Just as the ventilator needs to know when to start a breath, so it needs to know when to end it, and this is defined by the cycling variable. This setting determines which parameter is used to open the expiratory valve. Strictly speaking, multiple cycling variables may be active at any given time, but usually some of them (for example, pressure) take the form of alarm limits. The cycling variable is an important determinant of how comfortable your patient is going to be with their mechanical breaths, as it represents another point at which they can exercise control over their tidal volume.
- Cycling refers to the variable a ventilator uses to end inspiration.
- The ventilator measures this variable during the inspiratory phase.
- When the set parameter for this variable is achieved, the ventilator opens the expiratory valve, and expiration may begin.
- Typical methods of ventilator breath cycling include:
- Time-cycled ventilation
- Flow-cycled ventilation
- Pressure-cycled ventilation
- Volume-cycled ventilation
- Time-cycled ventilation is mainly used for sedated or paralysed patients, and is typical of mandatory modes
- Flow-cycled ventilation is mainly used for spontaneously breathing patients and is typical of spontaneous modes.
- Pressure cycling and volume cycling are largely historical, and were used in early ventilators.
In terms of peer-reviewed reading materials for this topic, probably the best reference is the 2011 article by Michael Gentile. That single reference would probably be enough to prepare the CICM primary or fellowship exam candidate for whatever the college throws at them. The next step up in sophistication would be to explore Dean Hess’ article from 2005, which explores the bottomless depths of troubleshooting a spontaneous mode of ventilation (for which flow cycling is a representative phase variable)
A breath is considered time cycled if the inspiratory phase ends when a predetermined time has elapsed. This is usually a feature of "mandatory" modes of ventilation. For instance, when one has set a respiratory rate of 20 and an I:E ratio of 1:2, it is understood that the inspiratory phase will last 1 second in a total 3-second breath cycle. Ergo, after 1 second the ventilator will terminate inspiratory flow and cycle to expiration.
The advantages of this method of breath cycling are:
- A careful control of minute volume can be achieved, with obvious advantages for scenarios where tight PaCO2 control is desirable (eg. traumatic brain injury)
- Ventilation is unaffected by changes in lung compliance or airway resistance because the timing of the breath is unrelated to any of the respiratory system parameters, instead being controlled by a timer
- Minute ventilation is not affected by an unreliable respiratory drive, making this method suitable for paralysed or deeply unconscious patients.
It also has disadvantages, which – if one were to list them, for example because somebody asked this in a written exam paper – would be basically the same as the disadvantages of a mandatory mode of ventilation.
- It is unsuitable for lightly sedated and awake patients, because such patients may become uncomfortable with a fixed and demand-independent inspiratory phase – perhaps some breaths they would like to go on for longer, whereas others they would prefer to terminate shorter.
- It may result in patient-ventilator dyssynchrony particularly if the patient tries to exhale before the cycle timer runs out, as discussed above. This could give rise to a phenomenon where pressure rises at the end of inspiration, either because the lung compliance has reached a point of overdistension, or (as in the example below) the spontaneously-breathing patient has decided that they want to exhale before the breath is over.
With flow-cycled ventilation, the ventilator cycles into the expiratory phase once the flow has decreased to a predetermined value during inspiration.
The flow cycling variable can be a fixed flow value in L/min or a percentage fraction of the peak flow rate achieved during inspiration. In some models it is probably something that can be pre-set at an absolute value, eg 5L/min. The Maquet SERVO-i expresses the flow cycling variable as a percentage of peak inspiratory flow and allows a range of settings (between 1% and 70%.), whereas the Puritan Bennett 840 only goes up to 45%.
A flow cycle setting of 1% would result in a breath which cycles to expiration only after the flow has essentially stopped, i.e. at a point of significant lung distension. Conversely, a flow cycle setting of 70% would terminate a breath very early in the inspiratory phase, and achieve an insufficiently low tidal volume. The default setting on the SERVO-i ventilator is 15% which corresponds to an inspiratory flow rate of 4.5L/min for a calmly breathing adult.
This is not an advantage or a disadvantage of flow cycling; merely on observation. This phase variable is an instrument, and like any instrument it has the potential for being used in some profoundly stupid fashion. For instance, in the mindless pursuit of somebody’s idea of perfect tidal volumes one might be tempted to drop this setting to zero for patients with poorly compliant lungs, thereby producing good volumes together with overdistension discomfort and increased work of breathing.
