Chapter 5: Ventilator Mode Classification

Objectives

1. Distinguish between the pneumatic and electronic powering systems of the ventilator.

2. Describe the variables, breath sequence, and targeting scheme used to control mechanical ventilator operation.

3. Compare pressure control, volume control, flow control, and time control.

4. Distinguish between trigger, limit, and cycle.

5. Define spontaneous and mandatory breath types.

6. Compare continuous mandatory ventilation, intermittent mandatory ventilation, and continuous spontaneous ventilation.

7. Compare set point, dual servo, adaptive, optimal, and intelligent targeting schemes.

8. Use the equation of motion to describe patient-ventilator interaction.

Introduction

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.

Ventilator Power Systems

Because ventilators deliver gas 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 microprocessor controlled. 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 for 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.

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. 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

In current-generation ventilators, 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 as numeric and waveform displays. Ventilator alarms are also controlled by the microprocessor.

Classification of Mechanical Ventilators

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 ...