Pressure vs Volume Controlled Ventilation
Figuring out a mechanical ventilator, and understanding the basic modes of ventilation can be a difficult task.
At the end of this brief summary, you should have a better understanding of what is meant by the some of the different terms used when ventilating a patient mechanically, and you should leave with a basic understanding of how pressure and volume controlled ventilation work.
For the purposes of this post, we will not be going into the different modes of ventilation, these will be covered in more depth in a different post, however, these definitions will set you up with a better understanding for all the ventilation chapters to follow.
First some definitions
Volume control
In this mode of ventilation, the parameter that is controlled is the volume administered to the patient (tidal volume)
Safe tidal volumes of around 4-8ml/kg are recommended for most patients, with somewhere near the 6ml/kg mark being a safe starting point
Pressure control
In this mode of ventilation, the controlled parameter is the pressure
The peak airway pressure is constant, and is used to achieve a tidal volume which may vary depending on many factors
Peak Inspiratory Pressure
This is the highest level of pressure applied to the lungs during inspiration
This is a factor of airway resistance, PEEP and compliance of the lungs
Pressure that is generally applied to the larger airways (those that are not involved in gaseous exchange) and the airways not capable of distending (conducting airways)
Ideally this pressure should not exceed 35cmH20 in lungs that are not obstructed. The ventilation of the patient with obstructive lung disease is discussed in a different post)
Mean Airway Pressure
This is the average pressure applied to the lungs throughout the respiratory cycle (on inspiration and expiration) – see the graph below
The mean airway pressure can be increased by allowing more time to be spent under higher pressures; increasing PEEP; increasing inspiratory time and increasing inspiratory pressure
PEEP – Positive END-Expiratory pressure
This is the baseline pressure left over in the lungs once expiration is complete. It can be set by the ventilator, or may be a reading of the intrinsic pressure that is left over due to air trapping in the asthma/COPD patient for instance
This is the pressure that allows the alveoli to stay partially open throughout the respiratory cycle (too much PEEP = too little tidal volume, too little PEEP – increased effort to open the alveoli and oxygenate.
Plateau Pressure
This is the pressure applied to the alveoli and is measured at the end of inspiration (or during an inspiratory hold). It is the pressure that is exerted on the actual tissues which can distend in the lungs.
Ideally this pressure should not exceed 30cmH20
Driving Pressure
This is a measurement of the difference between the plateau pressure and the PEEP in the system. The higher the driving pressure, the more the lung is placed under stress. A driving pressure of around 14cmH20 – 18cmH20 is recommended when following the “open lung” or lung protective ventilation strategy. The lower, the better (provided that the appropriate tidal volume is produced for the patient).
I:E Ratio
This is the time in the respiratory cycle that is allocated to the inspiration of air, and expiration of air, normally a person breathes at a ratio of 1:2 (inspiration is usually 1/2 as long as expiration). This is usually as a result of the active nature of inspiration (it is driven by actively moving muscle), whilst expiration is about relaxing muscle and so usually takes a bit longer.
In advanced ventilation, the inspiratory time can be made equal to or more than the expiratory time to maximize mean airway pressure and increase oxygenation, this is called inverse ratio ventilation and is NOT recommended in normal ventilation in the EM setting.
Tidal Volume
This is the volume of air required to distend the lungs in one single breath (normal breath without any active work to get more air in) and is normally around 7ml/kg when a person is at rest.
Pressure Support
This is a preset pressure value that is delivered when the patient triggers a breath. It is a pressure mode of ventilation, that assists an existing drive to breathe. It makes the work of breathing easier by decreasing the effort the patient has to put in to achieve an appropriate tidal volume.
Respiratory Cycle
The respiratory cycle includes two phases: inspiration of environmental air and the expiration of gases from inside the lung including carbon dioxide.
Trigger
This refers to the method used by the ventilator to decide when to give the next breath.
Time trigger:
uses time alone to determine when the next breath should be given. It does not really take the patient into account at all, and uses the respiratory rate to determine the time between breaths (example: rate set at 12b/min, a breath will be given every 5 seconds regardless of the patient’s desire to breath at a different time)
Pressure trigger:
this trigger is patient driven and uses the fact that spontaneous breaths create negative pressure. When a certain amount of negative pressure is generated in the ventilator circuit by the patient, then the ventilator will deliver the breath. Because the patient needs to generate a change in pressure, it requires that the patient work relatively hard to get a breath.
Flow trigger:
When the patient makes an inspiratory effort, some of the gas that was previously flowing continuously through the circuit is diverted to the patient. The ventilator senses the decrease in flow returning through the circuit, and a breath is triggered, this requires much less work than the pressure trigger but also requires a flow sensor to be calibrated and in place on the ventilator with constant monitoring
Pressure Modes
In a pressure controlled mode of ventilation, the inspiratory pressure is the control variable (this is the one you can set), and is maintained during the inspiratory phase. The volume will be variable depending on a number of different things and can vary on a breath to breath basis.
Why is pressure controlled ventilation good?
Pressure controlled ventilation means that:
- There is increased mean airway pressure which helps to assist in the improvement of oxygenation. The more time the airway (and specifically the alveoli) are exposed to pressure, the longer the time for oxygen movement across the membrane.
