Introduction

Acute hypoxemic respiratory failure (AHRF) is one of the hallmarks of acute lung injury (ALI) and acute respiratory distress syndrome (ARDS), which are caused by an inflammatory process initiated by any of a number of potential systemic and/or pulmonary insults. In these critically sick patients, optimizing the management of oxygenation is crucial. Airway pressure release ventilation, (APRV) is a relatively new mode of mechanical ventilation that has been shown to achieve recruitment and improve oxygenation and gas exchange while maintaining acceptable peak airway pressure (PIP) [1–5]. APRV commences at an elevated baseline pressure and follows with a deflation to accomplish tidal ventilation. The modality consists of a high-pressure setting (Phigh) and a low-pressure setting (Plow) [6, 7]. Recruitment and oxygenation occur during Phigh—which is a high-level CPAP—at a set time interval (Thigh). Ventilation is augmented by releasing airway pressure to a second CPAP level (Plow).

Previous studies and case reports in adult patients suggest that APRV provides a benefit by allowing oxygenation and ventilation to patients with acute lung injury (ALI) and ARDS [1–5]. There is limited data about the benefits of APRV in children [8–10]. In this paper, authors have presented three patients with AHRF whose oxygenation did not improve with conventional ventilation and were treated with APRV.

Case Reports

Three patients who had developed AHRF were treated with APRV. The underlying causes of the respiratory failure were different. The demographic data and AHRF etiologies are shown in Table 1. Before the application of APRV, patients were mechanically ventilated using pressure control ventilation (PCV) mode with ventilatory variables, with sequentially increased PIP and positive end-expiratory pressure (PEEP), in an attempt to achieve maximum oxygenation at a minimum fraction of inspired oxygen (FiO2). The ventilator settings before conversion to APRV are shown in Table 2. All patients were ventilated using Dräger Evita XL ventilator (Dräger, Lübeck, Germany). Ventilatory variables on APRV were set according to the Habashi et al’s clinical guide for pediatric patients [11].

Table 1 Demographics and AHRF causes
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Table 2 Conventional ventilation data of the patients pre-enrollment to APRV treatment
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Case 1

An 11-month-old girl with a past medical history of chronic obstructive lung disease, presented with an episode of wheezing and hypercapneic respiratory failure. She had recurrent wheezing attacks since last 6 months. During the hospitalization, she was intubated for hypercapneic respiratory failure and transferred to our pediatric intensive care unit (PICU). She was hemodynamically stable. Initially the patient was supported with PCV. At the 24th h of mechanical ventilation, the patient’s oxygenation deteriorated, partial arterial oxygen pressure/fraction of inspired oxygen (PaO2/FiO2) ratio was 85 torr, and the chest radiograph showed right patchy infiltrate and left hyperaeration. Escherichia coli was grown from blood cultures. The oxygenation was not improved by high PEEP, high FiO2, long inspiratory time (TI) ventilatory settings and prone positioning. On the 32th h of mechanical ventilation, further deterioration in the patient’s oxygenation warranted a trial of APRV. Although patients with chronic obstructive lung disease and acute asthma exacerbation maybe difficult to ventilate with APRV, severe hypoxemia warranted the attempt. APRV ventilation resulted in a marked improvement in oxygenation. The FiO2 level decreased gradually. The patient continued to be weaned from ventilatory support and at the fifth day of APRV, pressure targeted synchronized mandatory ventilation (SIMV) and pressure support (PS) were introduced. The hyperaeration on the left lung persisted, so chest computed tomography and bronchoscopy were performed, leading to the diagnosis of Swyer-James-MacLeod syndrome. The patient was extubated on the 13th day of mechanical ventilation.

Case 2

A three and half month old boy was transported to our PICU with acute encephalopathy and the suspicion of inborn error of metabolism. Severe metabolic acidosis was evident on arterial blood gas analysis at admission (pH: 6.97, bicarbonate level: 8 mmol/L, base deficit: 10 mmol/L). He was intubated and put on PCV. At the 24th h of mechanical ventilation, the oxygenation deteriorated and chest radiograph revealed bilateral diffuse ground glass opacities. The PaO2/FiO2 was 100 torr. Oxygenation did not improve and SpO2 was 80%, despite high PEEP, high FiO2, long TI ventilatory settings and prone positioning. APRV ventilation was started. Within an hour, marked improvement in oxygenation was noted and SpO2 increased to 93%. After the 33th h of APRV, ventilation the lungs were well expanded; pressure targeted SIMV and PS was started. He was eventually extubated on day 22.

Case 3

A 6-month-old girl, was treated for intractable seizures. She had nosocomial sepsis and hemodynamic instability. She was intubated and received aggressive fluid resuscitation and vasoactive medication infusions. Her hemodynamic condition improved, but at the 18th h of PICU admission, she became tachypneic and the SpO2 decreased to 88%. Chest radiograph showed diffuse bilateral infiltrates. She was ventilated with high FiO2, high PEEP and long TI ventilatory settings. SpO2 increased but the FiO2 remained close to 1.00. She was started on APRV at the 26th h after admission to PICU. Her oxygenation improved, oxygen requirements were decreased and ventilatory settings were weaned. After 7 days on APRV, she was put back on conventional ventilation and was extubated on day 9.

Mean time on conventional ventilation before conversion to APRV was 33 ± 10.8 h. The mean time spent on APRV was 108 ± 66.8 h. The ventilatory settings and blood gases of the patients pre-enrollment to APRV and after APRV are compared in Table 3. The PIP decreased, the average mean airway pressure (MAP) increased and release VT increased after the start of APRV. The pH increased and PCO2 level decreased with APRV (Table 3). FiO2 and partial carbon dioxide pressure (pCO2) level progressively decreased throughout the APRV period.

Table 3 Conventional ventilation versus APRV
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Discussion

Data obtained from present cases support the notion that APRV can improve oxygenation in some children with AHRF as in Krishan case series [9]. Krishan et al. showed that oxygenation index improved in several of their patients managed with APRV but there was no data about lung mechanics. In Shultz et al. study [10], the MAP and VTe in children were slightly lower in the APRV mode as compared to SIMV. The present findings differ from Shultz et al. study. In the present patients, the MAP increased to 53% and VTe increased to 59%, and PIP decreased to 9%, after the start of APRV. Thus the use of APRV in the present patients recruited the lungs by increasing MAP; optimized mean airway pressure/lung volume providing a greater surface area for gas exchange. The APRV modality could provide better oxygenation with lower peak airway pressure.

It is important not only to recruit the lung, but also to keep it open. APRV maybe viewed as a nearly continuous recruitment maneuver with Phigh providing 80% to 95% of the cycle time creating a stabilized “open lung” [5]. The progressive decrease in FiO2 throughout the APRV period in the present patients, support the hypothesis that APRV stabilized the “open lung”.

Also, authors experience, alveolar ventilation was adequate on APRV. PCO2 levels improved during APRV. Ventilation in APRV occurs during the release phase and during spontaneous breathing at any point in the cycle. APRV achieves equally effective alveolar ventilation with lower minute ventilation compared to PCV, suggesting less dead space ventilation.

Conclusions

APRV may offer potential clinical advantages for the ventilatory management of pediatric patients and maybe considered as an alternative mechanical ventilation mode in the pediatric population. Also, APRV may improve oxygenation in patients when conventional ventilation has failed to improve oxygenation. APRV is a relatively simple modality that can be implemented easily, particularly when high frequency oscillatory ventilation is not accessible. The effect of APRV on the duration of mechanical ventilation, duration of PICU stay, and mortality remains to be studied in pediatric patients.