Special Feature
Pressure Control Ventilation: Practical Steps for Patient Management
By Edgar Delgado, BS, RRT and Leslie A. Hoffman, PhD, RN
In the early 1970s, siemens1 introduced the use of a microcomputer to control respiratory valves. These servocontrolled valves provided the apparatus required for the development of the pressure-controlled, time-cycled mode of ventilation known as pressure control ventilation (PCV).2 Subsequently, PCV has gained popularity as a method to manage patients who require mechanical ventilation as a consequence of conditions such as acute lung injury (ALI) and acute respiratory distress syndrome (ARDS). Although PCV has been used extensively, a standard approach to patient management when using this ventilator mode has not been established. This essay provides a brief overview of the characteristics of PCV, a suggested approach for initiating PCV, and recommendations for patient monitoring.
Overview of PCV
PCV allows the clinician to preset a maximum level of pressure delivered to the airway, as well as a predetermined inspiratory time for each controlled mechanical breath. In contrast, during conventional volume-controlled ventilation (VCV), the clinician controls the minimum volume of gas delivered into the patient’s lung each minute by setting tidal volume (Vt) and frequency (f). The speed at which the gas is introduced into the airways, termed inspiratory flow rate, is also controlled by the clinician during VCV.2,3
The way in which gas is delivered to the lung is perhaps the most significant difference between VCV and PCV. A square-flow waveform is often used to deliver VCV. The flow rate and shape of this waveform are predetermined, regardless of patient effort. During PCV, the flow waveform and rate are not predetermined, thereby permitting variability of flow characteristics as a function of several factors. Flow of gas into the lung is determined by the set inspiratory pressure, the inspiratory-to-expiratory (I:E) ratio or length of inspiratory time, lung resistance and compliance, patient effort, and certain limitations within the flow control algorithm that are specific to the ventilator used.3
Also, the flow pattern is decelerating or "ramp-type" in nature. Inspiratory flow is initially high, reflecting the large gradient between alveolar and proximal airway pressure, and then decelerates as this pressure gradient falls. Some clinicians have theorized that the early and sustained increase in intra-alveolar pressure created by a decelerating flow pattern might facilitate gas exchange within the lung, but this potential has not been proven.2,4 Additional advantages reported with PCV include an increase in arterial oxygen tension (PaO2) while maintaining a lower peak airway pressure (Ppeak), FIO2, positive end-expiratory pressure (PEEP), and minute ventilation (VE).5-7
Several scenarios may prevent the set target inspiratory pressure from being reached during PCV. In situations when a long inspiratory time is used, flow will reach zero prior to time cycling (termination of inspiratory phase) of the mandatory breath. Here, the set target pressure is usually reached. In other scenarios (e.g., use of a short inspiratory time), flow may not reach zero prior to time cycling. In this situation, set target inspiratory pressure may not be obtainable. Therefore, the clinician needs to determine if the set pressure is actually reached.
Depending on dynamic ventilatory parameters, Vt may also vary. Generally, as compliance decreases or resistance increases, Vt will be reduced. Conversely, if compliance increases or resistance decreases, Vt will increase. Additional considerations relate to the development of intrinsic positive end-inspiratory pressure (auto-PEEP). During PCV, the Vt delivered to the patient is determined by the difference between the set inspiratory pressure and total PEEP (ventilator PEEP + auto-PEEP). Accordingly, any change in auto-PEEP will alter total PEEP and consequently VE unless set inspiratory pressure is increased. Similarly, any increase in ventilator (applied) PEEP will decrease the delivered Vt unless the set inspiratory pressure is increased.
Suggested Approach for Initiation of PCV
The following approach for implementation of PCV has been used at our institution (University of Pittsburgh Medical Center [UPMC]-Presbyterian) since 1992. Our approach is similar to that described by Howard.7
• Consider using PCV when Ppeak approximates 40 cm H2O with PEEP ³ 10 cm H2O.
• Obtain a baseline arterial blood gas (ABG).
• Set the ventilator mode to assist control, and match the f, FiO2, PEEP, and I:E ratio to the VCV settings.
• Set the initial inspiratory target pressure at 75% of the difference between Ppeak and PEEP while on VCV.
• Increase set inspiratory pressure until the desired Vt is obtained. For example, if Ppeak on VCV is 39 cm H2O and PEEP is set on 12 cm H2O, the difference is 27. Seventy-five percent of 27 is 20.25; therefore, the initial inspiratory pressure setting is 20 cm H2O. Increase the pressure in increments of 5 cm H2O until the desired Vt is obtained. There is no magic in this procedure. However, it establishes a conversion from VCV to PCV without exceeding the Ppeak during VCV, and in many situations permits a lower Ppeak than during VCV with comparable Vt.3
• Obtain an ABG within 30 minutes (or sooner) and make further adjustments as necessary.
