Helium-oxygen mixture does not improve gas exchange in mechanically ventilated children with bronchiolitis

Abstract

Varying concentrations of helium-oxygen (heliox) mixtures were evaluated in mechanically ventilated children with bronchiolitis. We hypothesized that, with an increase in the helium:oxygen ratio, and therefore a decrease in gas density, ventilation and oxygenation would improve in children with bronchiolitis. Ten patients, aged 1-9 months, were mechanically ventilated in synchronized intermittent mandatory ventilation (SIMV) mode with the following gas mixtures delivered at 15-min intervals: 50%/50% nitrogen/oxygen, 50%/50% heliox, 60%/40% heliox, 70%/30% heliox, and return to 50%/50% nitrogen/oxygen. The use of different heliox mixtures compared with 50%/50% nitrogen/oxygen in mechanically ventilated children with bronchiolitis did not result in a significant or noticeable decrease in ventilation or oxygenation. ) and work of breathing in children and adults with status asthmaticus. We hypothesized that, in mechanically ventilated children with bronchiolitis, increasing the ratio of helium:oxygen concentrations would improve both ventilation and oxygenation. To examine the effect of varying concentrations of heliox mixtures on ventilation and oxygenation in mechanically ventilated children with bronchiolitis. ). = 0.93, 0.98, and 0.96, respectively). The use of different heliox mixtures compared with 50%/50% nitrogen/oxygen in mechanically ventilated children with bronchiolitis did not result in a significant or noticeable decrease in ventilation or oxygenation.

Introduction:

) and work of breathing in children and adults with status asthmaticus. We hypothesized that, in mechanically ventilated children with bronchiolitis, increasing the ratio of helium:oxygen concentrations would improve both ventilation and oxygenation.

Objective:

To examine the effect of varying concentrations of heliox mixtures on ventilation and oxygenation in mechanically ventilated children with bronchiolitis.

Patients and methods:

).

Results:

= 0.93, 0.98, and 0.96, respectively).

Conclusion:

The use of different heliox mixtures compared with 50%/50% nitrogen/oxygen in mechanically ventilated children with bronchiolitis did not result in a significant or noticeable decrease in ventilation or oxygenation.

Introduction

Bronchiolitis is a disease that is epidemic during winter months worldwide. The pathogen that is most commonly found to be responsible for this illness is the respiratory syncytial virus. Each year, up to 2% of all infants in a community require hospitalization for bronchiolitis due to feeding difficulty, respiratory distress, apnea, or the need for supplemental respiratory support. Bronchiolitis in premature infants, infants less than 3 months of age, children with chronic lung disease, children with cardiac disease, and children with severe disease often mandates endotracheal intubation and mechanical ventilation.

in mechanically ventilated children with bronchiolitis.

Study population

We studied 10 pediatric patients with the diagnosis of viral bronchiolitis and respiratory failure mandating endotracheal intubation and mechanical ventilation who were admitted to the Critical Care Unit at Children's Hospital and Health Center, San Diego, California, USA. Bronchiolitis was defined by either a positive viral pathogen study or a clinical diagnosis with negative tracheal bacterial culture at the time of endotracheal intubation. Enrolment was from November 1996 to April 1998. The study protocol was approved by the Institutional Review Board for the Children's Hospital, San Diego, California, USA. Informed written consent was obtained from the patient's parent(s) before study enrolment. Enrolment criteria included the following: corrected gestational age greater than 38 weeks and less than 2 years at time of entry into the study; patient intubated more than 3 h and less than 48 h at onset of the study; no administration of brochodilators within 4 h of the initiation of the study; hemodynamic stability requiring no blood products or vasoactive medications before or during the study period; and no underlying congenital heart disease, chronic pulmonary disease, terminal illness, immune disease, or neuromuscular disease.

Study design

). None of these ventilator parameters were adjusted during the 75 min of the study. Before the initiation of the study, 50%/50% nitrogen/oxygen was delivered. All patients were sedated with a fentanyl infusion and were paralyzed with vecuronium during the trial. Demographic data collection, review of chest radiographs, and respiratory examination were performed by the primary clinical investigator before onset of the trial. Physiologic data collection was performed using the cardiorespiratory monitor (Marquette Transcope 12C, GE Medical Systems, Milwaukee, WI, USA) in conjunction with an arterial catheter for continuous blood pressure and intermittent blood gas analysis.

