Chapter 5: Ventilator Mode Classification

Current generation mechanical ventilators are sophisticated life support devices. The ventilator must be reliable, flexible, and relatively easy to use by the skilled clinician. This chapter describes the ventilator system, and then covers ventilator classification and breath types during mechanical ventilation.

Because ventilators deliver gases to the patient, they must have a pneumatic component. First-generation ventilators were typically pneumatically powered, using gas pressure to power the ventilator as well as ventilate the patient. Current generation ventilators are electronically controlled with the aid of a microprocessor. A generic block diagram of a ventilator is shown in Figure 5-1.

Pneumatic System

The pneumatic system is responsible for delivery of a gas mixture to the patient. Room air and 100% oxygen are delivered to the ventilator at 50 lb/in². The ventilator reduces this pressure and mixes these gases to provide a prescribed Fio₂ and flow into the ventilator circuit. The ventilator circuit not only delivers gas to the patient, but also filters, warms, and humidifies the inspired gas. During exhalation, gas flows through the expiratory limb of the circuit, a filter, the exhalation valve, and then into the atmosphere. The exhalation valve closes during inspiration to allow inflation of the lungs and is responsible for controlling positive end-expiratory pressure (PEEP). In the past, the exhalation valve was closed completely during the inspiratory phase. Newer-generation ventilators use an active exhalation valve during pressure-controlled ventilation, meaning that it opens if the pressure exceeds the pressure set during the inspiratory phase.

The pneumatic system can be either single circuit or double circuit. With single-circuit ventilators, the gas that powers the ventilator is the same gas that is delivered to the patient. With double-circuit ventilators, the gas delivered to the patient is separate from the gas that powers the pneumonic system.

Ventilators can be positive-pressure or negative-pressure generators. Positive-pressure ventilators apply a positive pressure to the airway. Negative-pressure ventilators apply a negative pressure to the chest wall. Critical care ventilators are positive-pressure generators. Negative-pressure ventilators are used infrequently, but may be used in some patients receiving prolonged mechanical ventilation.

Electronic System

Current-generation mechanical ventilators are microprocessor-controlled. The microprocessor controls the inspiratory and expiratory valves. It also controls the flow of information from the monitoring system of the ventilator (eg, pressure, flow, volume) and the display of that information. Ventilator alarms are also controlled by the microprocessor.

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).

Control Variables

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).

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.

Breath Sequence

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.

Operational Algorithms

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.

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).

Targeting Scheme

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.

Inflation of the lungs is explained by the equation of motion, which states that the pressure required to deliver a breath is determined by the elastic (volume and compliance) and resistive (flow and resistance) properties of the respiratory system. The pressure can be either that applied to the airway (P_vent) by the ventilator or the pressure generated by the respiratory muscles (P_mus), or a combination of both. Mathematically, this becomes:

During volume-controlled ventilation, flow and volume delivery from the ventilator are fixed. If the patient generates an inspiratory effort (increased P_mus) during volume-controlled ventilation, the airway pressure drops—a common sign of patient-ventilator asynchrony. During pressure-controlled ventilation, P_airway is fixed. If the patient generates an inspiratory effort during pressure-controlled ventilation, flow and volume delivery increase, which may improve patient-ventilator synchrony but might also contribute to overdistention lung injury.