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Instrumentation and Terminology 

Instrumentation and Terminology
Chapter:
Instrumentation and Terminology
Author(s):

John W. Kreit

DOI:
10.1093/med/9780190670085.003.0004
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General Design

Despite big differences in outward appearance, all mechanical ventilators have several basic features in common (Figure 4.1). All must be connected to high-pressure sources of oxygen and air. Once these gases enter the ventilator, they are blended to produce a clinician-selected fractional inspired oxygen concentration (FIO2). At the onset of inspiration, the demand valve opens to allow this pressurized gas to flow through a heater and humidifier, through the inspiratory limb of the ventilator circuit, through the endotracheal tube, and into the patient’s lungs. Inspiration ends when the demand valve closes and the expiratory valve opens. Exhaled gas travels back to the ventilator through the expiratory limb of the ventilator circuit and passes through a filter before being released into the atmosphere. Volume and flow are measured each time gas leaves and returns to the ventilator. Airway pressure (PAW) is continuously measured within the ventilator circuit.

Figure 4.1 Schematic diagram of the ventilator and the ventilator circuit.

Figure 4.1 Schematic diagram of the ventilator and the ventilator circuit.

* Indicates site of flow and volume measurement

+ Indicates site of airway pressure measurement

All ventilators have a user interface, which includes a prominent video display. An example is shown in Figure 4.2.

Figure 4.2 A representative ventilator–user interface.

Figure 4.2 A representative ventilator–user interface.

The user interface serves two important functions. It allows the clinician to easily choose from a wide variety of ventilator settings (Box 4.1), and it displays these settings, as well as important, real-time patient data (Box 4.2).

Terminology

Boxes 4.1 and 4.2 list most of the terms that you’ll need to use and understand when caring for mechanically ventilated patients. So this is a perfect time to tackle ventilator terminology. A few are self-explanatory, but the rest will be reviewed here. Don’t worry if you can’t remember all of these terms. This is just an introduction. You’ll hear much more about these terms (and others will be introduced) in later chapters.

Ventilator Settings (Box 4.1)

The mode of ventilation is the most basic setting because it determines how the patient interacts with the ventilator. In this book, I classify the ventilator modes as:

  • Continuous mandatory ventilation (CMV)

  • Synchronized intermittent mandatory ventilation (SIMV)

  • Spontaneous ventilation (SV)

  • Bi-level ventilation

Breath types are defined by their pressure, volume, and flow characteristics, by the signal that ends inspiration, and by the mode(s) with which they can be used. I divide mechanical breaths into five types:

  • Volume control (VC)

  • Pressure control (PC)

  • Adaptive pressure control (aPC)

  • Pressure support (PS)

  • Adaptive pressure support (aPS)

Chapter 5 is devoted entirely to ventilator modes and breath types.

As discussed in earlier chapters, positive end-expiratory pressure (PEEP) is the pressure (in cmH2O) that remains in the ventilator circuit, airways, and alveoli between mechanical breaths. PEEP increases end-expiratory lung volume and can improve oxygenation by preventing the collapse of alveoli that were “recruited” during inspiration.

Continuous positive airway pressure (CPAP) is a confusing term because its meaning changes depending on whether it’s used during invasive or non-invasive mechanical ventilation (Chapter 16). Just remember that in intubated patients, CPAP has exactly the same meaning as PEEP. That is, it’s the amount of pressure maintained in the airways and alveoli throughout expiration. So why do we use this term at all? Well, that’s a long story. But, suffice it to say that “CPAP” is sometimes used instead of “PEEP” in the spontaneous ventilation mode (see Chapter 5).

Triggering means that a signal has initiated inspiration by opening the demand valve and closing the expiratory valve. Mechanical breaths are triggered either by the patient (patient-triggered breaths) or, in the absence of sufficient inspiratory effort, by the ventilator (ventilator-triggered breaths).

Most ventilators allow the clinician to choose between two triggering signals that indicate patient inspiratory effort. When set to pressure-triggering, the demand valve opens once patient effort has lowered the measured airway pressure by a small, clinician-selected amount referred to as the pressure sensitivity. For example, as shown in Figure 4.3, if the trigger sensitivity is set at –2 cmH2O and expiratory PAW is zero (atmospheric pressure), a mechanical breath will be triggered whenever patient effort lowers PAW below –2 cmH2O. If PAW is 5 cmH2O during expiration (PEEP), triggering will occur when PAW drops below 3 cmH2O.

Figure 4.3 Airway pressure (PAW) versus time during a mechanical breath. When set to pressure triggering, the demand valve opens when patient inspiratory effort lowers PAW by the set pressure sensitivity. In this example, expiratory pressure is zero (atmospheric pressure) and sensitivity is set at –2 cmH2O. A breath is triggered when PAW falls below –2 cmH2O.

Figure 4.3 Airway pressure (PAW) versus time during a mechanical breath. When set to pressure triggering, the demand valve opens when patient inspiratory effort lowers PAW by the set pressure sensitivity. In this example, expiratory pressure is zero (atmospheric pressure) and sensitivity is set at –2 cmH2O. A breath is triggered when PAW falls below –2 cmH2O.

Figure 4.4 shows that when flow-triggering is used, gas continuously flows at a low rate (the base flow) through the ventilator circuit. When the patient inhales, some of this gas is diverted into the lungs, and the measured expiratory flow falls. A mechanical breath is triggered when the difference between measured inspiratory and expiratory flow exceeds the clinician-selected flow sensitivity. For instance, if the base flow is 3 liters per minute (L/min) and the sensitivity is set at 1 L/min, a breath will be triggered every time measured expiratory flow falls below 2 L/min.

