FEATURE NEONATOLOGY 013
INTRODUCTION Mechanical Ventilation (MV) of newborns is an important factor in improving survival of critically ill preterm and term infants. The introduction of mechanical ventilation in the 1960s was one of the major interventions in neonatal medicine. Over the past fifty years, mechanical ventilation has led to improved survival of preterm infants, along with other technologic advancements such as surfactant therapy. This has been of particular benefit to premature infants born less than 30 weeks gestation with immature lungs. Approximately two-thirds of all infants admitted to the neonatal
intensive care unit (NICU) and most Extremely Low Gestational Age Neonates (ELGAN), who are born before 28 weeks of gestation, require some type of respiratory support. Common indications for MV include:
Respiratory distress syndrome (RDS) Pneumonia Meconium aspiration syndrome Apnea of prematurity or Apnea secondary to perinatal hypoxia Congenital anomalies, such as congenital diaphragmatic hernia Post-operative recovery However, while mechanical ventilation is a life-saving therapy, its use
increases the risk of lung injury and developing Chronic Lung Disease (CLD). This happens particularly in ELGANs, in whom the incidence of CLD remains high.
CURRENT STATUS Ventilatory support in NICU can generally be divided into three main categories: Conventional Ventilation, which is based upon setting a ventilator breath rate less than 120 per minute, with continuous gas flow through the ventilator circuit. The mode commonly used is Pressure Limited Time Cycled ventilation where the operator sets Peak Inspiratory Pressure (PIP), Positive End Expiratory Pressure (PEEP), breathing rate, and inspiratory time (Ti). Ventilatory breaths either start according to the set rate or may be triggered by the patient’s own inspiratory effort, referred to as patient-triggered or synchronized ventilation (see table 1).
Pressure controlled ventilation Intermittent positive-pressure ventilation Synchronized intermittent mandatory ventilation Assist-control Pressure support ventilation Neurally adjusted ventilator assist Volume-controlled ventilation
Volume guarantee ventilation mode Volume-limited ventilation
Pressure-regulated colume control Volume-assured pressure support
High-frequency jet ventilation
High-frequency flow interruption High-frequency oscillatory ventilation
Abbreviation PCV
High frequency ventilation, which is based on the delivery of small volumes of air, which are equal to, or smaller than, the anatomic dead space, at an extremely rapid rate (300 to 1500 breaths per minute).
Non-invasive ventilation, which is based on providing ventilatory support without using an endotracheal tube (ETT). This can be achieved using different methods with nasal CPAP (Continuous Positive Airway Pressure), Nasal Synchronized Intermittent Mandatory Ventilation (N-SIMV), and Nasal IMV. Non-invasive ventilation can also be provided using a tube that ends in the pharynx. Prior to the advent of the current generation of neonatal ventilators,
conventional mechanical ventilation (CMV) was provided mainly with time-cycled pressure limited (TCPL) ventilators. This method, also known as intermittent mandatory ventilation (IMV), was the most common mode of ventilation. During IMV mechanical breaths of fixed duration are delivered at predetermined time intervals. This frequently leads to asynchrony depending on the phase of the spontaneous breath of the infant when these IMV breaths are delivered. Inspiratory asynchrony occurring when a mechanical breath is delivered at the end of and extends beyond spontaneous inspiration can produce an inspiratory hold. This limits the spontaneous respiratory rate and/or results in high pressure and excessive lung inflation. Expiratory asynchrony occurring when a mechanical breath is delivered during spontaneous exhalation, can delay lung deflation and elicit active expiratory efforts against positive pressure producing large fluctuations in intrathoracic pressure. Asynchrony can affect gas exchange, and has been linked to increased risk of air leaks and Intraventricular Haemorrhage in the preterm infant. As volume monitoring was lacking in most IMV devices, it was difficult to detect excessive lung inflation, gas trapping or hypoventilation. Avoiding asynchrony has been possible with technological advances
that allowed the development of ventilators that can measure flow of gas going through the ETT into the baby. Other signals that indicate the beginning of inspiration could also be detected to allow the ventilators to synchronize the mechanical breath with the baby’s spontaneous inspiration. Synchronization could also be extended to termination of
TABLE 1: Summary of different avaliable modes of ventilation with their particular settings Mode of ventilation
IPPV or time-cylsed, pressure-limited ventilation SIMV AC PSV NAVA VCV
VG or pressure-limited, volume-targeted time VLV
PRVC or autoflow VAPS
HFJV HFFI HFOV Settings
Set PIP and PEEP Set Rate independently from Ti and Te
Limitation of PIP and PEEP Set Ti at o.35 s.
Fixed respiration rate synchronized with child’s breathing within limit of ventilator setting
Assist each breath on the basis of PCV Set minimum sycles and short Ti
Set variables for support based on PCV Set minimum rate and trigger Threshold ± pressure slope
Set trigger to pick up the electrical diaphragmatic activity Adapt NAVA level to regulate pressure support
Fixed preset Vt and rate Limit high pressures with pop-off value or Ti Set the maximum PIP and PEEP, the desired exhaled Vt and Ti
Set upper volume limit And Pressure support variables
Flow rate will vary to adjust PIP to deliver target Vt
Combines volume and pressure-target ventilation Set target Vt and Pressure limit as in PSV
Short bursts with short Ti
High frequency (600/min) constant flow determines PEEP level Interrups at high frequency (20 Hz) continuous flow Adjust MAP, 1/E ration, Rate
PIP, peak inspiratory pressure; Ti, inspriratory time; Te, expiratory time; MAP, mean airway pressure; Vt, tidal volume. Table taken from: National ventilation, Habre W, Best Practise & Research Clinical Anaesthesiology 24 (2010) 353-364
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