Mechanical ventilation overview
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief: Vishnu Vardhan Serla M.B.B.S. [2]
Overview
In medicine, mechanical ventilation is a method to mechanically assist or replace spontaneous breathing when patients cannot do so on their own, and must be done so after invasive intubation with an endotracheal or tracheostomy tube through which air is directly delivered (in contrast to noninvasive ventilation). In many cases, mechanical ventilation is used in acute settings such as in the ICU for a short period of time during a serious illness. For some patients who have certain chronic illnesses that require long-term ventilation assistance, they are also able to do so at home or other nursing/rehabilitation institution with the help of respiratory therapists and physicians. The main form of mechanical ventilation currently is positive pressure ventilation, which works by increasing the pressure in the patient's airway and thus forcing additional air into the lungs. This is in contrast to the more historically common negative pressure ventilators (for example, the "iron lung") that create a negative pressure environment around the patient's chest, thus sucking air into the lungs. Although often a life-saving technique, mechanical ventilation carries many potential complications including pneumothorax, airway injury, alveolar damage, and ventilator-associated pneumonia, among others. Accordingly it is generally weaned off or to minimal settings as soon as possible.
Historical Perspective
Vesalius was the first person to describe mechanical ventilation by inserting a reed or cane into the trachea of animals and then blowing into this tube. The iron lung, also known as the Drinker and Shaw tank, was developed in 1929 and was one of the first negative-pressure machines used for long-term ventilation.
Types of Ventilators
Mechanical ventilation may be classified into non-invasive and invasive mechanical ventilation. Non-invasive mechanical ventilation can be further sub-divided into continuous positive airway pressure breathing (CPAP), bilevel positive airway pressure breathing (BiPAP) and mask ventilation. Mechanical ventilators may also be classified based on the basic underlying mechanics of the device and the clinical condition in which it is used. Ventilation may be delivered via bag valve mask, continuous flow, transport ventilators, ICU ventilators, NICU ventilators and PAP ventilators
Indications for Use
Mechanical ventilation can be used in patients who have labored breathing and are unable to maintain adequate gaseous excange leading to hypoxemia and/or hypercapnia. Common clinical indications of mechanical ventilation include moderate to severe dyspnea, respiratory rate (RR) > 24-30/min, signs of increased breathing, accessory muscle use for breathing and abdominal paradox. It may also be used in patients who have inadequate arterial partial pressure of oxygen or critically low PaO2 (PaO2 < 70 mm Hg), hypercapnia PaCO2 > 45 mm Hg and PaO2/FiO2 < 200. Patients suffering from acute exacerbation of COPD, asthma/asthmatic attack, neuromuscular disease that prevents chest movement to allow gas exchange, central nervous system depression (CNS depression due to drugs, cardiac arrest, trauma), chest injury, chest malformation, acute and chronic respiratory failure, heart failure and ventilation-perfusion mismatch may also be candidates for mechanical ventilation.
Ventilator Variables
Ventilator variables modulate the oxygenation achieved. They can be adjusted according to the clinical condition of the patient and to achieve specific goals of management. The variables include fraction of inspired oxygen (FiO2), tidal volume (Vt), respiratory rate(f), positive end expiratory pressure (PEEP), inspiratory time, inspiratory flow rate, peak inspiratory pressure (PIP) and plateau pressure (Pplateau). Tailoring the ventilator settings can help achieve specific goals, for example, to improve oxygenation, option include increasing the FiO2 and PEEP and to improve ventilation, the tidal volume (Vt), inspiratory pressure and respiratory rate (f) may be increased (this follows the basic principle of minute ventilation = Tidal volume x respiratory rate).
Choosing Amongst Ventilator Modes
Choice of ventilator mode depends upon the clinical condition of the patient. Choice of ventilator mode can be tailored to achieve specific goals of management and set to achieve spontaneous breathing, volume-targeted ventilation, pressure-targeted ventilation, or some combination. In some conditions, for example in case of spontaneously breathing patient, the patient sets the respiratory rate and generates the desired flow rate. Different modes of ventilation include pressure support ventilation (PSV), continued mandatory ventilation (CMV) or assist control mode (AC), synchronous intermittent mandatory ventilation (SIMV), proportional assist ventilation (PAV), dual control mode, high frequency ventilation, pressure and volume targeted modes.
Initial Ventilator Settings
Protocol
Candidacy for mechanical ventilation is based on specific criteria and clinical condition of the patient. Body weight of the patient and height also play important role in determining the optimal ventilator settings. Similar to initiation of mechanical ventilation there are specific criteria for weaning the patient off from the ventilator and doing a spontaneous breathing trial.
Complications
Complications of mechanical ventilation include, oxygen toxicity, ventilator associated pneumonia (VAP), laryngeal edema and ulceration, malnutrition, oversedation/delirium and ventilator induced lung injury..