Figure 4.4 (A) Gas flows continuously through the ventilator circuit (the base flow). Since no gas enters the patient’s lungs, the flow leaving (inspiratory flow) and returning to the ventilator (expiratory flow) are the same.(B) When the patient makes an inspiratory effort, some of the base flow is diverted into the lungs. The demand valve opens and a mechanical breath is provided when expiratory flow and inspiratory flow differ by more than the set flow sensitivity.

Figure 4.4 (A) Gas flows continuously through the ventilator circuit (the base flow). Since no gas enters the patient’s lungs, the flow leaving (inspiratory flow) and returning to the ventilator (expiratory flow) are the same.

(B) When the patient makes an inspiratory effort, some of the base flow is diverted into the lungs. The demand valve opens and a mechanical breath is provided when expiratory flow and inspiratory flow differ by more than the set flow sensitivity.

Mandatory breaths are delivered by the ventilator with or without patient inspiratory effort, so they are also referred to as “guaranteed” or “back-up” breaths. Cycling refers to the transition between inspiration and expiration, when the demand valve closes and the expiratory valve opens. Spontaneous breaths are patient-triggered breaths in excess of the set (mandatory) rate.

When patients receive volume control breaths, some ventilators require the selection of a maximum or peak inspiratory flow rate and a flow profile, which specifies how the flow rate changes during inspiration. The two common flow profiles are square-wave, in which the set peak flow is maintained throughout inspiration, and descending-ramp, in which the peak flow occurs early and then steadily falls throughout inspiration (see Chapter 1, Figure 1.11). Other ventilators allow only square-wave flow, which is determined by setting both the tidal volume and the inspiratory time.

The driving pressure is a constant, clinician-set pressure that is applied by the ventilator when patients are receiving pressure control and pressure support breaths.

The inspiratory time (TI) is simply the duration of inspiration. It is the interval between triggering and cycling.

Normally, the expiratory valve opens as soon as the demand valve closes and the ventilator then cycles from inspiration to expiration. Clinicians have the option of delaying the opening of the expiratory valve and holding the delivered volume in the lungs by specifying a plateau time (see Chapter 1, Figure 1.7).

Ventilator alarms are meant to notify health care providers about a potentially dangerous patient condition or machine malfunction. They’ve become a very common and, unfortunately, often-ignored sound in most intensive care units (ICUs). Although alarm parameters are typically set by respiratory therapists, it’s essential that physicians understand their meaning and potential implications (Table 4.1). Ventilator alarms are discussed in Chapter 6.

Table 4.1 Important Ventilator Alarms

Alarm

Common causes

Implications

High airway pressure

  • Coughing

  • Patient–ventilator asynchrony

  • ET tube or large airway obstruction

  • Pneumothorax

When the high PAW limit is exceeded, the expiratory valve opens, and the patient receives NO VENTILATION.

Low airway pressure

  • Leak in ventilator circuit

  • Vigorous inspiratory effort

Low tidal volume or excessive work of breathing.

High respiratory rate

  • Respiratory distress

  • Agitation

  • High ventilation requirements

High patient work of breathing. The patient may need more ventilator support.

Low respiratory rate

Impaired respiratory drive or effort

Ventilation is probably inadequate.

Low exhaled tidal volume

  • Impaired respiratory drive or effort

  • Leak in ventilator circuit

  • High PAW limit has been exceeded

Ventilation is probably inadequate.

PAW = airway pressure; ET = endotracheal

Patient Data (Box 4.2)

All ICU ventilators are capable of showing graphical displays of real-time patient data. The most commonly used are:

  • Airway pressure vs. time

  • Flow vs. time

  • Flow vs. volume

  • Volume vs. airway pressure

The user interface shows the set (mandatory) breath rate, the spontaneous breath rate, and their sum, the total breath rate. The inspired (delivered) and exhaled tidal volumes are also displayed and are normally, of course, very similar. An isolated drop in exhaled volume usually results either from a leak in the ventilator circuit or from incomplete exhalation. Inspired and exhaled minute ventilation are the product of the total breath rate and inspired and exhaled tidal volume, respectively.

Most ventilators also show inspiratory time, expiratory time (TE), and the ratio of inspiratory to expiratory time (I:E ratio). The expiratory time is the interval between mechanical breaths, so it’s also the maximum time that the patient has to exhale the delivered tidal volume. It’s determined by TI and the total breath rate. For example, if a patient breathes at a rate of 20 times per minute, the average respiratory cycle length is 60/20, or 3 seconds. If TI is 1 second, then TE must be 2 seconds. The I:E ratio is normally less than 1.

The peak airway pressure (PPEAK) is the maximum pressure reached during the preceding mechanical breath. The mean airway pressure (PMEAN) is continuously calculated by averaging airway pressure throughout the entire respiratory cycle (inspiration and expiration).

The end-inspiratory or plateau pressure (PPLAT) is measured and displayed after a clinician-set delay between closure of the demand valve and opening of the expiratory valve. This can be done on most machines by setting a plateau time (see preceding), or more commonly, by simply initiating a brief pause through the user interface. As discussed in Chapter 1, the plateau pressure is the total elastic recoil pressure of the respiratory system at the end of inspiration.

The end-expiratory pressure is measured and displayed when the clinician closes the expiratory valve just before the next breath. On all ventilators, this can easily be done via the user interface. This pressure, which is also referred to as total PEEP (PEEPT), is the sum of the set level of PEEP (extrinsic PEEP; PEEPE) and intrinsic PEEP (PEEPI), which results from incomplete exhalation and dynamic hyperinflation.