can be expressed by a variety of indexes, among which lung clearance index and mean dilution number are the most commonly used because of their sensitivity in detecting peripheral airway collapse [9, 16]. The regions of the lung that obtain less of the tidal volume than others have a slower clearance compared with those receiving a higher percentage of the tidal volume. The increase in ventilation homogeneity following caudal block in our patients might be explained by re-expansion of partially collapsed airways in the dependent regions of the lungs in the supine position after application of a caudal block [31]. As for FRC, the decrease in abdominal wall tension results in caudal displacement of the diaphragm [17] and recruitment of previously collapsed lung zones which improves ventilation homogeneity.

In the control group, functional residual capacity and ventilation distribution did not change during the observed study period of about 30 min. This corroborates findings in spontaneously breathing adults showing that the changes of functional residual capacity develop a short time after induction of anaesthesia and stabilise thereafter [32]. The changes of functional residual capacity and ventilation distribution observed in this study after caudal block with bupivacaine are therefore related to this intervention and not to time.

We are unable to draw any conclusions on how the observed increase in functional residual capacity would change using different concentrations of bupivacaine or other local anaesthetics that exhibit fewer muscle relaxant properties than bupivacaine (e.g. ropivacaine) [33]. In addition, the influence of caudal block on respiratory function in awake children cannot be predicted by this study performed under propofol anaesthesia. Propofol has blocking effects on the central part of the motor system and sodium channels in skeletal muscle, resulting in myorelaxation [34, 35]. Thus, the extent of a potential interaction between propofol and caudal block with bupivacaine cannot be quantified. In our study, all children received a standardised propofol target controlled infusion regimen to minimise intra- and intergroup differences.

Although we did not record blood pressure data in our study, it has been shown that blood pressure changes are minimal in healthy children undergoing caudal block [36]. Therefore, we do not expect blood pressure to be a confounding variable of functional residual capacity and ventilation homogeneity measurements.

The results of this randomised, controlled trial showed a significant increase in functional residual capacity and an improvement in ventilation homogeneity after caudal block. Under propofol sedation, functional residual capacity and ventilation distribution remained unchanged in the control group during the 30-min study period. Our results suggest that caudal block might have a beneficial impact on respiratory physiology in preschool children under general anaesthesia.

Acknowledgements

The authors thank all the children and their families who participated in this study, Damian M. Craig, M.H.S. (Department of Cardiovascular Surgery, Duke University, Durham, North Carolina, NC 27710, USA) for his advice with statistical analyses, and Joan Etlinger, B.A. (Department of Anaesthesia, University of Basel, CH-4031 Basel, Switzerland) for her help with manuscript preparation.

Funding was received from the Swiss Society of Anaesthesia and Resuscitation (SGAR) and the Department of Anaesthesia, University of Basel, Switzerland. A. Schibler is supported by Preston James Research Fund and the Golden Casket Research Fund (Australia).

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