To understand how ventilators work, one must have some knowledge of basic mechanics. A ventilator is simply a machine, which is a system designed to alter, transmit, and direct applied energy in a predetermined manner to perform useful work.⁴ Ventilators are provided with energy in the form of either electricity or compressed gas. The energy is transmitted or transformed (by the drive mechanism of the ventilator) in a predetermined manner (by the control circuit) to augment or replace the patient’s muscles in performing the work of breathing (the desired output). To understand mechanical ventilators, the following four basic functions of ventilators must be understood:

• Input power

• Power transmission and conversion

• Control system

• Output (pressure, volume, and flow waveforms)

This simple outline format can be expanded to add as much detail about a given ventilator as desired.

Rule of Thumb

For patient transport, you must use either a pneumatically powered ventilator or one that can run solely on batteries. Always take along a manually powered bag-valve mask, and for long transports be sure to have back-up power available (extra cylinders or batteries).

Power Transmission and Conversion

The power transmission and conversion system consists of the drive and output control mechanisms. The drive mechanism generates the actual force needed to deliver gas under pressure. The output control consists of one or more valves that regulate gas flow to the patient.

Drive Mechanism

The drive mechanism of the ventilator converts the input power to useful work. The characteristic flow and pressure patterns the ventilator produces are determined in part by the type of drive mechanism it contains. Drive mechanisms can be either (1) a direct application of compressed gas via a pressure-reducing valve or (2) an indirect application via an electrical motor or compressor. Descriptions of these devices are provided in textbooks devoted to respiratory care equipment.⁶

Output Control Valve

The output control valve regulates the flow of gas to the patient. It may be a simple on/off exhalation valve, as in the Newport E100i (Newport NMI Ventilators; Newport Medical Instruments, Newport Beach, CA). Alternatively, the output control valve can shape the output waveform, as in the Maquet SERVO-i (Maquet, Bridgewater, NJ). Commonly used output control valves include the pneumatic diaphragm, electromagnetic poppet/plunger valve, and proportional valve.⁶

Control System

Knowledge of the mechanics of breathing provides a good foundation for understanding how ventilators work. Specifically, the pressure needed to drive gas into the airway and inflate the lungs is important. The formula that relates these variables is known as the equation of motion for the respiratory system (Figure 42-1):⁷

FIGURE 42-1 The respiratory system can be modeled as a single-flow conducting tube connected to a single elastic compartment. This physical model can be described by a mathematical model called the equation of motion for the respiratory system. In this model, pressure, volume, and flow are variables (i.e., functions of time), whereas resistance and elastance (or compliance) are constants.

PVENT+PMUS=(E×V)+(R×V˙)+auto-PEEP Equation 42-1, A

Equation 42-1, A

where P_VENT is the pressure generated by the ventilator above positive end expiratory pressure (PEEP), P_MUS is the pressure generated by the ventilatory muscles to expand the lungs and chest wall, E is respiratory system elastance, V is the change in lung volume above functional residual capacity (FRC), R is respiratory system resistance, and is flow (usually zero unless auto-PEEP is present). Auto-PEEP is the difference between end expiratory airway pressure and end expiratory lung pressure. The combined muscle and ventilator pressures cause gas to flow into the lungs. Elastance (elastance = Δpressure/Δvolume) and resistance (resistance = Δpressure/Δflow) together constitute the impedance or load against which the muscles and ventilator do work. The equation of motion is sometimes expressed in terms of compliance (compliance = Δvolume/Δpressure) instead of elastance.

The auto-PEEP term in the equation of motion indicates that if auto-PEEP is present, more force is required by the ventilator or the muscles or both to generate a given tidal volume and flow. Under passive conditions (P_MUS = 0) for volume control ventilation, where the terms E × V and E × are constant because of the preset values of tidal volume and inspiratory flow, as auto-PEEP increases, peak inspiratory pressure increases. For pressure control ventilation, the airway pressure waveform is constant because of the preset value of inspiratory pressure. As auto-PEEP increases, both inspired tidal volume and peak inspiratory flow decrease. If inspiration is unassisted (P_VENT = 0), the equation shows that P_MUS must exceed auto-PEEP before inspiratory flow can begin. If the patient is connected to a ventilator, P_MUS must be greater than auto-PEEP to trigger an assisted breath. In these cases, auto-PEEP increases the patient’s work of breathing. However, auto-PEEP is not always undesirable, as in the application of airway pressure release ventilation where expiratory time is intentionally shortened to create gas trapping in the absence of a set PEEP. Auto-PEEP is also present as an unavoidable and perhaps desirable factor during high-frequency ventilation.

