Combination modes of ventilation: trying to please everyone.
Simply stated, VCV provides a guaranteed tidal volume (VT), but the pressure is variable. PCV, on the other hand, provides a set constant pressure with a variable VT.
In an attempt to please both sides of the aisle, ventilator manufacturers have developed combination modes that have the qualities of pressure ventilation with a volume guarantee. Yet these modes have not been found to be superior over volume or pressure-control ventilation, nor have they been proven to be better at weaning a patient off ventilatory support. It is many clinicians' opinion that these combination modes do have a place in our critically ill patients.
There are two types of control combination modes of ventilation available: those delivered by the "breath-by-breath" technique and those by the "within a breath" technique. Each brand of ventilator that offers a combination mode offers one type or the other. Ventilator brands that offer the same type of combination mode may have slightly different algorithms to achieve the same goal. It is essential that all RCPs understand the particular dynamics of the mode on the ventilator they use.
The first type of combination mode discussed is the combination pressure and volume ventilation mode delivered by the "breath-by-breath" technique. Examples of the modes available on current ventilators are: Pressure Regulated Volume Control (PRVC) on the Servo-I and 300 ventilators (Maquet), Volume Ventilation Plus on the Puritan Bennett 840 ventilator and Autoflow on the Drager ventilators. "Breath-by-breath" refers to the breath that continually attempts to achieve the set VT. For example, on the Maquet Servo-I ventilator, if 20 cm H2O is delivered to a patient and the result is 450 mL VT and the set or desired VT is 500 mL, the ventilator will automatically increase pressure by 1 to 3 cm H2O each breath until the 500 mL is reached. This is a continuous process on a "breath-by-breath" basis. The machine looks at the volume delivered and increases or decreases the pressure depending on the resultant volume from the previous breath. The breaths themselves are pressure type breaths. This means that each breath delivered by the ventilator has a square pressure waveform pattern with a variable decelerating flow. This gives the patient the benefit of a pressure breath with a guaranteed VT.
The mode of combined pressure and volume ventilation "within each breath" is the other technique. This initially gives a pressure breath. If the desired set VT isn't delivered by the time the flow has decelerated to a certain set point, the breath will convert into a volume breath and give the remainder of the volume desired. If the volume is delivered before the set flow, the breath acts as a pressure support breath. These breaths can be time or patient-triggered. Examples of this type of breath are Volume Assured Pressure Support (VAPS) and Pressure Augmentation.
There are a few situations where this type of ventilation should be used with caution. If the patient has a variable and or high respiratory drive, the ventilator may be confused and deliver too little pressure. Also the pressure waveform will not be square as it should be with any pressure breath. In this situation the patient is pulling against his respiratory impedance (resistance and compliance). Flow starvation will be experienced by the patient if the VT is not set high enough. Increasing the desired VT may help this situation by enabling the patient to receive more flow. If this is not an option in your patient, increasing sedation may be necessary.
Another issue with combination breaths is that the mean airway pressure may fluctuate due to the increasing and decreasing capability of the pressure for each breath. The pressure fluctuates due to the volatile airway resistance and lung compliance. Also, mean airway pressure is directly related to oxygenation. If oxygenation is a problem straight pressure control may be preferred.
There are also support modes of ventilation which deliver a pressure support breath in a "breath-by-breath" basis. Every breath must be patient-triggered as with stand alone pressure support ventilation. Examples of this are Variable Pressure Support (VPS) and Volume Support (VS). Volume support in the Servo-I ventilator works much like PRVC. The RCP sets a desired VT parameter. The ventilator continually looks at the previous volume achieved and the pressure automatically increases or decreases by 1 to 3 cm H2O in an attempt to reach this volume.
One avenue to provide optimal patient care in ventilator management is to attempt these combination modes. Just as it is important to understand each type of ventilation (volume and pressure), the RCP must understand their indications, hazards and contraindications. Both sides of the aisle may avoid gridlock and save face by utilizing these modes of ventilation.
In the next issue of Focus, we will continue our series covering types and modes of ventilation by reviewing the stand alone mode of pressure support ventilation. It has many uses and must be completely and accurately understood in order to assure optimal patient assistance and synchrony.
by Dana Oakes RRT-NPS & Sean Shortall RRT-NPS
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|Author:||Oakes, Dana; Shortall, Sean|
|Publication:||FOCUS: Journal for Respiratory Care & Sleep Medicine|
|Date:||Mar 22, 2005|
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