Pressure controlled ventilation (PCV) is commonly used to manage both adult and pediatric populations. However, there is very little evidence distinguishing the efficacy of PCV over that of volume controlled ventilation in the adult patient population. This gap in the literature may be due to the absence of a consistent and systematic algorithm for managing PCV.
To help gain a better understanding of how to treat this patient population, Lonny Ashworth, MEd, RRT, FAARC, and colleagues sought to determine the need among clinicians for a comprehensive, conceptual understanding of mechanical ventilation, pulmonary physiology, and interpretation of ventilator graphics. Their goal was to identify an efficient approach to managing PCV to provide a more generalizable and equitable approach to management of the ICU patient. Study results were published in the Journal of Critical Care.
An Algorithmic Approach
“This study provides an algorithmic approach to managing a patient being ventilated in pressure control – assist control (PC-AC),” Ashworth says. “It is hoped that this will enhance the understanding of the multiple options available to manage the patient-ventilator system.”
After discussing the options for ventilator changes during PC-AC, it became clear that many clinicians believe the options to increase minute ventilation are to increase peak inspiratory pressure or increase the respiratory rate and do not consider other important options, explains Ashworth. He and his colleagues believed that if clinicians have a thorough understanding of PC-AC, the mechanics of the specific ventilator, ventilator graphics and physiology, the options available to the clinicians will be increased. For this study, the authors intentionally included respiratory therapists, physicians, nurses, and physical therapists to draw upon the expertise of each.
“Managing a patient being ventilated in PC-AC requires a consistent, methodical approach to consider all possible options to optimize ventilatory support,” Ashworth says. “After evaluating arterial blood gasses, a decision is made as to whether or not there is the need to change minute ventilation. If the partial pressure of carbon dioxide (PaCO2) is too low or the pH is too high, methods to reduce minute ventilation need to be considered. If the PaCO2 is too high or the pH is too low, then non-ventilatory and ventilator options to reduce the PaCO2 need to be considered. Ventilatory methods to decrease the PaCO2 are guided after evaluating the plateau pressure, tidal volume, and inspiratory and expiratory flow waveforms.”
Ashworth and colleagues suggest several areas in which clear understanding is needed to provide optimal care during PCV:
- Understanding of mechanical ventilation
In order to properly manage a patient being ventilated in PC-AC, the clinician must understand general principles of mechanical ventilation. This includes modes of ventilation.
- Pulmonary physiology
An understanding of physiology is necessary to understand arterial blood gas interpretation, alveolar pressure, peak airway pressure, airway resistance, static compliance, and time constants. Many of the principles of patient management during PC-AC involve understanding the effects of flow, pressures, and volume delivery on alveolar volume and pressures. During PC-AC, an understanding of time constants is important to understand these interactions.
- Interpretation of ventilator graphics
One of the key principles of properly managing patients being ventilated in PC-AC is to understand the clinical interpretation of ventilator graphic waveforms. This allows the clinician to detect auto positive end expiratory pressure (AutoPEEP). It also allows the clinician to know when an increase in inspiratory time may increase or decrease the tidal volume, or when an increase or decrease in expiratory time may increase or decrease the tidal volume and minute ventilation.
The study authors also provide options to increase minute ventilation when desired (Table). Ashworth notes that physicians can increase respiratory rate by decreasing inspiratory time if the inspiratory flow waveform returns to baseline, decreasing expiratory time if there is no AutoPEEP, and decreasing inspiratory and expiratory time if inspiratory flow waveform returns to baseline and there is no AutoPEEP. “We would like to see clinicians use the algorithmic approach on each patient ventilated in PC-AC, and encourage all clinicians to do the same,” he says.