Mechanical ventilation overview
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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.
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 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
Initial ventilator settings should be modified and tailored according to the clinical condition of the patient and specific goals of management. Selection of ventilatory mode, sensitivity at flow trigger mode, tidal volume, rate, inspiratory flow, positive end expiratory pressure (PEEP), pressure limit, inspiratory time and fraction of inspired oxygen (FiO2) should be made according to the underlying etiology of hypoxemia/hypercapnia. Other factors for example, age of the patient, weight and height also play an important role in deciding the initial ventilatory settings. General rules that help physicians to choose the initial settings in a time-efficient manner include choosing a tidal volume of 12 mL per kg body weight delivered at a rate of 12 a minute (12-12 rule) in adults and adolescents. In infants and children without existing lung disease a tidal volume of 4-10 ml/kg may be delivered at a rate of 30-35 breaths per minute. With respiratory distress syndrome (RDS), a decreased tidal volume and increased respiratory rate sufficient to maintain pCO2between 45-55. Allowing higher pCO2 (sometimes called permissive hypercapnia) may help prevent ventilator induced lung injury. Ventilator triggered breaths may be initiated via either a pressure-triggered or a flow-triggered mechanism. Pressure triggered breaths are initiated when the ventilator senses a negative pressure (indicating that the patient is trying to initiate a breath). During the flow-triggered mechanism, a continuous flow is delivered and a ventilator-delivered breath is initiated when the return flow is less than the delivered flow.
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 of mechanical ventilation include, oxygen toxicity, ventilator associated pneumonia (VAP), laryngeal edema and ulceration, malnutrition, oversedation/delirium and ventilator induced lung injury..
Modification of Settings
In adults when 100% FiO2 is used initially, it is easy to calculate the next FiO2 to be used and easy to estimate the shunt fraction. When using 100% FiO2, the degree of shunting is estimated by subtracting the measured PaO2 (from an arterial blood gas) from 700 mmHg. For each difference of 100 mmHg, the shunt is 5%. A shunt of more than 25% should prompt a search for the cause of this hypoxemia, such as mainstem intubation or pneumothorax, and should be treated accordingly. Patients should have their ventilation considered for withdrawal if they are able to support their own ventilation and oxygenation, and this should be assessed continuously. Specific clinical, ventilator and oxygenation criteria should be met in order to do a trial of spontaneous breathing in mechanically ventilated patients.
Connection to Ventilator
There are various procedures and mechanical devices that provide protection against airway collapse, air leakage, and aspiration. Face mask, laryngeal mask airway, tracheal intubation, esophageal obturator airway, cricothyroidotomy and tracheostomy may be used to provide mechanical ventilation.