Commonly used modes available on most commercial ventilators include assist/control, pressure-controlled ventilation (PCV), pressure-support ventilation (PSV), and continuous positive airway pressure (CPAP). The setup and operation of these modes are described below.
When choosing ventilator settings, the mode refers specifically to the manner in which ventilator breaths are triggered, cycled, and limited. The trigger, either an inspiratory effort or a time-based signal, defines what the ventilator senses to initiate an assisted breath. Cycle refers to the factors that determine the end of inspiration. For example, in volume-cycled ventilation, inspiration ends when a specific tidal volume is delivered to the patient. Other types of cycling include pressure cycling, time cycling, and flow cycling. Limiting factors are operator-specified values, such as airway pressure, that are monitored by transducers internal to the ventilator circuit throughout the respiratory cycle. If the specified values are exceeded, inspiratory flow is immediately stopped, and the ventilator circuit is vented to atmospheric pressure or the specified positive end-expiratory pressure (PEEP). A list of types of assessments that should be considered for a patient undergoing mechanical ventilation are provided in Table 7-1.
Table 7-1Parameters Monitored During Mechanical Ventilation ||Download (.pdf) Table 7-1Parameters Monitored During Mechanical Ventilation
Arterial blood gases
Flow time waveform
Flow pressure curve
Lines and endotracheal tube position
Signs of barotrauma
Evaluation of lung parenchyma, pleural space, chest wall, subcutaneous tissue, cardiac silhouette (and hardware), vascular component
In assist/control mode ventilation (ACMV), an inspiratory cycle is initiated either by the patient's breathing effort or, if no patient effort is detected within a specified time window, by a timer signal within the ventilator based on user-specified parameters. Every breath delivered, whether patient- or timer-triggered, consists of the operator-specified tidal volume. Ventilatory rate is determined either by the patient or by the operator-specified backup rate, whichever is of higher frequency (Fig. 7-2). ACMV is used commonly for initiation of mechanical ventilation because it ensures a backup minute ventilation in the absence of an intact respiratory drive and allows for synchronization of the ventilator cycle with the patient's inspiratory effort.
Assist/control mode ventilation (ACMV) airway pressure and delivered tidal volume profiles. In ACMV ventilation, two types of breaths can occur. Assisted breaths are initiated by the patient and are fully supported by the ventilator, which delivers a user-specified tidal volume. Ventilator-controlled breaths are initiated by the ventilator at the backup rate specified by the user and are triggered by the timer system in the ventilator if the patient fails to initiate a breath after a specified period.
Problems can arise when ACMV is used in patients with tachypnea resulting from nonrespiratory or nonmetabolic factors such as anxiety, pain, or airway irritation. Respiratory alkalemia may develop and trigger myoclonus or seizures. Dynamic hyperinflation may occur if the patient's respiratory mechanics are such that inadequate time is available for complete exhalation between inspiratory cycles. This can limit venous return, decrease cardiac output, and increase airway pressures, predisposing to barotrauma. ACMV is not effective for weaning patients from mechanical ventilation because it provides full ventilator assistance on each patient-initiated breath.
PCV can be used to provide ventilator support either with ACMV triggering (PCV-ACM) or SIMV triggering (PCV-SIMV). In contrast to conventional ACMV or SIMV, which are volume-cycled and pressure-limited, PCV-ACM and PCV-SIMV are time-cycled and pressure-limited. During the inspiratory phase, a given pressure is imposed at the airway opening, and the pressure remains at this user-specified level throughout inspiration (Fig. 7-3). Since inspiratory airway pressure is specified by the operator, tidal volume and inspiratory flow rate are dependent rather than independent variables and are not user-specified. PCV is used commonly for patients with documented barotrauma because airway pressures can be limited, as well as for postoperative thoracic surgical patients, in whom the stress across a fresh suture line can be limited. When PCV is used, minute ventilation and tidal volume must be monitored; minute ventilation is varied by the user through changes in rate or in the pressure-controlled value.
Pressure-control ventilation (PVC) delivers airway inflation pressure using time cycling rather than a user-specified tidal volume using volume cycling. In this figure, all breaths are shown as timer-cycled, although PCV can be programmed to trigger according to an assist/control algorithm or synchronized intermittent mandatory algorithm.
PCV with the use of a prolonged inspiratory time is frequently applied to patients with severe hypoxemic respiratory failure. This approach, called pressure-controlled inverse inspiratory-to-expiratory ratio ventilation (PCIRV), increases mean distending pressures without increasing peak airway pressures. It is thought to work in conjunction with PEEP to open collapsed alveoli and improve oxygenation. In acute lung injury (ALI), PCIRV may be associated with fewer deleterious effects than conventional volume-cycled ventilation, which requires higher peak airway pressures to achieve an equivalent reduction in shunt fraction, but there are no convincing data to show that PCIRV improves outcomes in ALI or adult respiratory distress syndrome.8,9
PSV is a form of ventilation that is patient-triggered, flow-cycled, and pressure-limited; it is designed specifically for use in the weaning process but is also used commonly to ventilate patients with new-onset respiratory failure who are agitated and become asynchronous with other modes of ventilator support. During PSV, the inspiratory phase is terminated when the inspiratory flow rate falls below a certain level; in most ventilators, this flow rate cannot be adjusted by the operator. When PSV is used, patients receive ventilator assist only when the ventilator detects an inspiratory effort (Fig. 7-4). Thus it is mandatory that the patient have an intact respiratory drive to be maintained safely on this mode of ventilation without additional backup ventilator support.
Pressure-support ventilation (PSV) requires patient triggering for every breath. The ventilator assists every patient effort by applying a user-specified amount of positive pressure throughout the airway circuit. As the lung fills, inspiratory flow slows. When flow falls below a set value, inspiratory assist ceases, and the expiratory valve opens, allowing for exhalation. The size of each breath is dictated by the patient's inspiratory efforts, which augment ventilator assist. Respiratory rate is determined by the frequency of triggering by the patient.
PSV is well tolerated by most patients who are being weaned. PSV parameters can be set to provide full or nearly full ventilator support that can be withdrawn slowly over a period of days in a systematic fashion to gradually load the respiratory muscles and allow the patient to fully resume spontaneous breathing.
CPAP is not a true support mode of ventilation because ventilation occurs through the patient's spontaneous efforts. The ventilator provides fresh gas to the breathing circuit with each inspiration and charges the circuit to a constant operator-specified pressure that can range from 0 to 20 cm H2O (Fig. 7-5). CPAP is used to assess extubation potential in patients who have been weaned effectively and are requiring little ventilator support.
Continuous positive airway pressure (CPAP) charges the ventilator circuit to a user-specified continuous airway pressure but provides ventilator assist with respiratory efforts. All ventilation occurs through the spontaneous efforts of the patient.