Title: Ventilatory response to CO₂ with Read’s rebreathing method in normal infants

Authors: Yosuke Yamada, M.D.¹⁾, Nobuhide Henmi, M.D*.¹⁾, Hisaya Hasegawa, M.D., Ph. D.¹⁾, Shio Tsuruta, M.D.¹⁾, Satoko Tokumasu, M.D.¹⁾, Yusuke Suganami, M.D. Ph. D.¹⁾, Masanori Wasa, M.D.¹⁾

1) Department of Neonatology, Tokyo Women’s Medical University Medical Center East

Funding: No grant support, manufacturer support to report.

*Corresponding author: Nobuhide Henmi, 2-1-10 Nishiogu, Arakawa-ku, Tokyo, Japan, 116-8567, Phone: +81-3-3810-1111, Email:sukoyakahenmi07777@gmail.com,

Conflicts of interest: The authors have no conflicts of interest or financial disclosures.

Authorship contribution: Y. Y., N. H. and H. H designed the study; Y. Y, N. H, H. H, S. T, S. T and Y. S performed examination and collected data. Y. Y and M. W wrote the manuscript and N. H, H. H and M.W critically reviewed the manuscript. All authors read and approved the final manuscript.

Keyword: Ventilatory response to CO₂, rebreathing method, steady-state method, infants, respiratory center

Running title: VRCO₂ with the rebreathing in normal infants

Abstract

Background

Methods of evaluating the ventilatory response to CO₂(VRCO₂) of the respiratory center include the steady-state and the rebreathing method. Although the rebreathing method can evaluate the respiratory center more in detail, the steady-state method has been mainly performed in infants. The aim of this study was to investigate whether we could perform the VRCO₂ with the rebreathing method in normal infants.

Methods

The subjects were 80 normal infants. The gestational age was 39.9（39.3-40.3）weeks, and the birth body weight was 3,142 (2,851-3,451) grams. We performed the VRCO₂ with Read’s rebreathing method, measuring the increase in minute volume (MV) in response to the increase in EtCO₂ by rebreathing a closed circuit. The value of VRCO₂ was calculated as follow: VRCO₂ (mL/min/mmHg/kg) = ΔMV / ΔEtCO₂ / Body weight.

Results

We performed the examination without adverse events. The age in days at examination was 3 (2-4), and the examination time was 150±38 seconds. The maximum EtCO₂ was 51.1 (50.5-51.9) mmHg. The value of VRCO₂ was 34.6 (29.3-42.8). Tidal volume had a greater effect on the increase in MV than respiratory rate (5.4 to 14.3 mL/kg, 44.1 to 55.9 /min, respectively).

Conclusion

This study suggests that the rebreathing method can evaluate the ventilatory response to high blood CO₂ in a short examination time. We conclude that the rebreathing method is useful even in infants. In the future, we plan to measure the VRCO₂of preterm infants, and evaluate the respiratory center of infants in more detail.

Introduction:

Respiration is controlled mainly by the central and peripheral chemoreceptors ¹⁾. The central chemoreceptor located in the respiratory center of the medulla oblongata increases the amount of ventilation in response to pH change due to high blood CO₂. The peripheral chemoreceptor in the carotid body responds to hypoxemia for the increase in the amount of ventilation. Respiratory control is mainly regulated by the respiratory center in response to change in blood CO₂, because the responsiveness to the change in pH is more sensitive than the change in PaO₂. Therefore, chemical respiratory control is evaluated as the ventilatory response to CO₂(VRCO₂), and its evaluation methods include the steady-state and the rebreathing method ²⁾. In the steady-state method, subjects breath gas with a fixed CO₂ concentration (e.g. 0%, 2%, 4%), and the change in amount of ventilation is evaluated. In the rebreathing method, subjects accumulate blood CO₂ by rebreathing a closed circuit, and the amount of ventilation is evaluated continuously at each CO₂ concentration. The value of VRCO₂which evaluates the responsiveness to CO₂ quantitatively is calculated with this formula, VRCO₂ (mL/min/mmHg/kg) = ΔMinute volume (MV)/ΔEtCO₂/body weight.

