We have previously shown that children (average age 9 yrs) with mildly elevated obstructive apnoea/hypopnoea indices (OAHI) retained CO2 at rest. Here, we report the results of a 6-yr follow-up study on 14 children from that study.
Minute ventilation (V′E) and end-tidal CO2 partial pressure (PET,CO2) were measured during hypercapnic challenge.
OAHI decreased from 7.5±4.7 events·h−1 at age 9 yrs to 2.5±1.8 events·h−1 at age 15 yrs (p<0.001), despite an increase in body mass index from 20±4.6 kg·m−2 to 26±5.7 kg·m−2 (p<0.0001). Eupneic V′E increased from 4.1±0.31 L·min−1·m−2 to 5.9±0.4 L·min−1·m−2 (p<0.01), while PET,CO2 fell from 44.1±0.8 to 33±1.0 mmHg (p<0.001). The V′E–PET,CO2 obtained during hypercapnia was left shifted, such that V′E at a PET,CO2 of 50 mmHg increased from 24 L·min−1 at age 9 yrs to 36 L·min−1 at age 15 yrs. Central respiratory drive did not change.
We hypothesise that somatic growth of the pharynx coupled with a regression of tonsillar tissue mass with age leads to enlargement of the upper airway lumen, a reduction in airway resistance and increased respiratory airflow at a given level of ventilatory drive.
We have previously shown that resting end-tidal CO2 partial pressure (PET,CO2) was higher in 6–12-yr-old children with relatively high obstructive apnoea/hypopnoea indices (OAHI) compared with age-matched controls with lower OAHI 1. The initial studies were performed in 1999 when the average age of our subjects was ∼9 yrs. Here, we report the results of a 6-yr follow-up study of 14 children from this original sample, now with an average age of 15 yrs. All of the children in our sample have intact tonsils and adenoids. The mechanism of the CO2 retention is unknown, but could be due to increased upper airway resistance, to blunted central chemoreceptor sensing and/or to altered regulation of central ventilatory motor output.
We were able to re-study 14 children from this original sample in 2006–2007, and our results are reported herein. Preliminary analyses from the entire Tucson Childrens Assessment of Sleep Apnoea Study (TuCASA) cohort indicate that the OAHI improves with age in children, even in the absence of adenoidectomy and tonsillectomy (table 1⇓) 2. We tested the hypothesis that these improvements in the OAHI are associated with improvements in ventilatory control. We were particularly interested in changes in the resting minute ventilation (V′E), the resting PET,CO2, and changes in the sensitivity to inspired CO2. Our unique longitudinal study shows that although the body mass index (BMI) of the children increased with age, they also had lower OAHIs, a marked diminution in resting PET,CO2, and a substantial leftward shift in the V′E–PET,CO2 response curve.
MATERIALS AND METHODS
All methods used to recruit subjects and to collect the present data set were approved both by the University of Arizona Human Subjects Committee and the Tucson Unified School District Research Committee (Tucson, AZ, USA). In all cases, we obtained written informed consent from the parents, and assent from the children. Our initial sample in 1999 was composed of subjects recruited through the Tucson Unified School District, as described in previously 1. For the present study we selected and attempted to contact all 50 subjects from our original cohort and asked them if they would like to participate in a follow-up study. Subjects that had adenoidectomy or tonsillectomy were excluded, and a total of 14 subjects (28% of the original cohort) who met our criteria agreed to participate. Their anthropometric characteristics are given in table 1⇑. The remaining children could not be located, had tonsillectomies or adenoidectomies (n = 19) and/or refused to participate. These children, in comparison to the 14 subjects who participated in the present study, did not differ with respect to mean age or BMI, sex or ethnicity distribution, or percentage of habitual snoring.
