A typical ventilator circuit consists of those components that deliver gas from the ventilator to the patient and return the patient's exhaled gas to the atmosphere. In addition to gas delivery, the circuit conditions the inspired gases by filtering and humidification as discussed previously. Ventilator circuits can be sterilized and reused, but many are disposable single patient-use devices. There are three common configurations of ventilator circuits (Figure 12-4).
(A) Dual-limb circuit with separation on inspired and expired gases. This configuration is most commonly used with critical care ventilators. (B) Single-limb circuit with exhalation valve near the patient. This configuration is most commonly used with portable ventilators. (C) Single-limb circuit with a passive exhalation port. This configuration is used with noninvasive ventilations. With this configuration, flow through the circuit during exhalation must be sufficient to prevent rebreathing.
Compression volume is based on the internal volume of the ventilator, the volume of the humidifier, and the characteristics of the circuit tubing. The compression volume of the system is a function of the volume of the circuit, the compliance (elasticity) of the tubing material, and the ventilation pressure. The volume of gas compressed in the circuit is not delivered to the patient, which becomes clinically important with high pressures and low tidal volumes. The volume that leaves the exhalation valve of the ventilator includes the exhaled volume from the patient as well as the volume of gas compressed in the ventilator circuit. Unless volume is measured directly at the patient's airway, the exhaled volume displayed by the ventilator may overestimate the patient's actual tidal volume by the amount of the compressible volume. Most current generation ventilators correct volume for circuit compression volume, so that the displayed tidal volume is an estimate of the volume delivered to the patient.
The compressible volume is often expressed as the compression factor, which is calculated by dividing the compression volume by the corresponding ventilation pressure. If the compression factor is known, the compressible volume can be calculated by multiplying it by the ventilating pressure. The delivered tidal volume is the volume leaving the exhalation valve minus the compression volume:
where VTexh is the volume leaving the exhalation valve and VT is the tidal volume corrected for compression volume (Figure 12-5).
Illustration of compression volume. In this example, if airway pressure is 30 cm H2O, set tidal volume is 500 mL, and compression factor is 4 mL/cm H2O, then the actual tidal volume delivered to the patient is only 380 mL.
Consideration of compression volume is important for several reasons. Most importantly, it decreases the delivered tidal volume to the patient. Failure to consider compression volume results in overestimation of lung compliance. Auto-positive end-expiratory pressure (auto-PEEP) measurements are also affected by circuit compression volume:
where Crs is the compliance of the respiratory system, Cpc is the compliance of the patient circuit, and estimated auto-PEEP is the value that is measured. Compression volume also affects the measurement of mixed exhaled Pco2, and the following correction can be used:
where Pēco2 is the true mixed exhaled Pco2 and Pexhco2 is measured mixed exhaled Pco2 (including gas compressed in the ventilator circuit). To avoid the effect of compression volume on Pēco2, mainstream volumetric capnography can be used.
Ventilator circuits and endotracheal tubes increase the imposed work-of-breathing for the patient. Circuit resistance adds to the resistance of the endotracheal tube. Circuit resistance increases with the addition of a passive humidifier. The resistance through the expiratory limb of the circuit is primarily due to the exhalation valve and PEEP devices. Current generation ventilators use an exhalation valve with a large diaphragm that is electrically controlled, and thus produces a more consistent circuit pressure regardless of flow. Most ventilators use an active exhalation valve during pressure-controlled ventilation, reducing the risk of circuit overpressurization. An active exhalation valve opens and closes to keep pressure in the circuit at the target level set on the ventilator.
The circuit dead space is the volume of the circuit through which rebreathing occurs. It is called mechanical dead space and is functionally an extension of the patient's anatomic dead space. Mechanical dead space is the volume of tubing between the Y-piece and the artificial airway. It becomes particularly important when the patient is ventilated with a small tidal volume. During low tidal volume ventilation, such as is used as part of a lung-protective strategy, the volume of mechanical dead space should be minimized. Dead space is increased with the use of an HME.
Many current generation ventilators pass a bias flow of gas through the circuit during the expiratory phase. The purpose of this bias flow is to improve triggering during flow-triggered ventilation. Due to this bias flow, it is not possible to accurately measure tidal volume or exhaled gas concentrations by attaching flow and gas measuring sensors distal to the exhalation outlet on the ventilator.
Intubated mechanically ventilated patients are at risk for nosocomial pneumonia. In the past, the ventilator circuit has been implicated in the risk of ventilator-associated pneumonia. However, the source of contamination of the lower respiratory tract is usually aspiration of upper airway secretions from around the cuff. Ventilator-associated pneumonia (VAP) might be better called endotracheal tube-associated pneumonia. Ventilator circuits do not need to be changed on a scheduled basis. Circuit changes are only necessary between patients, if the circuit malfunctions, or when the circuit is visibly soiled. There is no strong evidence that the use of a heated wire circuits or an HME decreases the risk of VAP.
The patient-ventilator system should be evaluated periodically related to the technical aspects of the ventilator system and the pathophysiology of the patient. The patient-ventilator system check is a documented evaluation of a ventilator and the patient's response to mechanical ventilatory support. It should be performed at regular intervals and more frequently if the patient becomes unstable or requires ventilator adjustments. A flow sheet is typically used to record these assessments.
Perhaps the most troublesome aspect of ventilator troubleshooting is the detection and correction of circuit leaks. These must be corrected promptly to prevent patient harm due to hypoventilation. To avoid patient injury due to hypoxia (and possibly death), a disconnect alarm must be set at all times. The disconnect alarm is usually low exhaled volume or low airway pressure. A manual resuscitator should be at the bedside of all mechanically ventilated patients to allow ventilation in the event of a ventilator failure.
Between patients, all ventilators should be calibrated and an operational verification procedure should be conducted as recommended by the manufacturer. With current generation microprocessor ventilators, sophisticated integral computerized self-test diagnostics are used. At manufacturer-determined intervals, more complete ventilator preventive maintenance is required.