Respiratory control in infants is premature and its prematurity causes respiratory diseases such as apneic attacks. Many preterm infants need oxygen therapy or mechanical ventilation due to prematurity of the respiratory center. Prematurity is also one of the risk factors in brief resolved unexplained events (BRUE)³⁾. Therefore, it is important to evaluate the respiratory center in infants. The VRCO₂ was mainly evaluated by steady-state methods in infants so far. Some report low VRCO₂ of infants born to smoking and drug abusing mothers ^{4, 5)}. However, the VRCO₂ with the rebreathing method is rarely performed in infants. While the steady-state method evaluates the increase in the amount of ventilation only at several CO₂concentrations, the rebreathing method evaluates the amount of ventilation continuously in increasing CO₂concentrations with rebreathing ²⁾. The rebreathing method can evaluate the VRCO₂ in more detail. The aim of this study was to evaluate whether we could perform the VRCO₂ with the rebreathing method in normal infants, as a first step in evaluating the respiratory center in more detail.

Methods:

Subjects

The subjects were infants without congenital abnormality and maternal smoking history admitted to the normal newborn room between March and September 2011. Infants in the normal newborn room of our hospital need no medical support such as oxygen, infusion and photo therapy. We performed SpO₂ monitoring for 12 hours to infants in the normal newborn room and excluded infants with apneic attacks. The total number of subjects was 80 and we performed VRCO₂ with the rebreathing method to each subject. Basic characteristics are shown in Table 1. The gestational age was 39.9（39.3-40.3）weeks, the birth body weight was 3,142 (2,851-3,451) grams, Apgar score at 1 min and 5 min was 8 (8-9) and 9 (9-9) and the rate of caesarean delivery was 22.5%. This study was approved by the ethics committee of our institution. Written informed consent was obtained from the patient’s parents.

Ventilatory response to CO₂

We performed VRCO₂ with the rebreathing method based on Read’s method ⁶⁾ using ARFEL III (Aivison, Tokyo, Japan) (Figure 1). The closed circuit consists of a face mask, EtCO2 sensor (Oridion medical, Jerusalem, Israel), pneumotachograph (Hans Rudolph, Shawnee, Kansas) and a bag filled with 5% CO₂and 95% O₂. Tidal volume (TV), inspiration time (I), expiration time (E) and EtCO₂ were measured with a pneumotachograph and EtCO₂ sensor, and these data were transferred to and analyzed by ARFEL III. As in Figure1, we accumulated the subjects’ blood CO₂ by rebreathing the closed circuit during quiet sleep. Respiratory parameters of each breath were measured until the EtCO₂ elevated more than 2% from the beginning of the examination. We terminated the examination when the patient woke up. We monitored SpO₂ during the examination and confirmed that the subjects had enough spontaneous breathing to exhaust CO₂ after the examination.

The value of VRCO₂ was calculated from the relationship between EtCO₂ and MV. The formula is as follow: VRCO2 = ΔMV / ΔEtCO₂ / Body Weight. In addition to VRCO₂, we investigated TV, inspiratory time/ total respiratory time (I/I+E), and respiratory rate (RR) at the beginning and end of examination. The data were analyzed with JMP® 15 (SAS Institute Inc, Cary, NC, USA). The values are in mean±SD or in median (first quartile – third quartile) according to distribution. Wilcoxon test was used to compare RR, TV and I/E at the beginning and end of examination. P value < 0.05 was considered statistically significant.

Results:

We performed the examination without any adverse events in all cases. The age in days at examination was 3 (2-4), and the examination time was 150 ± 38 seconds. The maximum EtCO₂ was 51.1 (50.5-51.9) mmHg.

