Mechanical Ventilators

 

intensive care

 

Ever since the introduction of the Iron Lung in treating polio victims, mechanical ventilators have kept patients alive during times of respiratory failure. Mechanical ventilators are found in the Intensive Care Unit (ICU), where they are used to support respiratory function in patients with respiratory failure. Today’s hospitals utilize positive pressure ventilators that deliver a breath of air into the patient’s lungs. There are two general phases of each breath that the ventilator must simulate: inspiration and expiration. Inspiration begins when the diaphragm contracts and moves downward, causing negative pressure to develop in the pleural space. This pressure difference causes air to move into the lungs. Expiration is a passive process when the diaphragm relaxes, causing pressure to equalize in the chest and air to rush out of the lungs.

There are several types of settings in ventilators to adjust ventilation to the needs of the patient. Most of this terminology refers to inspiration; expiratory support is almost always via PEEP or CPAP, which is described later. First, the ventilator needs to know when to initiate inspiration. This is known as triggering. Triggering can occur at a set timed frequency. The ventilator can also “sense” the patient’s inspiratory effort by way of a decrease in baseline pressure, causing initiation of inspiration. Most modern ventilators today trigger inspiration by sensing inspiratory flow created by the patient. This mechanism requires less work by the patient than pressure triggering.

Ventilators need to know how much air to deliver to the patient with inspiration. This can be volume controlled or pressure controlled. In volume control, the ventilator is set to deliver a particular volume of air during inspiration. In pressure control, a particular pressure is targeted. Another parameter is known as cycling, which is how the ventilator switches between inspiration and expiration. Once the ventilator has reached the volume or pressure target, the machine has to know how long to stay there. Cycling can be time cycled, flow cycled, or volume cycled. Inspiration can also have a flow pattern. In spontaneous breathing, the flow pattern is sinusoidal. The flow can be constant, where flow is at the same rate throughout inspiration. Flow can be decelerating, as in pressure ventilation, where inspiration slows down as airway pressure increases. Finally, flow can be accelerating, flow increases progressively as the breath is delivered (accelerating is not used in clinical practice).

In mechanical ventilation, there are several modes or patterns of ventilation. What determines the mode is often dependent on the type of breaths the patient is taking. Breaths are classified as mandatory, assisted, or spontaneous. Mandatory breaths are set by the respiratory rate and unassisted. Assisted breaths are breaths where a patient can initiate the breath while the ventilator assists with inspiratory support. Spontaneous breaths are breaths that are completely unassisted.

The most common modes of respiration in short are conventional controlled ventilation (CMV), assist-control, intermittent mandatory ventilation (often used as Synchronized Intermittent Mandatory Ventilation or SIMV), pressure support, and high frequency ventilation. CMV is breathing completely dependent on a ventilator. Assist-control allows the patient to initiate inspiration while the ventilator delivers a controlled breath. In IMV, the patient can take spontaneous breaths with occasional mandatory breaths mixed in their cycle. SIMV is IMV that is synchronized by a computer to prevent stacking of breaths. In pressure support, the patient controls all aspects of breathing except for the pressure limit. High frequency ventilation is used to keep mean airway pressure constant by delivering a high frequency of short breaths.

As discussed earlier, most ventilator support is focused on supporting inspiration, with two main exceptions that support expiration: positive end-expiratory pressure (PEEP) and continuous positive airway pressure (CPAP). Both PEEP and CPAP are used to keep airway pressure positive to keep the alveolar airway open. A ventilator can supply PEEP (usually about 3 to 5 cm H2O) or the ventilator can be programmed to have a slightly shorter expiratory phase to cause air retention in the lungs. The latter is called intrinsic PEEP or “autoPEEP.” Intrinsic PEEP is often used by anesthesia during surgery when extrinsic PEEP cannot be used due to patient sedation. However, large amounts of PEEP can cause volume retention and lead to complications. When gas gets trapped in the airway from obstruction, CPAP treatment is often used to open up the airway. For instance, in individuals with a large neck circumference, the pharynx can collapse during sleep. CPAP delivers positive pressure that opens the pharynx so air can get into the lungs. CPAP ventilators can be unidirectional or bidirectional depending on the patient needs.

 

 

Source: http://www.medwow.com/articles/