Chapter 214 Advanced Mechanical Ventilation
VENTILATOR CONCEPTS
An understanding of ventilator function requires an appreciation of how the machine is generating and controlling a given breath. This requires knowledge of what is determining each phase of the breath and where the energy for that breath is derived (i.e., patient or ventilator).
Respiratory Cycle
The respiratory cycle can be divided into four phases: (1) the inspiratory flow phase, (2) the inspiratory pause phase, (3) the expiratory flow phase, and (4) the expiratory pause phase (Figure 214-1).1,2 By defining how each respiratory phase is determined, the ventilator mode can be described. The respiratory phases also provide a context in which to define common ventilator parameters. For example, peak airway pressure is the maximal airway pressure measured during the inspiratory flow phase. The plateau pressure is the airway pressure measured at the end of the inspiratory pause. The difference between the peak inspiratory pressure and plateau pressure is usually minimal in patients with normal lungs. Some pulmonary disease processes that increase airway resistance (e.g., asthma) can cause significant differences between the peak and plateau pressures.
Equation of Motion
Patient-ventilator interactions can be described by the equation of motion. This equation is built into the ventilator software and is the basis for machine operation (Box 214-1).2-4 The equation of motion states that the pressure required to deliver a breath depends on the tidal volume and flow of the breath in addition to the resistance and compliance of the system. The resistance and compliance are determined largely by the characteristics of the patient while pressure, volume, and flow are the three interdependent variables that may be manipulated by the machine. To understand a ventilator breath, knowledge of changes in airway pressure, volume, and flow during each respiratory phase is required.
DEFINING THE VENTILATOR MODE
The ventilator mode is defined by nature of the breath type and pattern, control variable, and phase variables used.
Breath Types
A ventilator breath is one of two major types: mandatory or spontaneous. During a spontaneous breath, the patient is responsible for both initiation and termination of inspiration. If the machine controls one or both of these factors, the breath is considered mandatory. When a mandatory breath is initiated by the patient, it is classified as an assisted breath. A spontaneous breath in which inspiration is augmented above baseline by the machine is considered a supported breath (Table 214-1).2-4
Control Variable
The control variable is the primary variable manipulated by the machine to generate an inspiration.2-4 Because flow and volume are interrelated, ventilator breaths are either volume controlled or pressure controlled. In a pressure-controlled breath the machine will maintain airway pressure at a constant, preset (by the operator) level, and inspiration ends when a preset inspiratory time is reached. The tidal volume and gas flow rate generated during this breath are dependent on the magnitude of the preset airway pressure and the resistance and compliance inherent to that system as per the equation of motion.
During a volume-controlled breath the flow and tidal volume are fixed to a level preset by the operator; the machine will maintain a constant gas flow, and the inspiration ends when a preset tidal volume is delivered. As the equation of motion describes, airway pressure reached during these breaths is dependent on the magnitude of the preset tidal volume and the resistance and compliance of the patient’s respiratory system.2-4 The basic waveforms for a pressure-controlled and a volume-controlled breath are shown in Figure 214-2. Exhalation is passive, and it can be seen that it has an exponential character.
Figure 214-2 Pressure, volume, and flow as related to time waveforms for pressure-controlled and volume-controlled ventilation. (Note the inspiratory portion of each waveform is in blue.) A, In pressure-controlled ventilation, airway pressure is maintained at a constant level throughout the inspiration, volume increases with time, and the flow rate of the breath decelerates as the lungs fill with gas. B, In volume-controlled ventilation, the flow rate is held constant throughout inspiration, but tidal volume and airway pressure both increase with time.
Phase Variables
The respiratory cycle helps define the four phases of a breath that can be controlled by the ventilator (see Figure 214-1): (1) the start of inspiration, (2) inspiration, (3) the end of inspiration, and (4) exhalation.2,3
Cycle Variable
This is the parameter by which inspiration is terminated.2-4 For example, to give a pressure-controlled breath, the ventilator maintains gas flow at a preset pressure for a given time. When that time has elapsed, inspiration is terminated and exhalation begins, so time is the cycle variable. Time is the most common cycle variable and will be determined by the preset respiratory rate and the inspiratory-to-expiratory (I:E) ratio. An inspiratory time of approximately 1 second is a common guideline.
Trigger Variable
This is the parameter that initiates inspiration. It is how the ventilator determines when to deliver a breath.2-4 In animals that are not making respiratory efforts of their own, the trigger variable will be time and is determined from the set respiratory rate. If the animal is making respiratory efforts, the trigger variable may be a change in airway pressure or gas flow in the circuit resulting from the patient attempting to initiate inspiration. The trigger sensitivity of the machine usually can be set by the operator. An airway pressure drop of 2 cm H2O or gas flow change of 2 L/min are usually effective trigger sensitivities. Appropriate trigger sensitivity is essential to ensure that ventilator breaths are synchronized with genuine respiratory efforts made by the patient. This increases comfort and allows the patient to increase its respiratory rate as required. The trigger variable can be too sensitive, such that nonrespiratory movements such as patient handling may initiate breaths, and this should be prevented.
Limit Variable
This is a parameter that the breath cannot exceed during inspiration, but it is different from the cycle variable because it does not terminate the breath.2-4 This variable may be found on modern intensive care ventilators. For example, a volume-controlled, pressure-limited breath means that the ventilator will generate the breath by delivering a preset tidal volume, but it will not exceed the limit set for airway pressure at any time during the delivery.
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