Pressure, volume, and flow all are changeable variables measured relative to their baseline or end expiratory values. When pressure, volume, and flow are plotted as functions of time, characteristic waveforms for volume-controlled ventilation and pressure-controlled ventilation are produced (Figure 42-2). The conventional order of presentation is pressure, volume, and flow from top to bottom. Convention also dictates that positive flow values (above the horizontal axis) correspond to inspiration and that negative flow values (below the horizontal axis) correspond to expiration. The vertical axes are in units of the measured variables (usually cm H₂O for pressure, L or ml for volume, and L/min or L/sec for flow). The horizontal axis of these graphs is time. Many ventilators have monitors that display pressure, volume, and flow waveforms, providing the clinician with information to evaluate ventilator-patient interaction.

Figure 42-2 shows that the expiratory lung pressure curves are the same shape for both volume and pressure control. This shape is called an exponential decay waveform (often called a decelerating waveform), and it is characteristic of passive emptying of the lungs (exhalation). Solving the equation of motion for lung pressure (assuming no auto-PEEP) provides the following expression:

PL=VTCe−t/RC Equation 42-2

Equation 42-2

where P_L is lung pressure during passive exhalation, V_T is tidal volume, e is the base of the natural logarithms (approximately 2.72), t is time (in this case, the time allowed for exhalation), R is respiratory system resistance, and C is respiratory system compliance. The product of R and C has units of time and is called the time constant. It is referred to as a “constant” because after a period of time equal to RC, the lung pressure changes by 63%. In the next period equal to RC, lung pressure changes another 63% and so on to infinity. When the expiratory time is equal to the time constant, the patient will have passively exhaled 63% of his or her tidal volume. This can be shown using Equation 42-2 by setting t equal to RC:

PL=VTCe−RC/RC=VTC2.72−1=VTC0.37 Equation 42-3

Equation 42-3

This expression shows that after an expiratory time equal to one time constant, only 37% of the lung pressure is left. Because volume is equal to pressure multiplied by compliance, multiplication of both sides of Equation 42-3 by compliance shows that only 37% of the tidal volume remains in the lungs. The implication is that 63% of the tidal volume has been exhaled. After two time constants (i.e., t = 2RC), exhalation is 86% completed, and after three time constants, exhalation is 95% complete. After five time constants, exhalation is considered to be 100% complete for practical purposes (Figure 42-3).

PL=VTC(1−e−t/RC) Equation 42-4

Equation 42-4

When this equation is graphed, it looks like the curved line showing lung pressure and volume during inhalation for pressure-controlled ventilation in Figure 42-2. These same equations govern the passive flow curves for volume-controlled and pressure-controlled ventilation. It is important to understand the concept of time constants to make appropriate ventilator setting adjustments. In any mode of ventilation, the expiratory time should be at least three time constants long to avoid clinically important gas trapping. Similarly, in pressure-controlled modes, inspiratory time (for passive inspiration) should be at least five time constants long to get the maximum tidal volume from the set pressure gradient (i.e., peak inspiratory pressure [PIP] − end expiratory pressure).

Control Circuit

To manipulate pressure, volume, and flow, a ventilator must have a control circuit. A control circuit is a system of components that measures and directs the output of the ventilator to replace or assist the breathing efforts of the patient. A ventilator control circuit may include mechanical, pneumatic, electrical, electronic, or fluidic components. Most modern ventilators combine two or more of these subsystems to provide user control.

Mechanical control circuits use devices such as levers, pulleys, and cams. These types of circuits were used in the early manually operated ventilators illustrated in history books.⁸ Pneumatic control is provided using gas-powered pressure regulators, needle valves, jet entrainment devices, and balloon-valves. Some transport ventilators use pneumatic control systems.

