Definition/Introduction
Pressure-controlled ventilation is a modality used in patients with an indwelling endotracheal or tracheostomy tube that allows the practitioner to ventilate at a maximal peak pressure. In contrast to volume-controlled ventilation, pressure-controlled ventilation involves selecting an inspiratory pressure rather than a tidal volume target. The setting of inspiratory pressure, as well as an associated positive end-expiratory pressure (PEEP), will allow a provider to control peak pressure, thereby protecting against barotrauma.
Issues of Concern
A skilled practitioner, familiar with pressure-controlled ventilation, in conjunction with a respiratory therapist, should manage patients on this mode, as it differs from conventional volume control in a very important way. In pressure control ventilation, the setting of inspiratory pressure, and therefore peak pressure, is protective against barotrauma. However, pressure control does have tidal volume as an independent variable. Excess pressure can cause a significantly elevated tidal volume, resulting in volutrauma; conversely, after setting an adequate and safe pressure, the tidal volume may be too low for a patient’s ideal body weight, resulting in increased dead space ventilation and ultimately ventilatory failure.[1]
Clinical Significance
Pressure-controlled ventilation is a common modality in both adults and pediatric patients. It is typically available in both pressure control-continuous mandatory ventilation (PC-CMV) and pressure control-intermittent mandatory ventilation (PC-IMV), which is also a feature of other commonly used modes. A clinician will set the driving pressure to aid in alveolar distention. However, the volume delivered may vary due to a patient’s intrinsic respiratory ability, thoracic compliance, airway resistance, and other ventilator settings, such as the I-to-E ratio and rise time.
While volume-controlled ventilation is the more common mode of ventilation, patients may better tolerate pressure-controlled ventilation. The critical difference in pressure control that may explain the difference in tolerance is the ability of the mode to maintain constant airway pressure by altering the inspiratory flow rates to meet the demand of the patient.[2] While tidal volume does vary in this mode, maintaining constant airway pressure permits improved alveolar distention universally rather than only in regions that are more readily available. This generalized distention may also improve dead-space ventilation, promoting active alveolar ventilation and resulting in improved comfort.
It bears mention that the waveforms on the ventilator screen are different than those seen in traditional volume control. In pressure control, the flow rate is dynamic and adjusts to the patient’s needs; however, the pressure will form a plateau. Figure 1 shows an example of the standard pressure control waveforms.[3] The display in Figure 1 shows a plateaued pressure waveform, indicating that after the rise time, the remainder of the inspiratory time is at constant pressure. The flow is dynamic to maintain this constant pressure. These forces, as well as other factors such as lung and thorax compliance, determine the inspired tidal volume.
Although pressure control has demonstrated improved ventilator synchrony, some findings on the ventilator display indicate that adjustments are necessary to maintain optimal conditions [3]. These abnormalities will usually present themselves in the flow waveforms as they are the dynamic portion of the cycle. Figure 2 demonstrates a shortened inspiratory time, as indicated by the flow waveform ending prematurely and failing to return to zero before exhalation. Most commonly, this will require a change in the I-to-E ratio, which may result from adjusting the respiratory rate or reducing airflow resistance during exhalation.
In contrast to insufficient inspiratory time, there is also the possibility of providing an excess of inspiratory time. In pressure control, the inspiratory time dictates how long a patient remains at the peak pressure; this determines the resulting tidal volume. However, space within the lung is limited, and providing additional inspiratory time does not necessarily confer any further benefit. Figure 3 demonstrates waveform findings indicating excess inspiratory time, and adjusting the I-to-E ratio can improve ventilator effectiveness. A flat component between the inhalation and exhalation phases of the flow waveform best represents the flow waveform.
As previously mentioned, excess inspiratory time is not equivalent to insufficient expiratory time. As above, in the flow waveform, the expiratory component returns to zero liters per minute, indicating that exhalation was permitted to continue until completion and not prematurely halted. Additionally, it is essential to note that the volume waveform returns to baseline, indicating that the volume provided was fully exhaled, preventing the possibility of auto-PEEP. Figure 4 demonstrates an insufficient expiratory time. It is important to note that the flow and volume waveforms are interrupted, which can create a life-threatening situation and require immediate adjustment of the I-to-E ratio or adjustment of the volume by modifying the driving pressure.
The ability to directly control airway pressure will aid in preventing lung injury. Also, using a pressure-based approach will ensure full distribution of ventilation across all lung units. This full distribution may contribute to V/Q mismatch in patients with poorly compliant lungs, as in ARDS.[4][5] In one particular study, researchers prospectively evaluated volume control against pressure control. This study also demonstrated that pressure-controlled ventilation might provide the same support with lower peak airway pressures and improve thoracic static compliance.[6]
Nursing, Allied Health, and Interprofessional Team Interventions
When using pressure-controlled ventilation, respiratory therapists and nursing staff should be actively involved in the co-management and surveillance of the patient on the ventilator in the ICU or emergency room. Discussion as a team should take place when deciding to place the patient on pressure control. Concerning features that require immediate intervention, the team should also be discussed at that time.
