MECHANICAL VENTILATION IN CHILDREN
Dr.Deilami.M , Anesthesiologist
The interaction between the ventilator and the infant is strongly dependent on the mechanical properties of the respiratory system. A pressure gradient between the airway opening and the alveoli must exist to drive the flow of gases during both inspiration and expiration.
There are no outcome data to recommend on any ventilatory or respiratory assist modes for children with or without lung pathology, cardiac children, or chronically ventilated children requiring escalation of support for acute exacerbations . Ventilator mode should be dictated by clinical experience and theoretical arguments, considering the pathophysiology of the disease
CONTINUOUS POSITIVE AIRWAY PRESSURE (CPAP)
CPAP has been an important tool in the treatment of neonates who have RDS. The mechanisms by which CPAP produces its beneficial effects include: 1) increased alveolar volumes, 2) alveolar recruitment and stability, and 3) redistribution of lung water . The results are usually an improvement in ventilation-perfusion matching. However, high CPAP levels may lead to side effects.
The most frequently used ventilators in neonates are time-triggered at a preset frequency, but because of the available bias flow, the patient also can take spontaneous . In contrast, patient-triggered ventilation (also called assist/control) uses spontaneous respiratory efforts to trigger the ventilator.
ASSIST- CONTROL VENTOLATION (ACV)
also known as continuous mandatory ventilation (CMV). Each breath is either an assist or control breath, but they are all of the same volume. The larger the volume, the more expiratory time required. If the I:E ratio is less than 1:2, progressive hyperinflation may result. ACV is particularly undesirable for patients who breathe rapidly – they may induce both hyperinflation and respiratory alkalosis.
SYNCHRONIZED INTERMITTENT MANDATORY VENTILATION
Guarantees a certain number of breaths, but unlike ACV, patient breaths are partially their own, reducing the risk of hyperinflation or alkalosis. Mandatory breaths are synchronized to coincide with spontaneous respirations.
This mode of ventilation achieves synchrony between the patient and the ventilator breaths. Synchrony easily occurs in most neonates because strong respiratory reflexes during early life elicit relaxation of respiratory muscles at the end of lung inflation.
Pressure-Controlled Ventilation (PCV)
Less risk of barotrauma as compared to ACV and SIMV. Does not allow for patient-initiated breaths. The inspiratory flow pattern decreases exponentially, reducing peak pressures and improving gas exchange [Chest 122: 2096, 2002]. The major disadvantage is that there are no guarantees for volume, especially when lung mechanics are changing. Thus, PCV has traditionally been preferred for patients with neuromuscular disease but otherwise normal lungs
Pressure Support Ventilation (PSV)
Allows the patient to determine inflation volume and respiratory frequency (but not pressure, as this is pressure-controlled), thus can only be used to augment spontaneous breathing. Pressure support can be used to overcome the resistance of ventilator tubing in another cycle (5– ۱۰cm H20are generally used, especially during weaning), or to augment spontaneous breathing. PSV can be delivered through specialized face masks.
Pressure Controlled Inverse Ratio Ventilation (PCIRV)
Pressure controlled ventilatory mode in which the majority of time is spent at the higher (inspiratory) pressure. Early trials were promising, however the risks of auto PEEP and hemodynamic deterioration due to the decreased expiratory time and increased mean airway pressure generally outweight the small potential for improved oxygenation
Airway Pressure Release Ventilation (APRV)
Airway pressure release ventilation is similar to PCIRV – instead of being a variation of PCV in which the I:E ratio is reversed, APRV is a variation of CPAP that releases pressure temporarily on exhalation. This unique mode of ventilation results in higher average airway pressures. Patients are able to spontaneously ventilate at both low and high pressures, although typically most (or all) ventilation occurs at the high pressure. In the absence of attempted breaths, APRV and PCIRV are identical. As in PCIRV, hemodynamic compromise is a concern in APRV. Additionally, APRV typically requires increased sedation.
PROPORTIONAL ASSIST VENTILATION
Both patient-triggered ventilation and synchronized intermittent mandatory ventilation are designed to synchronize only the onset of the inspiratory support. In contrast, proportional assist ventilation matches the onset and duration of both inspiratory and expiratory support.
Because of its potential to reduce volutrauma, there has been a surge of interest in high-frequency ventilation in the past few years. High-frequency ventilation may improve blood gases because, in addition to the gas transport by convection, other mechanisms of gas exchange may become active at high frequencies. There has been extensive clinical use of various high-frequency ventilators in neonates.