Electrical control circuits use only simple switches, rheostats (or potentiometers), and magnets to control ventilator operation. Electronic control circuits use devices such as resistors, capacitors, diodes, and transistors and combinations of these components in the form of integrated circuits. The most sophisticated electronic systems use preprogrammed microprocessors to control ventilator function.

Fluidic logic-controlled ventilators, such as the Bio-Med MVP-10 (Bio-Med Devices, Stanford, Connecticut) and Sechrist IV-100B (Sechrist, Anaheim, California), also use pressurized gas to regulate the parameters of ventilation. However, instead of simple pressurized valves and timers, these ventilators use fluidic logic circuits that function similar to electrical circuit boards.⁹ Fluidic control mechanisms have no moving parts. In addition, fluidic circuits are immune to failure from surrounding electromagnetic interference, as can occur around MRI equipment.

Control Variables

A control variable is the primary variable that the ventilator manipulates to cause inspiration. There are only three explicit variables in the equation of motion that a ventilator can control: pressure, volume, and flow. Because only one of these variables can be the independent variable, the others are dependent variables. In other words, only one variable can be directly controlled at a time, and a ventilator must function as a pressure, volume, or flow controller. Time is implicit in the equation of motion and in some cases serves as a control variable. Figure 42-4 illustrates the criteria for determining what variable the ventilator controls at any given time.

Figure 42-5 illustrates the important variables for volume-controlled modes. It shows that the primary variable we wish to control is the patient’s minute ventilation. A particular ventilator may allow the operator to set minute ventilation directly. More frequently, minute ventilation is adjusted by means of a set tidal volume and frequency. Tidal volume is a function of the set inspiratory flow and the set inspiratory time. Inspiratory time is affected by the set frequency and, if applicable, the set inspiratory-to-expiratory (I : E) ratio. The mathematical relationships among all these variables are shown in Table 42-1.

TABLE 42-1

Equations Relating the Important Parameters for Volume-Controlled and Pressure-Controlled Ventilation

Mode	Parameter	Symbol	Equation
Volume-controlled	Tidal volume (L)	V_T
	Tidal volume (L)	V_T
	Mean inspiratory flow (L/min)
	Mean inspiratory flow (L/min)
Pressure-controlled	Tidal volume (L)	V_T	V_T = ΔP × C × (1 − e^−t/τ)
	Instantaneous inspiratory flow (L/min)
	Pressure gradient (cm H₂O)	ΔP	ΔP = PIP − PEEP
Both modes	Exhaled minute ventilation (L/min)
	Total cycle time or ventilatory period (seconds)	TCT	TCT = T_I + T_E = 60 ÷ f
	I : E ratio	I : E
	Time constant (seconds)	τ	τ = R × C
	Resistance (cm H₂O/L/sec)	R
	Compliance (L/cm H₂O)	C
	Elastance	E
	Mean airway pressure (cm H₂O)
Primary variables	Pressure (cm H₂O)	P
	Volume (L)	V
	Flow (cm H₂O/L/sec)
	Time (sec)	τ
	Inspiratory time (sec)	T_I
	Expiratory time (sec)	T_E
	Frequency (breaths/min)	f
	Base of natural logarithm (≈2.72)	e

With pressure-controlled modes, the goal is also to maintain adequate minute ventilation. However (as the equation of motion shows), when pressure is controlled, tidal volume and minute ventilation are determined not only by the ventilator’s pressure settings but also by the elastance and resistance of the patient’s respiratory system. This additional variable makes minute ventilation (and gas exchange) less stable in pressure-controlled modes than in volume-controlled modes. Figure 42-6 shows the important variables for pressure-controlled modes. Tidal volume is not set on the ventilator. It is the result of the pressure settings and the patient’s lung mechanics and the inspiratory time. On some ventilators, the speed with which the PIP is achieved (i.e., the pressure rise time) is adjustable. That adjustment affects the shape of the pressure waveform and the mean airway pressure. Mean airway pressure is important because, within reasonable limits, as the mean airway pressure increases, arterial O₂ tension increases. Mean airway pressure is higher for pressure-controlled modes than for volume-controlled modes (at the same tidal volume) owing to the differences in the shapes of the airway pressure waveforms.