In both the study in 1999 and the present study, children underwent unattended, nocturnal home polysomnography 3 using the Compumedics PS-2 system (Abbotsford, Victoria, Australia). The following signals were obtained: C3/A2 and C4/A1 electroencephalogram, right and left electrooculogram, a bipolar submental electromyogram, thoracic and abdominal displacement (inductive plethysmography bands), airflow (nasal/oral thermocouple), nasal pressure, electrocardiogram (single bipolar lead), snoring (microphone attached to a vest), body position (Hg gauge sensor), pulse oximetry (Nonin, Plymouth, MN, USA) and ambient light (sensor attached to the vest to record on/off). Using Compumedics W-Series Replay, version 2.0, release 22, sleep stages were scored according to standard criteria 4. The respiratory disturbance index (RDI) was defined as the number of respiratory events (apnoeas and hypopnoeas) per hour of the total sleep time irrespective of any associated oxygen desaturation or arousal. Polysomnograms with <4 h of scorable oximetry were classified as failed studies and were repeated if the participant consented. Central apnoeas were scored if both airflow and thoracoabdominal effort were absent. However, central events that occurred after movement were not included. Obstructive apnoeas were identified if the airflow signal decreased to below 25% of the “baseline amplitude”. Hypopnoeas were scored if the magnitude of any ventilation signal decreased below ∼70% of the “baseline” amplitude, as described previously 1. Although more recent rules for scoring respiratory events have been published 5, we elected to score apnoeas and hypopnoeas using the same algorithm used in our 1999 study in order to make valid comparisons between the two time-points, in each instance using the thermistor and/or the inductance plethysmography signal to score respiratory events.
The RDI that we routinely compute includes central apnoeas as well as obstructive apnoeas and hypopnoeas 3. Based on the clinical and physiological uncertainty of central apnoeas in children 6, 7, we subtracted central events from the RDI to derive the OAHI. In our subjects this index primarily represents hypopnoeas. For example, in 1999 the number of frank obstructive events ranged from 0.1–0.8 obstructions·h−1, except for one subject that averaged 7 obstructions·h−1 in 1999. In 2006 there were no frank obstructions, except in one subject who had 2 obstructions·h−1.
Ventilatory control protocol
For this section of the study, participants were studied between 09:00 h and 16:00 h and were instructed to refrain from caffeinated beverages and food for 1 h prior to the time of their scheduled experiment. Subjects were studied while seated, and listened to music through headphones throughout the entire protocol. Analogue waveforms from transducers monitoring expiratory airflow, mask pressure and the fractional concentrations of O2 and CO2 were passed through an analogue-to-digital converter (Spike II; Cambridge Electronic Design, Cambridge, UK), sampled at 2,500 Hz per channel, and stored on the hard drive of an IBM-compatible computer (details of the measurements and equipment are given below). Estimated oxygen saturation of arterial blood was monitored and recorded manually in all studies with a pulse oximeter (Ohmeda, Madison, WI, USA).
The investigators were blinded to the OAHI status of the subject when conducting experiments and analysing data. For hyperoxic hypercapnia, the subjects breathed from humidified Douglas bags filled with 3, 5 or 7% CO2 in O2. Subjects started by breathing room air for 3–5 min, and then breathed, in succession, each of the three CO2-O2 mixtures for 3 min each, and recovered by breathing room air. In all conditions, airway occlusions were applied twice per minute, to obtain measurements of mouth occlusion pressures measured 100 ms after onset of inspiratory effort (P0.1).
Measurement of pulmonary ventilation, inspired and expired gas concentrations and P0.1
Subjects breathed through a tight fitting mask that covered the nose and mouth, and that allowed free breathing through either the oral or nasal airway (Hans-Rudolph paediatric rubber face mask; Hans Rudolph Inc., Shawnee, KS, USA). An additional rubber seal was created around the mask using Exaflex (GC America, Inc., Alsip, IL, USA). The mask was checked for leaks by instructing the subjects to hyperventilate, while an investigator looked for leaks by sampling CO2 around the mask seal. When leaks were noted, they were sealed with additional Exaflex. The PET,CO2 on inspiration returned to zero under all conditions, indicating that the system dead space was sufficiently low to prevent rebreathing.
