Basics of Neonatal Ventilation - Phase variable

Introduction

Assisted ventilation is defined as the movement of gas in and out of the lungs, driven by an external source connected directly to the neonate.

Broadly it can be divided into Invasive and Non-Invasive.

External sources generating gas flow can be – 

·  a resuscitation bag,

·  Continuous distending pressure device, 

·  or mechanical ventilator.

Attachment to the neonate can be – 

· Face masks, 

· Nasal prongs, 

· Endotracheal tubes, 

· or rarely tracheostomy or laryngeal mask airway (LMA). 

Based on the type of attachment used, assisted ventilation can be broadly divided into two:

1. Invasive: Assisted ventilation with endotracheal tube in situ is called invasive.

2. Non-invasive: Assisted ventilation where no endotracheal intubation has been done.

 

Assisted ventilation using mechanical ventilators or respirators is called- Mechanical Ventilation. 

Simply it is a device that assists ventilation by driving in gases with the help of a pressure gradient generated by the machine.

Goal 

·Facilitate alveolar ventilation.

·Maintain adequate oxygenation.

·Reduce the work of breathing.

·Support gas exchange in case of hypoventilation.

·Maintain airway in case of airway disease

Basic concept 

The first and foremost concept is that practically all mechanical ventilators are non-physiological.

i.e., In spontaneous breaths, the gas movement is affected by creating a negative intra-thoracic pressure while most mechanical ventilators push in gases under positive pressure.

Generally, each breath delivered by a mechanical ventilator is a cycle of three events – trigger, control, and termination.

1. Trigger

It is that stimuli that start the inspiration.

Based on the type of trigger, the ventilators can be classified into the following category:

Assistor type: If the mechanical breaths are triggered/ initiated by a neonate  

Controller type: If mechanical breaths are initiated by a machine only

Assistor-controller type: If both neonate and ventilator can initiate respiration

Most ventilators in neonates are Assistor- Controller type. 

It means that the device allows the neonate to initiate mechanical breaths; however, if the neonate fails to generate the triggering signal then the device delivers backup positive pressure breaths.

This feature of triggering helps to attain synchrony of neonates a spontaneous inspiratory effort with the positive pressure breaths delivered by the ventilator.

The different triggers are- Flow, pressure, chest wall movement, etc

Flow trigger- ventilator detects the positive flow of gas in the airways with the spontaneous breathing effort of the infant and delivers its breaths in synchrony with the infant's effort.

The pressure-triggered- ventilator detects the negative pressure in the airway generated by the infant's spontaneous efforts Movement triggered- ventilator detects the chest wall movement of the infant and synchronizes the mechanical breaths.

2. Control

It is that factor that controls the ventilation.

Indirectly it can be considered as the mode of ventilation.

The two most common controls or modes are - pressure control and volume control.

The older neonatal ventilators were pressure-controlled ventilators- where the pressure delivered by the device is set.

With the advent of newer ventilators and technology to reliably measure the small volume of gases delivered in neonatal ventilation- Volume-controlled modes have also emerged.

In volume-controlled modes – the volume of gases delivered by the device is set.

Presently hybrid modes combining both pressure and volume control are also available

Predominant CONTROL (MODE) of neonatal ventilation is Pressure control. This was because previously it was not possible to measure accurately the small volumes of gases that move in and out during ventilation of a neonatal lung.

For eg., the normal tidal volume of a neonate is 4-6 mL/Kg.

Hence, in a 3 kg baby approximately 18 ml should account for its tidal volume. Very often the older equipment is used to measure gas flows at the machine end.

So accurate measurement was not feasible with the volume of ventilator tubings etc. accounting for a greater proportion of the tidal volume. 

With the emergence of microprocessors and better flow sensors based on the hot wire anemometer principle or pneumotachograph, even small gas volumes are measurable at the neonate end instead of the device end.

Hence volume control ventilation is practically feasible even in small preterm babies.

This has led to the emergence of HYBRID modes of ventilation- where features of both pressure control and volume control are integrated.

Currently, many new ventilators have an option for controlling the volume of gases delivered

e.g. Volume guarantee (VG), Targeted Tidal Volume (TTV), Volume Assured, Pressure Regulated Volume Control (PRVC), etc.

To simplify, whatever mode is used- the aim of ventilation is an optimum gas exchange and some volume of gases moving in is achieved at a price which is the pressure gradient.

3. Cycling or Termination

The method by which the inspiratory cycle is terminated is called Cycling. The different types of cycling are

Volume cycling- Inspiration ends when a certain volume is delivered

Pressure cycling- inspiration ends when a preset pressure is reached

Time cycling- Inspiration ends when the preset time is reached

Flow cycling- Inspiration ends when flow reduces below a critical limit Mixed cycling- (two or more independent cycling mechanisms available in the same device.)

Summary 

thus these are four stages of mechanical ventilation. There is the trigger phase, the inspiratory phase, the cycling phase, and the expiratory phase; which compiles a mechanical breath.

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