# Mechanical Ventilators

## How Ventilators Work

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:

### Control System

Equation 42-1, A

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=VTCet/RC Equation 42-2

Equation 42-2

Equation 42-3

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

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

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.

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.

Jun 12, 2016 | Posted by in RESPIRATORY | Comments Off on Mechanical Ventilators

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