Phase Variables

A complete ventilatory cycle or breath consists of four phases: (1) the initiation of inspiration, (2) inspiration itself, (3) the end of inspiration, and (4) expiration. To understand a breath cycle, the clinician must know how the ventilator starts, sustains, and stops inspiration and must know what occurs between breaths.

The phase variable is a variable that is measured and used by the ventilator to initiate some phase of the breath cycle. The variable causing a breath to begin is the trigger variable. The variable limiting the magnitude of any parameter during inspiration is the target variable. The variable causing a breath to end is the cycle variable. To describe what happens during expiration, the baseline variable that is in effect must be known. Figure 42-7 shows the criteria for determining phase variables.

Trigger Variable

On most modern ventilators, either the machine or the patient can initiate a breath. If the machine initiates the breath, the trigger variable is time. If the patient initiates the breath, pressure, flow, or volume may serve as the trigger variable. Manual or operator-initiated triggering is also available on most ventilators.

Time Triggering

When triggering by time, a ventilator initiates a breath according to a predetermined time interval, without regard to patient effort. In the past, time triggering was the only method available to initiate a ventilator cycle. At the present time, time triggering is most commonly seen when using the intermittent mandatory ventilation (IMV) mode.

Specific systems for setting a breathing rate vary from ventilator to ventilator. A rate control is the most common approach, which divides each minute into equal time segments, allotting one time segment for each full breath. When a rate control is used, inspiratory and expiratory times vary according to other control settings, such as flow and volume. An alternative approach is to provide separate timers for inspiration and expiration. Changing either or both of these timers alters the breathing rate.

Pressure Triggering

Pressure triggering occurs when a patient’s inspiratory effort causes a decrease in pressure within the breathing circuit. When this pressure decrease reaches the pressure-sensing mechanism, the ventilator starts gas delivery. On most ventilators, the pressure decrease needed to trigger a breath can be adjusted. The trigger level is often called the sensitivity. Typically, the trigger level is set 0.5 to 1.5 cm H₂O below the baseline expiratory pressure. Setting the trigger level to a higher number, such as 3 cm H₂O, makes the ventilator less sensitive and requires the patient to work harder to initiate inspiration. Conversely, setting the trigger level lower makes the ventilator more sensitive to patient effort. How fast the ventilator mechanism responds to patient effort is called the response time. It is important for the ventilator to have a short response time to maintain optimal synchrony with the patient’s inspiratory efforts. Either a large pressure decrease (i.e., low sensitivity setting) or a delay in flow delivery can increase a patient’s work of breathing.

Flow Triggering

Using flow as the trigger variable is more complex. A ventilator that uses flow triggering typically provides a continuous low flow of gas through its circuit. The ventilator measures the flow coming out of the main flow control valve and the flow through the exhalation valve. Between breaths, these two flows are equal (assuming no leaks in the patient circuit). When the patient makes an inspiratory effort, the flow through the exhalation valve falls below the flow from the output valve. The difference between these two flows is the flow trigger variable.

Flow Trigger Variable

To adjust the sensitivity of a flow-triggered system, the clinician usually sets both a base continuous flow and a trigger flow level. Typically, the trigger flow level is set to 1 to 3 L/min (below baseline). If the base continuous flow is set at 10 L/min and the trigger is set at 2 L/min, the ventilator triggers a breath when the output flow decreases to 8 L/min or less. An alternative approach used with some ventilators is simply to measure the flow at the “wye” connector and start breath delivery on that signal.

Compared with pressure, using flow as the trigger variable decreases a patient’s work of breathing.^11,12 However, ventilators that use a flow-triggering mechanism tend to be highly susceptible to circuit leaks or movement caused by turbulence or gas flow through condensed water. Either of these conditions can cause spurious breaths, which can disrupt patient-ventilatory synchrony and increase the work of breathing. The perception exists, with regard to the most recent generation of ICU ventilators, that pressure and flow triggering are equally effective. Using volume as the trigger can help overcome synchrony problems caused by circuit leaks, but at the present time only the Draeger Babylog (Draeger Medical, Telford, Pennsylvania) uses true volume triggering.