An example of the result is shown in Figure 2. At the beginning of the examination, RR was 44.7 /min, TV was 5.4 mL/kg and I/I+E was 0.47. MV increased in response to the continuous increase of EtCO₂ by rebreathing. At the end of the examination, RR was 60.1 /minute, TV was 15.0 mL/kg, I/I+E was 0.44 and EtCO₂ was 50.7 mmHg. The value of VRCO₂in this case was 36.5 mL/kg/min/mmHg. TV contributed to the increase in MV more than RR.

All data is shown in Table 2. The value of VRCO₂ was 34.6 (29.3-42.8) mL/kg/min/mmHg. RR and TV significantly increased through the examination (44.1 to 55.9 /min, P<0.001, 5.4 to 14.3 mL/kg, P<0.001, respectively). The rate of increase was higher in TV than RR. There was no significant change in I/I+E through the examination (0.48 to 0.47, P=0.14).

Discussion:

The aim of this study was to perform the ventilatory response to CO₂ with the rebreathing method in normal infants to evaluate the respiratory center. The examination was performed without any adverse events and the value of VRCO₂ was 34.6 (29.3-42.8) mL/kg/min/mmHg. The median examination time was 150 seconds and the median maximum EtCO₂ was 51.1mmHg. The increase in TV had a greater effect to the increase in MV than that of RR. To the best of our knowledge, this is a study with the largest number of subjects evaluating the VRCO₂ with the rebreathing method in normal infants.

The steady-state and rebreathing method are the main methods for evaluating VRCO₂. However, some studies have reported that evaluation with the rebreathing method is more precise and has higher reproducibility than the steady-state method.

Berkenbosch et al. performed the VRCO₂ with the steady-state and rebreathing method in 10 males, and reported that the value of VRCO₂ with the rebreathing method was 1.85 times higher than that of the steady-state⁷⁾. They described that the difference between EtCO­₂ and the CO₂ partial pressure in the central chemoreceptor in the rebreathing method was less than the difference in the steady-state method, causing the difference in VRCO₂ between the 2 methods. In brief, the error between EtCO₂ and blood CO₂ of the respiratory center was larger in the steady-state method. As a result, the VRCO₂ with the steady-state method could be lower than that of the rebreathing method. In the study of Mohan et al., the VRCO₂ with the steady-state method was significantly lower than that of the rebreathing method in 5 normal adults⁸⁾. This was because the examination time was longer and the VRCO₂ was evaluated at a lower blood CO₂ in the steady-state method. The long examination time causes respiratory muscle fatigue, and the increase in the amount of ventilation demanded by the ventilatory response of the respiratory center cannot be generated, which may result in an underestimation of the VRCO₂. Hasegawa et al. demonstrated that the tolerance for respiratory load in infants was lower than in adults, and respiratory muscle fatigue was more likely to occur in infants⁹⁾. We think that the short examination time is one of the advantages of the rebreathing method in infants. The increase in EtCO₂ with the steady-state method could be insufficient, because the subjects breathe gas with a fixed CO₂ concentration repeatedly in the steady-state method. In our study, we increased the maximum EtCO₂ to up to 51 mmHg and we were able to evaluate the VRCO₂ at a high CO₂ concentration.

Mannee et al. performed the VRCO₂ with the steady-state and rebreathing method multiple times in 20 normal adults and evaluated the intraclass correlation coefficients (ICC) of the VRCO₂ ¹⁰⁾. While the ICC of the rebreathing method was as high as 0.89, the ICC of the steady-state method was 0.56. They found that the VRCO₂ with the rebreathing method had higher reproducibility than that of the steady-state method. They assumed that the cause of this difference was due to the discordance between EtCO₂ and blood CO₂ of the respiratory center in the steady-state method, as in the study of Berkenbosch et al. The high reproducibility is more preferable in infants, because it is difficult for them to follow our orders and to be examined multiple times. This high reproducibility is also an advantage of the rebreathing method.

