Measuring Work-of-Breathing in Mechanically Ventilated Children

1. INTRODUCTION AND RATIONALE The need for mechanical ventilation for respiratory failure is one the most common indications for children to be admitted to a centralized paediatric intensive care unit (PICU) in the Netherlands. Up to 64% of all admitted children need mechanical ventilation for at least 24 hrs (1,2). Hence mechanical ventilation is a key feature in the management of critically ill children. Nevertheless, numerous issues related to the use of mechanical ventilation in children remain unsolved. Much of the current clinical practice is based upon anecdotal experience and data obtained from studies performed in critically ill adults (3). However, the respiratory system is physiologically different between small babies, children and adults implying that all data obtained from adults cannot be easily extrapolated to children (4). For instance (to name but a few), the elastic properties of the lung increases during childhood contributing to increased lung compliance. Furthermore, over the remainder of childhood the lung continues to grow and mature: at the age of 8 years the alveolar surface is about half of that of an adult. Interalveolar pores develop during pre-school years, whereas broncho-alveolar pores begin to develop at the age of 6 - 8 years. The absence of these collateral pathways places children at risk for the development of atelectasis and resulting ventilation/perfusion inequality. Next to this, the resistance of peripheral airways decreases profoundly with increasing age. However, the resistance in an 8-year old is still four times higher than in an adult. Tidal volume (Vt) is comparable between children and adults (about 5 - 7 ml/kg ideal bodyweight), but functional residual capacitity (FRC) in young children is much smaller than in adults. And finally, chest wall compliance is also profoundly different between young children and adults. The chest wall compliance decreases with increasing age because of ossification of the rib cage and an increase in mucular tone of the intercostal muscles.

Although mechanical ventilation is often life saving, it can be associated with complications such as ventilator-induced lung injury and nosocomial pneumonia as recently nicely summarized by Newth et al (5). Endotracheal tubes (ETT) are uncomfortable for patients and increase the need for sedatives. An ETT in the upper airway can be associated with airway injury, particularly in mobile young patients. Furthermore, positive pressure ventilation may contribute to cardiovascular instability from heart-lung interactions. Therefore, it is important that MV be discontinued as soon as the patient is capable of sustaining spontaneous breathing. However, the experience in adults suggests that premature extubation may also be problematic and result in emergent reintubation with attendant complications, including the potential of catastrophic morbidity. A high mortality rate has been documented in both pediatric and adult patients who have required reintubation after extubation failure. Extubation failure is independently associated with a five-fold increased risk of death in pediatric patients. Consequently, although expeditious weaning and extubation are the goal, premature extubation can be lethal. Over 50% of ventilated PICU patients will have been extubated by 48 hrs after admission, but the rest often require prolonged ventilatory support. Both premature and delayed extubation increases morbidity and mortality as well as costs.

Initiation of weaning and timing of extubation have been largely neglected in the pediatric literature (5). Weaning is the transition from ventilatory support to completely spontaneous breathing, during which time the patient assumes the responsibility for effective gas exchange while positive pressure support is withdrawn. There is no standard method of weaning. Indeed, there is disagreement about when the onset of weaning actually occurs and no validated, objective criteria as to when a patient can be extubated (6-9).

The most common approach to weaning infants and children is gradual reduction of ventilatory support. Weaning with intermittent mandatory ventilation (IMV) or synchronized IMV (SIMV) occurs by reducing the ventilatory rate. With pressure support (PS) ventilation, the inspiratory pressure is initially set to provide the required support and then reduced gradually. PS is often combined with IMV/SIMV during weaning (SIMV-PS). Alternatively, another approach to weaning is attempted with alternating periods of complete ventilatory support and graded spontaneous breathing with assistance. This "sprinting" is performed on the theory that the respiratory muscles can be slowly trained to sustain complete spontaneous breathing. Also, theoretically this "sprinting" allows a better distribution of the tidal volume in the lung. Interestingly, both approaches are used simultaneously: the decision to use either one or both approaches is dependent upon the preferences of the attending physician as described in many observational single center studies.

Importantly, there is no data comparing the "sprinting approach with more traditional approaches of weaning in children with respect to patient work-of-breathing. Work-of-breathing is defined by the physiologic work a patient has to deliver to expand the lungs and the chest wall. It can be assessed by various means:

• Bedside: tachypnoea and the presence of nasal flaring and intercostal and/or interjugular retractions indicate increased work of breathing
• Clinical surrogate parameters: the ratio of the inspiratory time to total breathing cycle time, the oesophageal pressure - rate product (PRP), oesophageal pressure - time product (PTP) and expiratory airway resistance (i.e. the difference in transpulmonary pressure and compliance, divided by flow). These clinical surrogate parameters require the presence of an oeosphageal catheter to measure the pressure. These catheters are routinely present in ventilated patients as they are used for nasogastric tube feeding; modern catheters can also measure the oesophageal pressure (i.e. double function).
• Mathematically: the area under the oesophageal pressure - volume curve. This is the classic and traditional approach to measure work of breathing. Its normal value is within the range of 0.5 - 1.0 J/L. The variable is measured by a commercially available ventilator (AVEA, CareFusion, Yorba Linda, CA, USA). To measure this parameter, an oesophageal catheter is necessary.