Changes in ventilation and oxygenation with changes in gas mixture in 10 infants with bronchiolitis on mechanical ventilation

I:E, ratio of ventilator inspiratory to expiratory times; IT, ventilator inspiratory time; RSV, respiratory syncytial virus.

No patient had tracheal suctioning within 20 min before or during the study. The only change made to the respiratory circuit during the study was to the gas mixture, as defined below. The different heliox concentrations provided by the ventilator were obtained by adjusting the relative flow rates of 100% oxygen to 100% helium gas supplied to the ventilator until the desired ratio was obtained, which was determined by measuring the oxygen fraction (55/90 oxygen analyzer, Hudson RCI, Ventronics Division, Temecula, CA, USA) in the inspiratory limb of the ventilator circuit. The ventilator was calibrated to deliver a constant tidal volume to each patient on the different gas mixtures. Protocol required that the trial be terminated in the event of any pulmonary toilet, other ventilator adjustment, interruption of the respiratory circuit, or acute deterioration in patient status requiring other interventions.

ratios were obtained at study onset and at 15-min intervals, just before changing the gas mixture. For each patient, data sets were collected 15 min apart while the patient was receiving the following sequence of gas mixtures: 50%/50% nitrogen/oxygen, 50%/50% nitrogen/oxygen, 50%/50% heliox, 60%/40% heliox, 70%/30% heliox, and 50%/50% nitrogen/oxygen.

Statistical analysis

< 0.05 was considered statistically significant.

Results

Ten patients, seven male and three female, with a mean age of 4 ± 3 months (range 1-9 months), were enroled into the study. Six patients were positive for respiratory syncytial virus on viral pathogen study. On average, the patients had upper respiratory symptoms for 2 days and increased work of breathing for 1 day before intubation. All patients required mechanical ventilation for respiratory failure as indicated by refractory tachypnea with fatigue or apnea.

are the averages of the initial two baseline 50%/50% nitrogen/oxygen data sets (obtained 15 min apart) and the final data set (obtained at 75 min from the onset of the study period) on return to 50%/50% nitrogen/oxygen. All patients' hemodynamic, respiratory status, and blood gas determinations were stable at the onset and on return to baseline 50%/50% nitrogen/oxygen at the end of the study period. There was no statistical or clinically significant difference between any of these three data sets on 50%/50% nitrogen/oxygen for an individual patient. Therefore, any change in ventilation or oxygenation seen with the different heliox gas mixtures should be the result of the change in gas mixture.

Statistical analysis of the changes in ventilation and oxygenation with changes in gas mixture in 10 infants with bronchiolitis who were on mechanical ventilation

= 0.96, respectively).

= 0.93, 0.98, 0.96, respectively).

All patients who were enroled completed the study, with no complications or departures from the protocol. Only three out of the 10 patients were continued on heliox after the study period at the discretion of the primary care team.

Discussion

]. This gradient is diminished when either the airways are narrowed or obstructed, or both, as in asthma or bronchiolitis.

(where f is frictional factor). Note that laminar flow is directly dependent on the gas viscosity μ, whereas turbulent flow is directly dependent on the gas density ρ.

].

). The patient with bronchiolitis compensates for the marked increase in airway resistance by increasing both the force of respiration and the respiratory rate to maintain minute ventilation and gas exchange. This can result in muscle fatigue and ultimately respiratory failure.

].

with heliox administration might have occurred.

).

= 10); the patients had only mild to moderate lung disease; the mode of ventilation (ie SIMV); and an ineffective concentration of helium.

ratios. Heliox was well tolerated by all patients, and no adverse effects were noted. Further studies are needed to determine whether helium can improve ventilation and oxygenation, and allow for lower ventilating pressures in patients with more severe lung disease or with higher concentrations of helium.

Acknowledgement

This study was supported in full by the Children's Hospital and Health Center, San Diego, California, USA.

Keywords

• bronchiolitis
• helium
• mechanical ventilation
• pediatrics
• respiratory failure