Sunday, August 26, 2012

Volume-Control to Pressure Control

Image 1: The Settings of VC-CMV. The settings highlighted in yellow are associated with patient-ventilator asynchrony. 


VC-CMV is likely the most utilized mode of mechanical ventilation. This is due to a few a reasons:

     1. VC-CMV is a standard mode on almost every intensive care ventilator (table 1).

     2. VC-CMV is one of the first modes of mechanical ventilation.

     3. VC-CMV is easy to understand in both theory and operation.

     4. VC-CMV is the standard of care when ventilating patients with Acute Respiratory Distress
          Syndrome (ARDS) and Acute Lung Injury (ALI).




Table 1: MANUFACTURER NAMES for VC-CMV
Manufacture
Ventilator
Name
Cardinal
AVEA
Volume A/C

Bear 1000
Assist CMV

VELA
Volume A/C
Draeger
Evita 4
CMV

Evita XL
CMV
Hamilton
Galileo
CMV

Raphael
n/a

G5
CMV
Maquet
Servo-i
VC

Servo-s
VC
Puritan Bennett
PB 840
Assist/Control Volume
Philips/Respironics
Esprit
VCV-A/C



VC-CMV has one main advantage, that it can deliver a preset tidal volume (Vt) and minute ventilation (VE).

 The advantages of this include:

     1. Providing a precise Vt that is lung protective, to meet ARDSnet guidelines (e.g. 4-6 ml/kg/IDBW).

     2.  Providing a Vt that will prevent or decrease the incidence of atelectasis. Conversely, this can also be accomplished with Positive End-Expiratory Pressure (PEEP).

     3. Providing a minimum VE that allows for optimal alveolar ventilation.

     4.  Since VE and PaCO2 are mathematical inverses one can predict changes in PaCO2 and pH with changes in minute ventilation.


Problem

VC-CMV may be an easy to use mode that has advantages during the acute phase of mechanical ventilation; however I believe it has no utility for the spontaneous breathing patient. 

VC-CMV is associated with eight out of the ten patient ventilator asynchronies (Table 2) and shifts additional work to the patient during periods of high inspiratory demand.


Table 2 : PATIENT-VENTILATOR ASYCHRONIES ASSOCIATED with VC-CMV
Breath Phase
Asynchrony
Yes (Y) or n/a
Trigger Phase
Auto-Triggering
Y

Trigger Delay
Y

Ineffective Efforts
Y

Double Triggering
Y
Inspiratory Phase
Flow Mismatch
Y

Low Driving Pressure
n/a

Pressurization Rate
n/a
Cycling Phase
Premature Cycling
Y

Delayed Cycling
Y
Expiratory Phase
Intrinsic PEEP (PEEPi)
Y


Just switching to a pressure control mode can decrease dyspnea in the spontaneous breathing patient [1]. 

So how does one switch to Pressure-Control Continuous Mandatory Ventilation?

First, Evaluate the current settings on VC-CMV, I'm sure you want to keep the same respiratory rate, FiO2, & PEEP. 

Second, if you want to generate the same tidal volume, evaluate the measured plateau pressure. You are going to use the plateau pressure measurement as a starting point for your pressure control setting (see video below).



Third, switch to pressure control and enter all the parameters similar with your previous settings on VC-CMV. Set the pressure control level the same as the previously measured plateau (e.g. plateau pressure 20, then set PC to 20). 
Note- some ventilators when changing modes will match the previous settings for you. 

Fourth, evaluate measured tidal volumes and titrate pressure control setting for appropriate target tidal volume. 

Fifth, assess the flow waveform to see if inspiratory time or I:E ratio is set appropriately (see link).


Sixth, evaluate the pressure waveform to determine if pressurization rate is set appropriately (see links)



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