(see also Lung Volumes; Ventilator Management; Weaning Parameters)
In the past, classifying ventilator modes was an easy task; they were either controlled mode or assist mode. That is not the case today. Rapid advancements in computer technology have caused the number of ventilator modes now available to grow exponentially. To add to the confusion, manufacturers have introduced different modes (often trademarked) that are actually similar. It is beyond the scope of this book to address all the modes currently available, but the alphabetical list that follows attempts to address some of the more common modes and briefly discuss their clinical use.
Adaptive Pressure Control (APC) This mode transitions between volume-targeted pressure control and pressure support breaths. The ventilator will automatically adapt the volume/rate to the patient's mechanics. Support is proportional to the patient's effort. Patients unable to trigger the ventilator are given pressure control breaths. Patients that can trigger are given pressure support.
Airway Pressure Release Ventilation (APRV) This is a pressure controlled, spontaneous breathing mode and a variant of CPAP that uses a high- and low-pressure limit as well as a time limit. This mode uses a higher airway pressure to increase arterial oxygenation by recruiting alveoli. The lower pressure will “release” CO2 during exhalation. ARPV is a good choice for patients with poor lung compliance, such as those with ALI or ARDS. ARPV is a reverse I:E ratio because it uses a short expiratory time for the pressure release.
Assist Control (AC) Patient initiates all breaths, but the ventilator cycles in at breath initiation to make sure the patient gets a preset tidal volume or achieves a preset peak pressure. The patient controls the rate, but always received a full machine breath. A minimum “backup” rate is usually set to be sure the patient doesn't become apneic and a maximum rate can be set (though used infrequently) if the patient is breathing too rapidly.
Bilevel Positive Airway Pressure (BPAP) Used during noninvasive ventilation to deliver an independent positive airway pressure to both inspiration and expiration. This mode is helpful in preventing intubation of end-stage COPD patients and in supporting those patients with chronic ventilator failure. Often BPAP is incorrectly referred to as BiPAP, which is the name of a portable ventilator manufactured by Respironics Corp. and is just one of the many vents that can deliver BPAP.
Continuous Positive Airway Pressure (CPAP) Pressure is maintained throughout inspiration and expiration during spontaneous breathing, usually set at 5 to 15 cm H2O. The patient may generate the rate of breathing and tidal volume; thus, CPAP can be used alone or in conjunction with another mode (typically IMV) at low rates for weaning. It decreases the work of breathing by improving compliance and preventing atelectasis.
Controlled Mandatory Mechanical Ventilation (CMV) The frequency of breaths is determined by the ventilator alone, with no option available to the patient. It is an uncomfortable mode for patients who are conscious and therefore indicated for use in only unconscious or paralyzed patients, those who have experienced a neurologic event, or those receiving continuous sedation.
Intermittent Mandatory Mechanical Ventilation (IMV) Requires the ventilator to deliver a predetermined number of breaths and a set tidal volume. It allows the patient to breathe spontaneously between machine breaths, but the patient receives only their own spontaneous tidal volume. Breath stacking is a complication, since the ventilator will always deliver a breath, even if the patient is exhaling.
Inverse Ratio Ventilation (IRV) The normal inspiratory/expiratory ventilation ratio (I:E ratio) in most ventilators is 1:1.5 to 1:3, producing a short inspiratory time and a long expiratory time. This increases venous return and right atrial filling and allows time for air to leave the lungs. IRV reverses this ratio to produce an inspiratory time equal to or even longer than expiratory time, as high as 4:1. Although it is useful in ARDS (by expanding stiff alveoli slowly and preventing their collapse with a rapid expiratory time), this mode is unnatural and requires the patient to be either heavily sedated, or more frequently, medically paralyzed.
Mandatory Minute Ventilation (MMV) While the patient is able to breathe spontaneously, the ventilator continually adjusts the number of mechanical breaths so that the sum of spontaneous breaths plus mechanical breaths, multiplied by the tidal volume, equals the desired set minute ventilation.
Neurally Adjusted Ventilatory Assist (NAVA) A positive pressure mode of mechanical ventilation that permits the respiratory center in a patient's brain to control the ventilator by means of electrodes mounted on an NG tube and positioned in the esophagus at the level of the diaphragm. The electrical activity of the diaphragm is captured, fed to the ventilator via a computer, and used to assist the patient's breathing, both with a given breath as well as assist between breaths. This means that the patient's respiratory center is in direct control of ventilation and any variation in demand is responded to immediately. This method strives to increase patient comfort and reduce the risk of iatrogenic hyperinflation, respiratory alkalosis, and hemodynamic impairment. Training is required to learn to position the electrodes, adjust the trigger, and determine the level of ventilator assist.
Positive End-Expiratory Pressure (PEEP) This is not a stand-alone mode, but used in conjunction with other modes. The positive pressure added at the end of expiration prevents closure of the alveoli and promotes better oxygen exchange. Usually set at 5 to 20 cm H2O. High levels of PEEP may cause barotrauma to weak lungs and result in a tension pneumothorax or may decrease cardiac output by increase intrathoracic pressure, causing compression of the heart and decreased venous return. AutoPEEP (intrinsic PEEP) can occur when a patient has inadequate time to exhale and is typically signaled by end-expiratory flow of >0 before the next breath is delivered. (See AutoPEEP, p. 223.)
Pressure Control Ventilation (PCV) Pressure and respiratory rate are predetermined. The ventilator delivers a flow of gas until the preset pressure is attained. This mode can be used with inverse ratio ventilation to allow alveoli a prolonged duration of positive inflation. This, in turn, shortens expiratory time and prevents loss of the patient's tidal volume.
Pressure-Regulated Volume Control (PRVC) This mode requires rate and FiO2 and pressure limit to be preset. The patient may take spontaneous breaths. During breathing, if the ventilator senses that the entire tidal volume cannot be delivered without violating the pressure limit, it will alter the flow to allow the entire tidal volume to be delivered. Breath-by-breath, the ventilator continuously adapts the inspiratory pressure to change in the volume/pressure relationship. This mode is appropriate for patients who initially need high-flow rates to open up closed lung compartments, those in whom unnecessarily high airway pressures should be avoided, and those with lung conditions such as asthma, chronic obstructive bronchitis, and lung injury.
Pressure Support Ventilation (PSV) This is purely a spontaneous mode. The ventilator is triggered by the patient's own breath and delivers support on inspiration with a preset pressure value. The patient can control the length and the flow of each breath, thus allowing for flexibility in ventilator support.
Proportional Assist Ventilation (PAV) Adjusts airway pressure in proportion to a patient's respiratory effort. The patient determines the inspired volume and flow rate, and support changes on a breath-by-breath basis, depending on the patient's effort and lung dynamics. PAV requires a continuous measurement of resistance and compliance, in order to determine the amount of pressure support to deliver. It is designed to decrease the work of breathing and improve ventilator synchrony.
Synchronized Intermittent Mandatory Ventilation (SIMV) A time-triggered mode, essentially the same as IMV, but in this mode, the ventilator is synchronized so that if a patient initiates a breath, a ventilator breath is not delivered at the same time. If the patient is breathing spontaneously between the mandatory breaths and begins to inspire just prior to the vent trigger, a mandatory breath will be delivered as an assisted patient triggered breath. This mode is more comfortable for the patient than IMV and is often combined with pressure support.
Volume-Assured Pressure Support (VAPS) A support mode that provides a constant tidal volume during regular breathing by automatically adjusting the pressure support. It is a combination of pressure control and volume control. Used most often in patients with neuromuscular disorders or for those with obesity hypoventilation syndrome that have a suboptimal response to BPAP.