Combination modes of ventilation: trying to please everyone.
As discussed in our last article in the Winter 2005 issue of Focus, there are clinicians who prefer volume-control ventilation (VCV VCV Vlaams Centrum Voor Volkscultuur
VCV Vowel-Consonant-Vowel (speech synthesis)
VCV Variance Covariance
VCV Volume Control Ventilation
VCV Verse Chorus Verse (Nirvana song) ) over pressure-control ventilation (PCV PCV packed-cell volume.
packed-cell volume, the volume of packed red cells in milliliters per 100 ml of blood. ) and visa versa. It is comparable to the political scene in the United States. The conventional volume ventilation proponents are much like the conservative republicans and the pressure control advocates are not unlike the liberal democrats. Each side is attempting to do good for the nation or in this case patient, but they are convinced their way is the right (or left) way to accomplish the common goal. It is the authors' opinion that both types of ventilation should be mastered and be an option for each particular clinical situation.
Simply stated, VCV provides a guaranteed tidal volume tidal volume
The volume of air inspired or expired in a single breath during regular breathing. Also called tidal air.
n (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 weaning,
n the period of transition from breast feeding to eating solid foods.
the act of separating the young from the dam that it has been sucking, or receiving a milk diet provided by the dam or from artificial sources. 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 Pressure Regulated Volume Control (PRVC) is a ventilator mode where the breaths are delivered mandatorily to assure preset volumes, with a constant inspiratory pressure continuously adapting to the patient's condition. The flow pattern is decelerating. (PRVC PRVC Pressure Regulated Volume Control
PRVC Princess Royals Volunteer Corps (UK)
PRVC Pacific Regional Visitor Center (Waikiki, Hawaii)
PRVC Pennsylvania Reserve Volunteer Corps ) 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 VAPS Video Arcade Preservation Society
VAPS Variable Assist Power Steering
VAPS Virtual Avionics Prototyping System
VAPS Virtual Applications Prototyping System
VAPS Virginia Power Shift (student conference on global climate change) ) 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 Sedation Definition
Sedation is the act of calming by administration of a sedative. A sedative is a medication that commonly induces the nervous system to calm.
The process of sedation has two primary intentions. 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 airway resistance Lung physiology A measure of the resistance–in cm H2O to the flow–in L/min of air in upper airways, the result of natural recoil–resiliency of anatomic structures–oro- and nasopharynx, larynx, and nonrespiratory and lung compliance lung compliance See Compliance. . Also, mean airway pressure is directly related to oxygenation oxygenation /ox·y·gen·a·tion/ (ok?si-je-na´shun)
1. the act or process of adding oxygen.
2. the result of having oxygen added. . 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 (1) (Vectors Per Second) The measurement of the speed of a vector or array processor. See vector, vector processor and array processor.
(2) (Virtual Private Server) See OS virtualization. ) and Volume Support (VS). Volume support in the Servo-I ventilator works much like PRVC. The RCP (networking, tool) rcp - (Remote copy) The Unix utility for copying files over Ethernet. Rcp is similar to FTP but uses the hosts.equiv user authentication method.
Unix manual page: rcp(1). 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 Gridlock
A government, business or institution's inability to function at a normal level due either to complex or conflicting procedures within the administrative framework or to impending change in the business. 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 synchrony /syn·chro·ny/ (-krah-ne) the occurrence of two events simultaneously or with a fixed time interval between them.
atrioventricular (AV) synchrony .
by Dana Oakes RRT-NPS & Sean Shortall RRT-NPS