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Jeffrey M. Feldman, MD
Consulting Medical Director North American Drager

Pressure controlled ventilation (PCV) has been a mode of ventilation that has been used in the intensive care unit for many years. Recently, this mode of ventilation has become available on ventilators designed for use in the operating room. At the recent meeting of the Society for Pediatric Anesthesia, some questions were raised about the implementation of PCV on different anesthesia ventilators and the flow waveforms that result. This article should clarify the relationship between pressure and flow waveforms during PCV.

Pressure controlled ventilation is defined as a ventilation mode whereby the user selects a desired peak inspiratory pressure which is sustained throughout the entire inspiratory phase. When using PCV, the volume that is delivered to the patient is determined by the compliance and resistance properties of the breathing circuit and the patient's lungs. This mode of ventilation can offer advantages over volume controlled ventilation for some patients including limiting the maximum pressure to which the lungs are exposed and providing better ventilation of portions of the lung with longer time constants.

Once the user selects a desired inspiratory pressure, respiratory rate, I:E ratio and peak inspiratory flow, the inspiratory pressure is achieved rapidly at the onset of inspiration and maintained at the selected value throughout the inspiratory portion of the breathing cycle. The maximum inspiratory flow of the ventilator and the control algorithm will determine how rapidly the desired inspiratory pressure is achieved.

At the start of each breath, the ventilator will deliver gas up to the maximum set inspiratory flow. Once the set inspiratory pressure is achieved, the ventilator controller will reduce, or decelerate, the gas flow throughout the remainder of the inspiratory time to maintain a constant inspiratory pressure at the preset value. As the lung fills, the pressure gradient between the ventilator and the lung will diminish and progressively less flow will be required to maintain the set inspiratory pressure. The ultimate shape of the flow waveform will depend upon the interaction between the ventilator and the patient's lungs. (Figure)

Consider an example where the desired inspiratory pressure is 25 cmH2O, the respiratory rate is 10 breaths per minute, the I:E ratio is 1:2 and the maximum inspiratory flow is 75 liters per minute. Each breath will take 6 seconds to deliver with an inspiratory time of 2 seconds and an expiratory time of 4 seconds. At the onset of inspiration, the ventilator will deliver a flow up to 75 liters per minute until the inspiratory pressure reaches 25 cmH2O. At that point, flow will begin to diminish (decelerate) to whatever value is necessary to maintain the desired inspiratory pressure for the remainder of the inspiratory cycle. Flow will then cease and expiration will begin. After four seconds of expiratory time, the next inspiration will begin. The shape of the flow waveform that the ventilator delivers will depend in large part upon the compliance of the patient's lungs and the breathing circuit. If lung compliance is low, the peak pressure will be reached rapidly and the flow will diminish rapidly since very little volume is needed to maintain the set pressure. If lung compliance is high, more flow will be required to maintain the desired pressure, the flow waveform will not diminish quite as rapidly and a greater volume will be delivered to the patient. (Figure) Airway resistance is a factor in generating pressure but becomes less important as the flow decreases.

When selecting an anesthesia ventilator for use with children, the capabilities of the ventilator should be evaluated relative to the clinical needs. Compensation for circuit compliance to insure accurate tidal volume delivery and independence of tidal volume from fresh gas flow changes are desirable features when using volume controlled ventilation. With regard to PCV, maximum flow and pressure specifications are sometimes used as an index of performance capabilities. One must be careful however about using published specifications as indices of clinical performance. Maximum flow specifications are not reported under conditions of imposed load nor are they reported at the patient's airway. For bellows type ventilators, the maximum flow is typically reported for gas leaving the valve which controls the movement of the bellows. For piston type ventilators, the maximum flow specification is reported for the maximum piston movement. For the clinician, the key question is how a particular ventilator will deliver gas to a patient. The only way to answer this question is to measure pressure and flow at the patient's airway under defined conditions of resistance and compliance. (Figure)

Can one expect differences in performance between different ventilators when using PCV under the same clinical conditions? Certainly there are differences in design between ventilators. Some ventilators are designed with feedback control algorithms which control the flow delivered to the circuit based upon the pressure changes in the circuit. Careful testing will reveal small differences between the actual pressure in the circuit and the set pressure to be delivered throughout inspiration as the control algorithm adjusts flow to achieve the desired pressure. When using a well designed ventilator, these pressure differences should be small and not impact clinical performance significantly.

All anesthesia workstations currently marketed by North American Drager are equipped with ventilators which are capable of creating a square wave pressure waveform at the patient's airway under challenging ventilation conditions. By definition, the flow waveform which results will have a decelerating pattern. The Julian and Narkomed 6000 anesthesia workstations both provide user selectable PCV. The Narkomed workstations configured with an AV2+ ventilator with a pressure limit control (GS, 4 and later models 2C), are capable of being adjusted to provide PCV. (See Pressure Mode can be Preset on the AV2+ Ventilator. Draeger Clinical Practice Bulletin #2, October 1997) Irrespective of the ventilator used, when using PCV, the shape of the flow waveform will depend primarily upon the ventilator settings and the properties of the breathing circuit and the patient's lungs.

LEGEND: Airway pressure, flow and volume waveforms obtained using the Narkomed 6000 to ventilate a pediatric test lung. Settings were an inspiratory pressure of 25 cm H2O, rate of 10 breaths per minute and I:E ratio of 1:2. Waveforms are shown for three breaths at different lung compliances where the desired inspiratory pressure is unchanged. In all cases the airway resistance was set to 50. Lung compliance settings were 1, 4 and 8 mls/cmH2O for breaths from left to right respectively. In all cases, the ventilator delivers a decelerating flow pattern which is determined by the desired inspiratory pressure and the properties of the patient's lungs and the breathing circuit. ( See also Macintyre NR. Graphical Analysis of Flow, Pressure and Volume during Mechanical Ventilation. 3rd ed. Bear Medical Systems, Riverside, CA. 1991.)