Wednesday, November 4, 2015

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?


The device operator selects a mode labeled “Volume Control” and the breath delivered actually resembles a Pressure Control or Pressure Support breath. This is due to the modes “targeting scheme”. A targeting scheme is the third building block for constructing a mode of mechanical ventilation the first is “Ventilator Control Variable” and second is the “Breath Sequence” [3]. The targeting scheme is usually a closed-looped feedback control system (similar to “If then statements” in conditional computer programming) which tries to achieve a specific ventilatory pattern. Thus making the targeting scheme the culprit for the change in breath pattern.

Volsko and colleagues [2] demonstrated this well when comparing differences in VC-CMV(s) and VC-CMV dual targeting (VC-CMV(d)). The researchers used an ASL 5000 lung model (Ingmar Medical) and simulated the pulmonary mechanics of an adult patient with ARDS during active and passive ventilation. During the simulated spontaneous breathing VC-CMV(s) delivered a consistent tidal volume similar to operator settings. However, VC-CMV(d) delivered a breathing pattern similar to pressure support resulting in higher flow rates and delivered tidal volume.

The deception is buried in the manufactures naming of the mode. In this study researchers used the “Volume Control” mode on the Servo-i ventilator for the dual targeting scheme arm of the study. The name “Volume Control” and the generic settings (fig. 3) associated with the mode on the Servo-i misleads the operator to believing that the mode is simply VC-CMV(s). The Dual Control targeting scheme happens behind scenes in which the device operator cannot control.  

Figure 3. Settings for Volume Control on the Servo-i ventilator. 

The mode mimics the standard set-point functionality associated with the traditional VC-CMV(s), until the patient makes an inspiratory effort that decreases the inspiratory pressure by 3 cmH2O. If this happens “the ventilator switches to pressure control and, if the effort last long enough, flow cycles the breath” similar to pressure support [4]. Thus delivering a higher inspiratory flow resulting in a larger tidal volume than predicted (fig. 4).

Figure 4. Screen shot demonstrating the dual control targeting scheme of Volume Control in the Servo-i, the inspiratory flow increases based on patient effort resulting in a higher tidal volume. 


As mentioned in previous posts Adaptive Pressure Control Ventilation (APC) may not provide consistent tidal volumes during rapid changes in pulmonary mechanics and vigorous inspiratory efforts [5,6]. In 2007 Jaecklin demonstrated that APC in a variety of ventilators regularly delivered excessive tidal volume in response to sudden increases in patient compliance or decreases in resistance potentially putting neonates at risk for lung injury [7].

 Why do I mention APC? Isn’t it obvious that APC is an entirely different mode then VC-CMV(s)?

APC actually classified as Pressure Control-Continuous Mandatory Ventilation with an “Adaptive” targeting scheme or PC-CMV(a) for short is much different than VC-CMV(s). The control variable, breath sequence, and targeting scheme are unrelated, furthermore the names for some of these modes “Pressure Regulated Volume Control (PRVC)” and “Adaptive Pressure Ventilation (APV)” make it difficult for the operator to believe that one of these modes is actually VC-CMV(s).

Unfortunately, the naming of a mode or modality adjunct can confuse the operator making them believe the mode is actually a volume controlled mode. I presented earlier how one may confuse “Volume Control” on the Servo-i with VC-CMV(s) based on the name and how the breath is delivered in a passive patient. Another example of this is in the PB840 ventilator with a mode called “Volume Control Plus (VC+)” this mode in fact is PC-CMV(a).

The mode adjunct “AutoFlow” on Draeger Series ventilators may deceive likewise.
Some consider AutoFlow an individual mode of ventilation, but I view it as an adjunct because it resides as an option among the additional settings in volume targeted modes.

Figure five shows the traditional settings for volume control ventilation on a Draeger V500 ventilator and figure six displays the additional settings rooted in the VC-CMV(s). 

Figure 5. VC-CMV settings on the Draeger V500.

Figure 6. AutoFlow highlighted under additional settings of VC-CMV. 

From this page the operator can turn on AutoFlow which switches VC-CMV(s) to PC-CMV(a). After selecting AutoFlow the displayed mode name does not change the word AutoFlow is added to it (e.g. VC-CMV, AutoFlow) (fig. 7).

Figure 7. Mode still labeled VC-CMV after AutoFlow activated. 

If the operator does not know that AutoFlow actually changes the mode to PC-CMV(a) then they may assume that they are still in a volume control mode and that a constant volume breath pattern will be delivered.

Volume Control-Continuous Mandatory Ventilation with set-point targeting is a safe and simplistic mode of ventilation that allows the operator to manually preset a minimum minute ventilation and quickly trouble shoot during adverse events. Unfortunately, due to no industry standards for ventilator mode taxonomy one can be misled using a mode labeled “Volume control”, “Volume Control Plus”, or selecting a mode adjunct expecting a volume control breath.



1. Williams, K. et. al. (2011). Control of Breathing During Mechanical Ventilation: Who is the Boss? Respiratory Care. 56 (2): 127-138.

2. Volsko, T. et. al. (2012). The Effect of Targeting Scheme on Tidal Volume Delivery During Volume Control Mechanical Ventilation. Respiratory Care. 57 (8): 1297-1304.

3. Chatburn, R. et. al. (2014). A Taxonomy for Mechanical Ventilation: 10 Fundamental Maxims. Respiratory Care. 59 (11)

4. Chatburn, R. (2012). Standardized Vocabulary for Mechanical Ventilation. Mandu Press Ltd.

5. APC: Variances in Delivered Tidal Volume.

6. Adaptive Pressure Control Ventilation: Vigorous Inspiratory Drive.

7.  Jaecklin, T. et. al. (2007). Pressure Control Volume Targeted Modes of Modern Neonatal Ventilators: How Stable is the Delivered Tidal Volume? Intensive Care Medicine. 33 (2): 326-335.