A low dead-space non-rebreathing valve (Hans Rudolph 2600; Hans Rudolph Inc.) was attached to the mask, and a short length of tubing and a pneumotachometer (Hans-Rudolph 4700; Hans Rudolph Inc.) were placed on the expiratory side of the breathing valve for the measurement of expiratory airflow. The pressure drop across the pneumotachometer was measured with a differential pressure transducer with a ±2.5 cmH2O diaphragm (Validyne MP 45; Validyne Engineering Corp., Northridge, CA, USA). The pneumotachometer was calibrated with a precision Matheson Rotameter before each experiment. The respiratory period was measured from the flow signal, and used to compute breathing frequency. Expired flow was integrated by the computer offline to derive expired tidal volume (VT), which was converted from ambient temperature, pressure and humidity conditions to body temperature, pressure and humidity conditions, with the assumption that body temperature was 37°C. V′E was computed offline as the product of VT and breathing frequency. Breath-by-breath values for CO2 and O2 were measured with a rapidly responding analyser (Raytech Instruments, Cerritos, CA, USA), which was connected to the non-rebreathing valve by small-bore tubing. The output of the analyser was digitised and used to compute PET,CO2 and the end-tidal level of O2. Mask pressure was measured by connecting a short length of PE 200 tubing to the centre of the non-rebreathing valve, and attaching the opposite end to a differential pressure transducer with a ±56 cmH2O diaphragm (Validyne MP 45).
For the measurement of P0.1, a Hans-Rudolph Inc. automated balloon valve was attached to the inspiratory side of the non-rebreathing valve. The balloon was connected to a compressed air source, and was inflated or deflated with a solenoid valve and vacuum pump, respectively. The computer controlled the activation and deactivation of the solenoid valve and pump. The inspiratory port was occluded during expiration, and the occlusion was maintained for ∼200 ms into the ensuing inspiration, allowing sufficient time to obtain measurements of P0.1 8. The computer measured the drop in mask pressure exactly 0.1 s after the onset of the occluded inspiratory effort, and denoted this value as P0.1.
Data processing and statistical analysis
Six to 10 breaths obtained over the last 30–40 s of control conditions and at each level of hyperoxic hypercapnia, were used to calculate average values of VT, breathing frequency, V′E and the end-tidal partial pressure of O2 and PET,CO2. All P0.1 values obtained in each condition were averaged, so that each subject had a single P0.1 measurement for each of the experimental conditions. All P0.1 values were expressed in units of cmH2O.
The V′E values for each subject were divided by the subject’s body surface area (BSA; m2) to account for the large differences in size (and hence VT) over the 6-yr age range of our paediatric subject population. This analysis allowed us to compare the hypercapnic ventilatory responses of our subjects with previously published results in children of various ages.
To determine if the severity of sleep-disordered breathing (SDB) correlates with ventilatory drive, we first plotted both V′E and P0.1 against PET,CO2 for each subject, and computed the slope of the relationship as an index of hypercapnic ventilatory drive using linear regression analysis. We then plotted each subject’s hypercapnic response slope against their OAHI, and subjected the data to a linear regression analysis. To determine if resting CO2 retention was correlated with the OAHI, we plotted the resting level of PET,CO2 for each subject against his or her OAHI. We used a simple linear regression model followed by ANOVA (Sigma Stat 3.0) to determine if the relationship was statistically significant, with significance defined as p<0.05.
As shown in table 1⇑, the subject’s average RDI and OAHI values were significantly lower in 2006 compared to 1999. Figures 1a⇓ and 1b⇓ show the RDI and OAHI values for each subject, and it is clear that these values fell with age in all but three subjects, in whom the values were very low to begin with. Weight, height, BMI and BSA all were significantly higher in 2006 compared to 1999 (table 1⇑). The BMI percentile adjusted for age, sex and ethnicity, calculated from the US Centers for Disease Control and Prevention growth charts 9, was higher in 2006 than in 1999 for 11 out of the 14 subjects (fig. 1c⇓). Total sleep time and arterial oxygen saturation nadir measured in the 1999 and 2006 nocturnal sleep studies did not differ significantly (table 1⇑).