After focusing on the difference between methods, we focused on the difference in results of the same rebreathing method. The value of VRCO₂ (ΔMV/ΔEtCO₂) in three studies with the rebreathing method in normal adults were 1.2, 4.6, 1.7 L/min/mmHg, respectively ^{7, 8, 10)}. In addition, Akiyama reported the value of VRCO₂ in 2 studies, 2.2 for 28 patients¹¹⁾ and 1.9 for 17 patients ¹²⁾, respectively with the rebreathing method in normal adults. It was assumed that this difference was due to the different range in EtCO₂ evaluated in the examination although the same rebreathing method was adopted. In the future, if we were to evaluate other pathologies using data of this study as reference, we should use the same protocol as of this study. It was difficult to compare our normal infants’ data with normal adults’ data, because the protocol and equipment were not the same and the data of normal adults’ lacked adjustment for body weight or body surface area. However, the data of 17 normal adults reported by Akiyama et al. included respiratory parameters (e.g. RR, TV) during measurement ¹²⁾, so we compared those parameters with our data. TV and RR at the beginning of examination were 0.27 L and 17.0 /min, and those at the end of examination was 1.53 L and 23.6 /min. The increase in TV was larger than that of RR, which was consistent with our data.

The VRCO₂ of the respiratory center has been mainly evaluated with the steady-state method in infants, and the VRCO₂ of infants born to smoking and drug abusing mothers has been performed so far ^{4, 5)}. Data of the VRCO₂ in normal infants was measured as the control for comparison. Wingkum et al. showed that the VRCO₂ in 12 infants of drug abusing mothers was lower in comparison with 12 normal infants⁴⁾. In this study, EtCO₂ and MV were measured with the steady-state method in which subjects breathed room air and 4% CO₂ gas for 5 minutes each. The maximum EtCO₂ was 40.43 mmHg, which was within normal range. The examination time of our study was 2.5 minutes, which was the shortest in all the studies quoted in this article. Furthermore, the maximum EtCO₂ in our study was 51.1 mmHg, so we were able to evaluate the VRCO₂ at high blood CO₂levels.

Ali et al. compared the VRCO₂ among 34 infants born to smoking mothers, 22 infants born to drug abusing mothers and 22 normal infants. They showed that the increase in ventilation was significantly lower in order of infants born to drug abusing mothers, smoking mothers and normal infants ⁵⁾. In this examination, MV was measured when breathing 0%, 2%, and 4% CO₂ gas, and the VRCO₂ was evaluated by the relationship between MV and F_ICO₂ rather than EtCO₂. The VRCO₂ using F_ICO₂ could be underestimated because the error tended to be larger between F_ICO₂ and blood CO₂ of the respiratory center than the error between EtCO₂ and blood CO₂, as in the study by Mannee et al.¹⁰⁾. In fact, the values of VRCO₂(mL/min/mmHg/kg) in normal infants were a median of 34.6 in our study and an average of 48.7 in the study by Wingkun et al.⁴⁾ compared to a low average of 10.9 in the study by Ali et al.⁵⁾

This study has some limitations. Normal adults were selected after respiratory function tests and interviews^{11), 12)}. However, it was difficult to perform spirometry test to infants, so we selected newborns that did not need any medication or monitoring as normal infants. This way of selection was the same as in other studies for normal infants ^{4, 5)}. Next, we measured the VRCO₂ once in this study. Some studies of normal adults studied the average measuring the VRCO_2­ multiple times^{7), 11)}. We designed to measure the VRCO₂ once in this study, because this physiologic test is difficult to perform in infants and the reproducibility of the rebreathing method is high ¹⁰⁾. We plan on evaluating the respiratory center of infants in more detail by performing the VRCO₂ to preterm infants, infants with apneic attacks and infants treated with caffeine, using the data of normal infants obtained from this study.

Conclusion:

We measured the VRCO₂ with the rebreathing method in normal infants. The value of VRCO₂ was 34.6 (29.3 - 42.8) mL/kg/min/mmHg. This study suggested that the rebreathing method can evaluate the ventilatory response in high blood CO₂concentrations in a short examination time. We conclude that the rebreathing method is useful even in infants. In the future, we plan on measuring the VRCO₂ of preterm infants, infants with apneic attacks, and evaluate the respiratory center of infants more in detail.

Reference: