Ventilator classification describes how the ventilator works. The classification schemes described here are general enough to be applied to any commercially available ventilator. The components of a ventilator classification system are the control variables, breath sequence, and targeting scheme (Table 5-1).
Table 5-1Ventilator Classification System ||Download (.pdf) Table 5-1 Ventilator Classification System
• Control variable
• Breath sequence
– Continuous mandatory ventilation (CMV): actual rate may be greater than the set rate with the patient-triggered breaths; backup rate is the minimum value in case of apnea.
– Intermittent mandatory ventilation (IMV): spontaneous breaths allowed between mandatory breaths; backup rate is the minimum value if apnea occurs.
– Continuous spontaneous ventilation (CSV): all breaths are patient-triggered.
• Targeting scheme
– Set point
Control variables describe how the ventilator manages pressure, volume, and flow during the inspiratory phase. The control variable remains constant as the ventilatory load changes. Specific control variables are pressure, volume, flow, and time (Figure 5-2). Modern ventilators control either flow or pressure during the inspiratory phase. Moreover, the ventilator can only control flow or pressure at any time. Dual control occurs in situations where inspiration starts out as volume control and then switches to pressure control before the end of the breath (or vice versa).
Criteria used to determine the control variable during inspiration. (Reproduced with permission from Chatburn RL. Classification of mechanical ventilators. Respir Care. 1992; Sep; 37(9):1009-1025.)
A ventilator is a pressure controller if the pressure waveform is not affected by changes in resistance and compliance. If the volume waveform remains unchanged with changes in resistance and compliance, the ventilator can be either a volume controller or a flow controller. The ventilator is a volume controller if volume is measured and used to control the volume waveform. If volume is not used as a feedback signal, but the volume waveform remains constant, then the ventilator is a flow controller. A ventilator is a time controller if inspiratory and expiratory times are the only variables that are controlled.
Two clinically different breath types can be provided during mechanical ventilation: mandatory or spontaneous breaths (Figure 5-3). A spontaneous breath is both initiated and terminated by the patient. If the ventilator determines either the beginning and/or the end of the breath, it is mandatory. The three types of breath sequences during mechanical ventilation are continuous mandatory ventilation, intermittent mandatory ventilation, and continuous spontaneous ventilation (Figure 5-4). All ventilator modes can be identified by one of breathing patterns given in Table 5-2.
Table 5-2Ventilation Modes Identified by Breathing Patterns ||Download (.pdf) Table 5-2 Ventilation Modes Identified by Breathing Patterns
|Breath-control variable ||Breath sequence ||Acronym |
|Volume || |
Continuous mandatory ventilation
Intermittent mandatory ventilation
|Pressure || |
Continuous mandatory ventilation
Intermittent mandatory ventilation
Continuous spontaneous ventilation
PC-CSV (ie, pressure support ventilation
Criteria used to determine breath types during mechanical ventilation. (Reproduced with permission from Chatburn RL. Classification of mechanical ventilators. Respir Care. 1992; Sep; 37(9):1009-1025.)
Breath sequence during mechanical ventilation. CMV, continuous spontaneous ventilation; CSV, continuous spontaneous ventilation; IMV, intermittent mandatory ventilation. (Adapted from Chatburn RL. Classification of ventilator modes: update and proposal for implementation. Respir Care 2007; Mar; 52(3):301-323.)
Phase variables are used to initiate some phase of the ventilatory cycle. Specifically, these are the trigger, limit, and cycle variables (Figure 5-5). The trigger variable initiates inspiration. If time-triggered, the ventilator initiates inspiration at a clinician-determined interval. For example, the ventilator will initiate inspiration every 3 seconds if the rate is set at 20/min. Initiation of inspiration can also be triggered by the patient. Patient triggering can be recognized by the ventilator as a pressure signal, as a flow signal (Figure 5-6), or as a signal from diaphragmatic activity. Pressure triggering occurs when patient effort causes a drop in airway pressure to a clinician-preset level (sensitivity setting). Flow triggering occurs when the patient's inspiratory flow reaches a clinician-preset level. A type of flow triggering is Auto-Trak, in which a shape signal is produced by offsetting the actual patient flow signal by 15 L/min and delaying it by 300 milliseconds. This causes the ventilator-generated shape signal to be slightly behind the patient's flow. A sudden change in patient flow will cross the shape signal causing the ventilator to trigger to the inspiratory phase or cycle to the expiratory phase. With neutrally adjusted ventilatory assist, the breath is triggered by the electrical activity of the diaphragm.
