Changing Flow Patterns vs. Changing Ventilator Modes

Figure 1: Various flow patterns within VC-CMVs on the Hamilton G5 Ventilator.

When a device operator thinks about changing the inspiratory flow pattern while administering a Volume-Controlled breathing pattern they do not assume it will change the mode of ventilation. 

However newer software in the Servo-i allows the operator to change the flow pattern 

from the traditional constant flow pattern, to either a fully decelerating  flow pattern (similar to PC-CMV) or to adaptive flow (Fig. 2).  The Adaptive Flow pattern was the default in older software which makes Volume Control a Dual Control mode [1].  The operator now has a choice of using Volume Control as a traditional VC-CMV mode by selecting the square waveform or providing a Dual Control breathing pattern by selecting the Adaptive Flow icon. 

Figure 2: Flow patterns available on the Servo-i, courtesy of Robert Chatburn. 

For more information on flow patterns and Dual Control see the below links.

DUAL CONTROL

Dual Targeting Schemes

FLOW PATTERNS

Flow Patterns & Flow Rates

RELATED POSTS

1. The Volume Control Ventilation Fallacy

    Targeting Schemes

HYPERCAPNEA & ATELECTRAUMA: MIMICKING APRV

In a previous post "APRV in the operating room is it practical?"  I argue that bringing a ICU ventilator into the operating room to utilize APRV is not practical and may lead to hypoventilation and hypoxia due to administration of anesthetic agents [1].

During surgical procedures the patient is maintained in stage 3 of anesthesia known as the "surgical stage". Stage 3 is broken down into four distinct planes, "from onset of automatic respiration to respiratory paralysis" [2]. The patient is usually maintained in Plane 3 (intercostal muscle paralysis) or Plane 4 (diaphragmatic paralysis) leading to the cessation of spontaneous breaths.

One key advantage of APRV is that the patient may breathe spontaneously contributing to the overall minute volume, with the termination of spontaneous efforts the patient will become severely hypercapnic.

Below is an image (fig 1) I captured from a "Pressure Control Ventilation Simulator" [3] demonstrating an ARDS patient on APRV.

Figure 1. Pressure Control Ventilation Simulator, notice patients PaCO2 at 100.7 mmHg.

Notice in figure 1 the outcome for a patient not contributing to the overall minute ventilation the PaCO2 would be 100.7 mmHg. 

Another example of how APRV maybe harmful in the operating room is when trying to mimic APRV with a anesthesia delivery system.

The Volume Control Ventilation Fallacy

Volume Control- Continuous Mandatory Ventilation with a “Set-point” targeting scheme (VC-CMV(s)) is likely the most utilized mode of mechanical ventilation in adult patients in North America. This is due to a few a reasons:

1.      VC-CMV is a standard mode on almost every intensive care ventilator and anesthesia delivery system.

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).

5.      VC-CMV is the standard of care for adult patients intraoperatively.

The key advantage of VC-CMV(s) is the safety and simplicity of the set-point targeting scheme. The operator can manually set all parameters of the volume/flow waveform and adjust the minimum minute ventilation parameters (relating to frequency and tidal volume). “One can quickly trouble-shoot a patient’s situation, so during a change the operator can diagnose the problem and intervene rapidly”. [1]

When one sees a mode of ventilation labeled “Volume Control”, “VC”, “Volume A/C”, or “CMV” it affirms that the breathing pattern delivered to the patient will consist of a constant tidal volume and inspiratory flow waveform (fig. 1) 

Figure 1. Volume Control Ventilation Breath Pattern.

Figure 2. Volume and Flow waveform remains constant even-though compliance decreased to 25, compared to Figure 1.  

regardless of changes in a patient’s respiratory system mechanics and/or inspiratory drive (fig. 2) [2]. Conversely, due to no industry standard for ventilator mode taxonomy and medical device manufacturers marketing schemes the actual breath delivered to the patient does not resemble the predicted breath pattern and may result in a tidal volume much larger than the expected preset value.

How does this happen?

Ventilator Mode Map App

Ventilator Mode Map Mobile App

Due to no industry standard for ventilator mode taxonomy and over 170 unique names for modes of ventilation.  Learning and understanding ventilator mode classification can be problematic.   Ventilator Mode Map provides a easy to use handheld solution.   With this app one can choose from 12 different vendors,  37 different models (with pictures) and provides hundreds of definitions. One would have to review various text books and journal articles to compile this knowledge base.

This app is for android devices, I have a copy on my Samsung Galaxy tablet & Motorola phone, works fine on both devices. Available on Google Play see link below.

GOOGLE PLAY

Ventilator Mode Map

Free Virtual Mechanical Ventilator

OPENPediatrics website provides a free virtual mechanical ventilator that allows users to review mechanical ventilator knowledge, play scenarios, and test their skills with case simulations.

One just needs to register for free on their "Clinician Community Site" to utilize the simulator. 

LINKS

OPENPediatrics

OPENPediatrics simulator

OPENPediatrics Clinician Community  Site

RELATED POST

Creating Flow Mismatch with a Simulator