The absolute pulmonary ventilation rate measured under resting conditions increased with age (fig. 2a⇓), but was the same when it was expressed as a function of BSA (p = 0.26; fig. 2b⇓). The rise in resting ventilatory output was associated with a much lower PET,CO2, from 44.1±0.8 mmHg in 1999 to 33±1.0 mmHg in 2006 (p<0.001; fig. 2c⇓). In contrast to the increase in lung ventilation with age, the resting P0.1 showed marked variability at both ages, and the mean values were not significantly different (fig. 2d⇓). It is important to point out that our baseline P0.1 values are within the range reported for children of similar age by Marcus et al. 10.
We estimated central CO2 sensitivity by conducting steady-state CO2 response tests at 3, 5 and 7% inspired CO2. The individual data points that were used to define the slope of the V′E–PET,CO2 response for all subjects are shown in figure 3a⇓. It is clear from this figure that although the slopes in 1999 and 2006 are not different (y = 1.28(x)−35.6 and y = 1.31(x)−28.6 in 1999 and 2006, respectively; t = 0.08, p = 0.94), the curve is markedly left shifted, such that V′E was much higher at any given PET,CO2 in 2006 compared to 1999 (fig. 3a⇓). For example, at a PET,CO2 of 50 mmHg the V′E estimated from the data shown in figure 3a⇓ would be 28.4 L·min−1 in 1999 and 36.9 L·min−1 in 2006. Similarly, the average x-intercept calculated from the slopes, the so-called apnoea point 11, was significantly different (33.1±1.1 and 26.4±2.7 in 1999 and 2006, respectively; t = 2.41, p = 0.03). Figure 3b⇓ shows that the left shift in the V′E–PET,CO2 response curves is due to a corresponding left shift in the VT–PET,CO2 response (y = 0.05(x)−1.08 and y = 0.07(x)−0.34 in 1999 and 2006, respectively; t = 1.43, p = 0.16), coupled with a frequency response that was identical in 1999 and 2006 (data not shown). To correct for changes in V′E secondary to growth, we also examined the V′E–PET,CO2 response curves with V′E expressed as a function of the BSA. Again, there were no significant differences in the slope of the relationship (y = 0.65(x)−19.6 and y = 0.72(x)−15.3 in 1999 and 2006, respectively; t = 0.36, p = 0.72).
Interestingly, the P0.1–PET,CO2 slopes in 1999 and 2006 (fig. 4⇓) were statistically identical (y = 0.23(x)−5.7 and y = 0.22(x)−5 in 1999 and 2006, respectively; t = 0.16, p = 0.87), and were only slightly left shifted, such that the apnoea point was the same in 1999 and 2006 (26.7±2.2 and 23.7±1.8 in 1999 and 2006, respectively; t = 1.24, p = 0.24). Individual V′E–PET,CO2 and P0.1–PET,CO2 slopes for each subject are shown in figures 5a⇓ and 5c⇓. With a few exceptions, the slopes did not change with age, and the group average slopes were the same in 1999 and 2006 (V′E–PET,CO2: 1.7±0.24 and 1.65±0.22, in 1999 and 2006, respectively, t = 0.14, p = 0.89; P0.1–PET,CO2: 0.28±0.04 and 0.32±0.05 in 1999 and 2006, respectively, t = 0.6969, p = 0.49). The individual apnoea points for each subject are shown in figure 5⇓ b and d. Statistical analysis of the individual apnoea points revealed significant differences for the V′E–PET,CO2 curves (35.5±1.3 and 24.6±3.2 in 1999 and 2006, respectively; t = 3.16, p = 0.0082), but not for the P0.1–PET,CO2 curves (29.9±2.5; 2 and 23.7±2.0 in 1999 and 2006, respectively; t = 1.4, p = 0.19).