BUT because it is not “normal” for the time of inspiration to be longer than the expiratory time, this benefit is mostly achieved through the use of PEEP (we tend not to ventilate a patient with I:E ratios that favor prolonged inspiratory time unless we run out of options)
- Increased alveolar recruitment occurs with pressure ventilation as the pressure is applied to the alveoli from the start, meaning they open up earlier and remain open longer, exposing them to oxygen for a longer period.
The pressure is exerted evenly across the alveoli and not directed down the path of least resistance (as sometimes happens with volume-controlled ventilation), so de-recruited alveoli are more likely to distend with this mode of ventilation.
- Pressure is limited to the set level, and the risk of barotrauma is limited, however changes in compliance can result in changes in tidal volume, so volume trauma, or hypo-ventilation are still possible.
- Patient comfort may be improved because the flow curve simulates “normal” negative pressure ventilation the closest, with a decelerating waveform, offering the highest flow at the start of the inspiration and tapering off towards the end of inspiration
- Leaks in the circuit can be accounted for as the pressure that is required will be achieved by adding more pressure (this will accommodate for a leak)
Why is pressure controlled ventilation not so good?
- Tidal volume is completely variable depending on a lot of different things. If the patient has any desynchrony then the ventilator will not be able to deliver the breath (the patient will breath against the ventilator and the pressure limit will be reached without any tidal volume being delivered).
If the patient is ventilated for a longer time, there needs to be a close watch on the tidal volume and minute volume achieved to ensure adequate control of the carbon dioxide levels. As compliance decreases, so the pressure increases and the tidal volume decreases.
- Because of the fact that tidal volume is totally variable, there are risks for the sudden increase in tidal volumes if compliance improves, this may risk damage to the elastic recoil of the lungs
- Flow rates set too high may cause the pressure limitation to be reached (the high pressure alarm) especially if there is an increase in airway resistance, this would mean a breath would not be delivered and the patient would not receive any tidal volume on that breath.
Examples of pressure controlled modes:
- CPAP
- BiPAP
- Pressure assist control
- Pressure Support
- Pressure controlled SIMV
- APRV
These and more will be discussed in detail in a different post.
Volume Modes
In volume-controlled modes of ventilation, the tidal volume is the control variable (this is the one you can set), depending on a whole lot of different parameters, the pressure will be variable from breath to breath.
On all ventilators there are three specific parameters/graphs that indicate how a breath has been delivered, based on the pressure, flow and volumes.
The graphs below are representative of a volume-controlled mode of ventilation for the following reasons:
- The volume graph is exactly the same for each breath, as the set tidal volume is achieved (the only time it will look different is if the pressure alarm limit is reached before the volume is fully delivered)
- The flow waveform is square as the flow is delivered in the same way with each breath, exactly the same flow throughout inspiration
- The pressure waveform is variable, as different breaths will result in different pressures depending on the patient at the time of the breath
- The pressure waveform slopes up early in the breath and then plateaus at the end of the breath
- Plateau pressure can be measured here if an inspiratory hold is completed (pushing the hold button at the end of inspiration)
In theory, or at least long ago when ventilators only had the ability to control one or the other parameter, this was dangerous as high levels of pressure could be achieved.
Why is volume controlled ventilation good?
- Tidal volume is set and won’t change and is helpful in cases where the carbon dioxide levels need to be carefully maintained (patient with increased ICP (intra-cranial pressure) for instance)
- Minute volume also does not change unless respiratory rate changes even if there are large changes in lung compliance, the ventilation and tidal volumes and well as minute volumes remain fairly static
- Flow rate is lower at the start of inspiration and can reduce the risk of reaching a pressure limitation (meaning early termination of the breath is less likely and the patient is more likely to receive their full tidal volume as required than if pressure control was used (especially in cases with higher airway pressures)
Why is volume controlled ventilation good?
- Mean Airway pressure is not as high as with pressure-controlled ventilation due to the variable pressure waves and time under the curve. For the hypoxic patient this might be a concern as the increased mean pressure results in increased oxygenation
- Volume-controlled ventilation tends to ventilate the “easier” to ventilate alveoli, those that are already open and able to distend easily. This might result in a ventilation perfusion mismatch as the “good” alveoli are over ventilated whilst the others are under ventilated. Recruitment of alveoli is better achieved with pressure than volume.
- If there is a leak, the volume-controlled mode is not as good at accommodating for it as the pressure-controlled mode. The volume will leak into the open space and be counted as delivered even though it didn’t go to the patient.
- If there is not enough flow at the start of inspiration, the patient may become agitated and air-hungry. Flow is not always a setting than can be adjusted on all ventilators and so might be difficult to get the patient settled if they are requiring more flow at the start of inspiration.
Examples of pressure controlled modes:
- Volume Mandatory controlled ventilation
- Volume assist control
- Volume controlled SIMV
These and more will be discussed in detail in a different post.
References
- Deden, K. (2015) ‘Ventilation Modes in Intensive Care’, Dräger, p. 72.
- Modes of Mechanical Ventilation (no date).
- Simplifying Mechanical Ventilation – Part I: Types of Breaths – REBEL EM – Emergency Medicine Blog (no date).
- Yartsev, A. 2015. Practical differences between pressure and volume controlled ventilation. Deranged Physiology.