Pressure Control with Inverse Ratios
In scenarios where recruitable lung units are present, sustained elevations in airway pressure tend to be more effective for the recruitment of alveoli than transient elevations.4 Conceptually, pressure control-inverse ratio ventilation (PC-IRV) permits sustained pressure elevation and facilitates recruitment of collapsed alveolar units, subsequently decreasing dead space ventilation. With the decrease in dead space ventilation, it may be possible to reduce VE (by decreasing Vt or f while maintaining the desired PaCO2). Potentially, this adjustment may translate into a reduction in Ppeak.
At UPMC-Presbyterian, we use the following approach to initiate PC-IRV:
• Provide sedation and/or paralysis as indicated.
• Obtain a baseline ABG.
• Keep f the same as PCV without an inverse ratio. Set the FIO2 to 1.0 for several minutes. Set the I:E ratio to 1:1 prior to inverting the ratio.
• Set the I:E ratio at 2:1.
• Assess the effect of this change on mean airway pressure. If the change in mean airway pressure is not desirable, decrease ventilator PEEP to match the mean airway pressure prior to inverting the ratio.
• Obtain an ABG and make further adjustments as necessary.
Monitoring During PCV
Monitoring during PCV does not differ substantially from routine management of the mechanically ventilated patient in an ICU. However, several parameters require particular attention. As adjustments to the preset inspiratory pressure, I:E ratio, or inspiratory time, frequency, and/or PEEP are made, mean airway pressure and VE may be affected. In addition, auto-PEEP may develop as a result of shorter expiratory times. Therefore, it is imperative to monitor these ventilatory parameters and make certain that the changes produced provide adequate oxygenation and ventilation with the lowest possible mean airway pressure.
In situations when PC-IRV is used, sedation and paralysis are strongly recommended. Patients typically do not tolerate PC-IRV without sedation and paralysis because it is not compatible with a normal breathing pattern. If patients are allowed to breathe above the set ventilatory rates, hemodynamic compromise and barotrauma are likely complications.
To reduce exposure of the lung to high airway pressure in patients with ALI or ARDS, mechanical ventilation should be conducted with sufficient PEEP to prevent end-expiratory collapse and tidal recruitment, and with a small tidal volume to avoid ventilator-induced lung injury. This goal may be facilitated by use of VCV or PCV. One advantage of PCV is the ability to easily designate a set inspiratory pressure that represents a limit alveolar pressure cannot exceed under conditions of passive ventilation. Consequently, PCV is commonly used in our institution when managing the care of patients with ARDS. Use of PCV requires an understanding of the complex interrelationships that occur when using this ventilatory mode and careful patient monitoring to ensure that the desired goals are met. (Edgar Delgado is Education, Research, and Quality Improvement Coordinator in the Respiratory Care Department at the University of Pittsburgh Medical Center.)
References
1. Ingelstedt S, et al. A servo-controlled ventilator measuring expired minute volume, airway flow and pressure. Acta Anaesthesiol Scand Suppl 1972;47:7-27.
2. Papadakos PJ, et al. Pressure-controlled ventilation: Review and new horizons. Clin Pulm Med 1998;5: 120-123.
3. Delgado E. Pressure controlled-inverse ratio ventilation. Crit Care Nurs Q 1996;19:23-35.
4. Al-Saady N, Bennett ED. Decelerating inspiratory flow waveform improves lung mechanics and gas exchange in patients on intermittent positive-pressure ventilation. Intensive Care Med 1985;11:68-75.
5. Lain DC, et al. Pressure control inverse ratio ventilation as a method to reduce peak inspiratory pressure and provide adequate ventilation and oxygenation. Chest 1989;95:1081-1088.
6. Gurevitch MJ, et al. Improved oxygenation and lower peak airway pressure in severe adult respiratory distress syndrome. Treatment with inverse ratio ventilation. Chest 1986;89:211-213.
7. Howard WR. Pressure-control ventilation with a Puritan-Bennett 7200a ventilator: Application of an algorithm and results in 14 patients. Respir Care 1993;38: 32-40.
During PCV, set inspiratory pressure is likely to be reached if:
a. inspiratory time is long.
b. inspiratory time is short.
c. inspiratory time is allowed to vary on a breath-to-breath basis.
d. patient breaths are interspersed with machine breaths.
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