This is the first longitudinal study of developmental alterations in the ventilatory control of children with SDB in early childhood. Our subjects were, on average, 9 yrs of age in the initial study and 15 yrs of age when the current study was completed. Our main finding is that subjects now have much lower OAHIs and retain much less CO2 than they did in 1999, despite similar mass-specific resting ventilation rates and a substantial increase in BMI. We also showed that, on average, the slopes of the V′E–PET,CO2 or P0.1–PET,CO2 curves were not significantly different in 1999 and 2006, indicating that CO2 sensitivity during steady state hypercapnic challenge was unchanged. However, the V′E–PET,CO2 response was markedly left shifted in 2006 compared to 1999, indicating greater V′E at a given PET,CO2, despite no change in CO2 sensitivity. This left shift also resulted in much lower apnoea points 11. Interestingly, there was no left shift in the P0.1–PET,CO2 curve. As will be discussed later, our observations are most likely explained by changes in mechanical factors, e.g. a decrease in airflow resistance, rather than changes in the central control of breathing. The fact that the left shift in the V′E–PET,CO2 response curve was the result of changes in VT rather than breathing frequency also supports the idea that the effects were mechanical and not central.
In theory, the increased lung ventilation rate at a given PET,CO2 with no change in the sensitivity to CO2 could be explained by a reduction in airway resistance. This is due, in a large part, to somatic growth of the airway as children grow taller 12. Using the regression equations derived by Zapletal and Chalupova 12, we estimate that nasopharyngeal resistance in our subjects would have fallen by 23% on the basis of the change in height alone (136 cm tall in 1999 versus 163 cm tall in 2006, table 1⇑). In addition, it is well known that tonsil size declines with age after reaching a peak size between 4 and 8 yrs of age 13. There have been anecdotal suggestions that enlarged tonsils, and thus a narrow pharyngeal airway, predispose children to nasal breathing, which in turn leads to hypoventilation through the high-resistance nasal pathway. When our subjects were initially studied, in 1999, we found that their resting PET,CO2 during wakefulness was significantly elevated, and that it was correlated with the OAHI 1. Our previous magnetic resonance imaging (MRI) studies showed that a sub-sample of the group studied in 1999 had large adenoids, tonsils and soft palates 14. We analysed those data by computing the sum of the cross-sectional areas of these soft tissue structures and expressed the sum as a percentage of the nasopharyngeal cross-sectional area. We found that the children with SDB had a narrow nasopharynx as a result of increased soft tissue mass. The present data suggest that this ratio is now smaller. In other words, if their pharyngeal airway grew at a faster rate than the surrounding soft tissue structures, the lumen of the nasopharynx would be enlarged, leading to lower airway resistance. Although we do not have MRI data as part of this study, the increased ventilation and lack of CO2 retention in the absence of a change in sensitivity to PET,CO2 is consistent with a larger upper airway lumen. The results of this study and our earlier one are consistent with other data showing hypoventilation and CO2 retention in young children with enlarged tonsils and SDB 15, 16.
We observed that both the RDI and OAHI decreased over the approximate 6-yr interval between polysomnograms (PSGs) in these subjects. Preliminary examination of data from the entire TuCASA cohort confirms this finding 2. In contrast to our findings, a previous study in Thai children showed that five out of seven children with obstructive sleep apnoea (OSA) had a higher OAHI over a 3-yr interval 17. However, PSG was performed in these children because they had symptoms of OSA, and thus there may have been some selection bias. As discussed previously, we suspect that in our study the observed decrease in RDI and OAHI is related to somatic growth of the pharynx, coupled with regression of tonsillar tissue with age. At the initial TuCASA examination, children were studied between the ages of 6 and 11 yrs. This is the age range where some children will have large tonsils resulting in an elevated RDI and OAHI. With normal regression in tonsil size as they become adolescents, there should be a decrease in RDI and OAHI as we found.
Although our observations are consistent with changes in airway resistance, the exact mechanism of the CO2 retention during quiet breathing at an average age of 9 yrs, but not approximately 6 yrs later remains unknown. One possibility is that the young children “chose” to hypoventilate rather than fight the increased flow resistance and, thus, higher work of breathing, that would have been required to drop their PET,CO2. This is consistent with the strategy employed by highly trained athletes during peak exercise, wherein they allow themselves to become hypoxic and relatively hypercapnic rather than consume the extra energy that would be required to elevate alveolar ventilation sufficiently to fully correct the blood gas and acid base derangements 18.
