MECHANICAL VENTILATOR

A machine that helps in the process of breathing mechanically by helping in movement of air into and out of the lungs is called as mechanical ventilator. It helps to sustain breathing in a patient who is unable to breathe properly (has respiratory failure). Ventilators are of 2 types: positive pressure ventilators and negative pressure ventilators. Negative pressure ventilators (iron lung machines are no longer used now).

Absolute indication for intubation and mechanical ventilation are:
- Emergency
- Apnea and severe irregularity of spontaneous breathing
- Severe respiratory failure
- Ineffectiveness of oxygen therapy by mask, CPAP or non-invasive ventilation

Advantages of mechanical ventilation:
- Correct ventilation of both lungs and progressive improvement of lung pathology
- Improvement of oxygenation and reduction of hypercapnia
- Reduction of respiratory fatigue and oxygen consumption
- Protection of airways and efficacious bronchial suctioning if tracheal intubation is performed.

Modes of ventilation: Mechanical ventilation using positive pressure can need airway invasion using an endotracheal tube or, as is being more frequently seen, non-invasion of airways can be performed (use of facial and nasal masks).

CPAP, Pressure or Volume Support Ventilation) or ventilation can be totally or partially controlled (Volume and Pressure Controlled Ventilation, Synchronized Intermittent Mandatory Ventilation). Each model has precise indications which allow better application on the one hand, while on the other avoid side effects.

Continuous Positive Airway Pressure (CPAP): CPAP is a mode of ventilation, which enables the elevation of end-expiratory pressure to levels above atmospheric pressure to increase total lung volume, and functional residual capacity, thus favoring improved oxygenation.

Pressure Support Ventilation (PSV): Pressure support ventilation (PSV) is designed to support spontaneous breaths during inspiratory phase. It is primarily designed to assist spontaneous breathing and therefore the patient should have an intact respiratory drive. Cycles are pressure limited and there is no pre-set tidal volume.

Volume Support Ventilation (VSV): The ventilator, breath by breath, adapts inspiratory pressure support to changes in the mechanical properties of the lung and the thorax in order to ensure that the lowest possible pressure is used to deliver pre-set tidal and minute volume that remain constant.

Controlled Mechanical Ventilation (CMV): This mode of ventilation controls the patient's respiratory activity completely. Introduction of gases into the lung, inspiration, is obtained using positive pressure, which pushes gases into the lung.

SPECIAL MODES OF VENTILATION:

1. High Frequency Ventilation - HFV: The most fundamental difference between high frequency ventilation (HFV) and intermittent positive pressure ventilation (IPPV) is that with HFV the tidal volume (Vt) required is approximately 1-3 ml/kg body weight, compared with 6-10 ml/kg with intermittent positive pressure ventilation (IPPV). The increase in ventilation rate to frequencies of 60 b.p.m. or more in HFV is obviously mandatory if even comparable minute volume ventilation is to result.

Reference:
Marraro GA. Practical Guidelines for Mechanical Ventilation. Pediatric Oncall [serial online] 2005 [cited 2005 January 1];2.

MECHANICAL VENTILATOR

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