Criteria used to determine the phase variables during a mechanical ventilation breath. (From Chatburn RL. Classification of mechanical ventilators. Respir Care. 1992; Sep; 37(9):1009-1025.)
Flow (A) and pressure (B) triggering. With flow triggering, the ventilator responds to a change in flow. With pressure triggering, the ventilator responds to a decrease in airway pressure.
The limit variable is the pressure, volume, or flow that cannot be exceeded during the inspiratory phase. Inspiration is not necessarily terminated when the limit variable is reached. Pressure-controlled ventilation is pressure limited because the pressure limit is reached before inspiration ends. For some ventilators, the inspiratory flow, inspiratory time, and tidal volume are set separately. In this case, the ventilator is volume limited because tidal volume is delivered before inspiration ends.
The cycle variable is the pressure, volume, flow, or time that terminates inspiration. First-generation ventilators were typically pressure-cycled. With pressure support ventilation, the breath is usually flow-cycled. With volume-controlled ventilation, the breath is volume or time-cycled. With pressure-controlled ventilation, the breath is time-cycled. The baseline variable is what is controlled during the expiratory phase, and is the PEEP or continuous positive airway pressure setting.
Conditional variables are used by the operational logic system of the ventilator to make decisions on how to manage control and phase variables. Conditional variables are if/then statements. For example, if minute ventilation is below the set threshold, then deliver a mandatory breath (eg, mandatory minute ventilation). Computational logic is a description of the relationship between ventilator settings, feedback signals, and breathing pattern to add detail about how the mode operates that is not given in the other components of the mode specification (eg, adaptive support ventilation).
The targeting scheme determines the feedback-control algorithm used by the ventilator. For set point targeting, the clinician adjusts specific static set points such as the pressure limit, tidal volume, and inspiratory flow. Set point targeting occurs with conventional modes such as volume-controlled ventilation, pressure-controlled ventilation, and pressure support ventilation. For set point targeting, the clinician sets all parameters of volume and flow waveforms (volume control modes) or the pressure waveform (pressure control modes). Dual targeting occurs when the breath starts out in volume control, but switches to pressure control within the breath if the patient makes a vigorous inspiratory effort.
With servo control, ventilator output follows and amplifies the patient's inspiratory flow, as occurs with proportional assist ventilation and neutrally adjusted ventilatory assist. With adaptive control, as occurs during pressure-regulated volume control, the ventilator adjusts the pressure control to maintain tidal volume with changes in lung mechanics. Optimal control is an advanced form of adaptive control, as occurs with adaptive support ventilation, where the ventilator tries to achieve a breathing pattern that minimizes the work rate of breathing. Intelligent control uses rule-based expert systems, such as SmartCare, in which the ventilator adjusts its output based on parameters set by the clinician.
Closed-loop control refers to the use of a feedback signal to adjust the output of a system. Closed-loop control of ventilation is commonly used in current-generation ventilators. Ventilators use closed-loop control to maintain consistent pressure and flow waveforms in the face of changing conditions. This is accomplished by using the ventilator output as a feedback signal that is compared to the input set by the clinician. The difference is used to drive the system toward the desired output. With negative control, the ventilator attempts to minimize the difference between target and actual values. A simple example is pressure-controlled or pressure support ventilation, where the ventilator adjusts flow to maintain a constant airway pressure. Another example of negative feedback control is adaptive modes in which there is a change in pressure control to minimize difference between actual and target tidal volume. Positive-feedback control increases the difference between actual and target values. Examples of positive-feedback control include proportional assist ventilation and neutrally adjusted ventilatory assist.