The left-shifted ventilatory response to PET,CO2 is often considered to be due to an “extra” stimulus to breathe. If this were the case, we would have expected a left shift in the relationship between P0.1 and PET,CO2, which we did not observe (fig. 4⇑). Complicating the relationship between P0.1 and PET,CO2 is that the former can be influenced by both respiratory muscle strength and end-expiratory lung volume. Weak inspiratory muscles lead to lower P0.1 values during hypercapnic challenges, although the effects are small until the PET,CO2 exceeds 60 mmHg 19. This would have little or no impact on our data, as the PET,CO2 values were <60 mmHg in every case (figs 3⇑ and 4⇑). Inspiratory muscle strength increases by ∼20% from age 9 yrs to 15 yrs in males 20, suggesting that changes in strength alone as the subjects grew would result in slightly higher P0.1 values in 2006 compared to 1999 (fig. 4⇑), but this was not seen.
The P0.1 can also be influenced by changes in end-expiratory lung volume, with lower volumes associated with a greater P0.1, due to improved muscle length-tension properties and thus improved mechanical advantage 21, 22. However, the pertinent issue is the end-expiratory lung volume as a percentage of an individual’s total lung capacity, as this dictates the length–tension relationship of the respiratory muscles for that particular system 23. End-expiratory lung volume as a percentage of total lung capacity increases from ∼46% in 9-yr-olds (the average age of our subjects in 1999), to 53% in 15-yr-olds (average age in 2006), corresponding to a volume increment of ∼400 mL 24. It has been shown that an increase in functional residual capacity of 500 mL reduces respiratory muscle pressure development by ∼10% 23. In our subjects this would translate into, at most, an 8% decrease in the P0.1 (400/500×10%), which would result in only a negligible shift in the P0.1–PET,CO2 curve (fig. 4⇑). Obesity can reduce end-expiratory lung volume independently of age and height, although the effects are small and variable 25, 26 except in severe obesity 27. Most of our subjects were in a higher BMI percentile in 2006 than they were in 1999 (fig. 1c⇑), with some of them exhibiting severe obesity (i.e. BMI values ≥95th percentile). This could also contribute to a slight leftward shift in the P0.1–PET,CO2 relationship, but again, this was not observed. Finally, although it is possible that hypercapnia could increase airway resistance to a variable extent across the subject population, the P0.1 is uninfluenced by airway resistance and behavioural adjustments in ventilatory output 8, 23. Taken together, our longitudinal data support the contention that the elevated resting PET,CO2 and the left-shifted V′E–PET,CO2 curve in younger children is the result of reduced flow resistance, and probably not due to the addition of an “extra” excitatory stimulus to breathe.
The functional consequences of the elevated eupneic PET,CO2 when the children were younger are unknown. Given that the apnoea point was significantly higher in 1999 than in 2006, one might surmise that the tendency for apnoea was greater in the young children. However, the difference between the apnoea point and the eupneic PET,CO2 was the same in 1999 and 2006 (1999: 7.7±1; 2006: 6.9±3; p = ns). This difference has been called the CO2 reserve, and it has been suggested that a smaller reserve increases the propensity for apnoea in adult human subjects 28. Our subjects had higher RDI values in 1999 than in 2006 despite a similar CO2 reserve, suggesting that the CO2 reserve may not predict a predisposition to apnoea in children.
In conclusion, we have examined changes in the control of breathing from childhood to adolescence in a group of subjects that had mild SDB as young children. The main finding is that the rate of pulmonary ventilation at a given PET,CO2 was much higher, and the eupneic PET,CO2 much lower at an average age of 15 yrs compared to an average age of 9 yrs. This occurred in the absence of changes in sensitivity to inspired CO2, suggesting that upper airway resistance decreased as the children grew, leading to improved alveolar ventilation in the absence of significant changes in central ventilatory drive.
This study was supported by a National Institute of Health grant (HL 62373).
Statement of interest
- Received November 28, 2008.
- Accepted June 23, 2009.
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