Flow-cycled ventilation has many advantages:
- It is more comfortable for the patient by preventing frustrated expiratory efforts; if the patient needs to terminate a breath and exhale the inspiratory flow ceases and the ventilator cycles to expiration rapidly. With conventional settings, the inspiratory time is rarely uncomfortably prolonged. This is the main advantage of this method.
- It is limited by changes in lung compliance and airway resistance, which could theoretically prevent inadvertent ventilator-induced lung injury (i.e. with poorly compliant lungs, the ventilator will cycle to expiration rather than continue to apply distending pressure).
There are also some disadvantages:
- Tidal volumes may be poor in patients with poor lung compliance, resulting in inadequate minute volume
- Patient comfort depends on intelligent settings; inappropriately low and inappropriately high settings could result in uncomfortably deep and prolonged inspiration or "double triggering" due to insufficient inspiratory time and tidal volume.
During pressure-cycled ventilation, inspiration ends when a certain user-prescribed pressure value is achieved. This is old-school – to the extent that no modern ventilator pressure-cycles, and most articles about this are from the 1960s. Pressure cycling is a feature of historical ventilator models (eg. the Bird Mark 8) and has become largely obsolete in the modern era of microprocessor-controlled solenoid valves.
In honour of this rich history, instead of the conventional vector diagram, here is a drawing of a pressure-cycled ventilator waveform generated by the Bird Mark 8, from a 1965 paper by H. Herzog
Advantages of this method are few; they include:
- Minimal mechanical requirements. One requires nothing more than a spring-loaded diaphragm valve to act as the pressure cycling mechanism – when the prescribed pressure is reached, the valve opens and triggers expiration.
- Safety from pressure-related lung injury. Because you’ve set a pressure as the parameter which open the expiratory valve, the circuit can never over-pressurise to damage the patient’s lung (eg. if the patient decides to cough mid-inspiration, instead of blowing a pneumothorax the ventilator will just let them exhale).
- Decelerating ramp pattern for the flow waveform, which supposedly results in a more even distribution of gas in lung units with different time constants (more on this in the chapter on the interpretation of the flow waveform, and on the pressure and volume controlled modes of ventilation). One would probably temper this with the statement that lots of other mechanisms of cycling also co-exist a decelerating flow waveform pattern.
- Compliance determines cycling, which can be viewed as a patient-centred spontaneous feature. If the patient’s respiratory compliance has changed, whether through pathological processes or because they are trying to exhale, the pressure in the circuit at the end of inspiration will increase, which will open the expiratory valve. In short, expiratory effort triggers expiration, which is very spontaneous-sounding.
Which is a nice segue to the disadvantages:
- Volume is determined by compliance: if the patient has poor lung compliance, the volume will be low. This is common to all mechanical ventilation models where the pressure is a fixed variable in some way.
- Respiratory rate may fluctuate. Because airway pressure is determined by airway resistance and lung compliance, this method of cycling can give rise to variable tidal volumes and respiratory rates, particularly in patients who have wildly fluctuating respiratory physiology (eg. asthmatics).
- It may increase the respiratory effort. The task of exhaling forcefully against the pressure valve in order to trigger expiration is not likely to be well tolerated, and the whole process may give rise to an increased work of breathing.
- Pressure cannot be a control variable. One is basically committed to a ramp-like pressure waveform, which will decrease the mean airway pressure. It would not be possible to have a pressure-cycled mode with a square pressure waveform by using pressure as a control variable or a limit (target) variable, because it would never reach the cycling threshold.
The inspiratory phase of a volume-cycled breath ends when the specified volume has been delivered. The volume will therefore remain constant, even though the characteristics of the patient's respiratory system may change. As a result, the pressure in the system and the flow will vary depending on lung compliance and airway resistance. As with pressure cycling, this method of terminating a breath is largely historical. Older models of piston ventilators were volume cycled, i.e. the ventilator would cycle to expiration when the piston delivered a pre-determined volume. Most modern ventilators do not make use of this method.
Its major disadvantages are related to safety. The risk of pneumothorax was probably the biggest thing. This cycling mode has the propensity to generate high peak airway pressures when the lung compliance deteriorates. The problem was at its worst in the bad old days. Steier et al (1974) reported on a scenario where the ICU started using volume-cycled ventilators and developed a shocking 43% increase in the rate of pneumothorax, where specifically the patients ventilated with on volume-cycled modes were 28 times more likely to develop complications.