The trigger phase variable is said to determine whether a mode of ventilation can be described as "mandatory" or "spontaneous", but there is more to the “spontaneousness” of a mode. Generally, the patient is also given some control over how the breath is terminated (i.e the cycling variable is anything other than time-cycled). There may be even more input from the patient (for example, NAVA proportions the level of ventilator support according to diaphragmatic contraction). In contrast, mandatory modes of ventilation take this control away, in return for a totally effortless respiratory experience where you just lie back and let the blower do the work.
To compare mandatory and spontaneous modes of ventilation on any level would therefore seem quite stupid, given that they are completely different in every way. However, it is important to know advantages and disadvantages of either, because this has implications for one’s decisions, particularly in scenarios where either mode seems appropriate
From an exam point of view, this topic is rather irrelevant for the purposes of the CICM primary and does not appear in their 2017 syllabus. It does however have some importance for CICM WCA competency “Ventilation” which expects the trainees to be at least “adequate” at “outlines the differences between mandatory and spontaneous modes of ventilation”. This chapter addresses this apparent need.
In the event that this topic is of any interest to anybody, the action of reading about it should be rewarded with the recommendation of some peer-reviewed resources. No article anywhere does anything directly comparable because presumably all authors in the field have come to the conclusion that these matters are so self-explanatory that nobody would ever need to have this material presented to them. If one needed to read anything “official” about this topic, one would probably benefit most from the 2016 article by Kacmarek and Branson, titled “Should Intermittent Mandatory Ventilation Be Abolished?” The authors debate the merits of IMV, which is a largely obsolete mode of ventilation; but their comparison of IMV with spontaneous ventilation is informative.
The advantages of a mandatory mode of ventilation
- There is a guaranteed respiratory rate:
- This favours patients whose respiratory drive is either non-existent or unreliable. Those who are paralysed with the use of neuromuscular junction blockers or patients who are heavily sedated obviously will not be breathing by themselves and spontaneous modes are not an option for them.
- Patients who are expected to have intermittent sedation, for example in the setting of procedural sedation, probably also fall into this category. A mandatory mode allows one to busy oneself with the actual procedure without having to worry about watching the ventilator.
- CO2 removal is controlled better
- The guaranteed respiratory rate, when combined with a guaranteed tidal volume, offers a control over minute volume, which is the major determinant of CO2 removal. A time-triggered mode is the preferred mode of ventilation in scenarios where tight PaCO2 control is important; a classic example of this is the management of the neurosurgical patient with raised ICP.
- Timing and volume of breaths can be contrary to the patients’ respiratory drive
- This has importance in scenarios where the patient would benefit from a pattern of respiration which is completely the opposite of what their medulla is telling them. Good examples are ARDS and severe asthma. In one, the volumes need to stay ow (much lower than what the patient might like); in the other the I:E ratio needs to be extremely long (while the patient would much rather be tachypnoeic). In a spontaneous mode, one has no control over the rate, and usually little control over the volume.
- The patient expends no effort in triggering a breath:
- The effort of breath triggering may be substantial in patients with severe lung pathology. The metabolic requirements of the respiratory muscles may constitute a substantial workload for the myocardium in conditions where lung compliance is significantly reduced (for instance, in the context of pulmonary oedema). The use of a mandatory mode of ventilation can greatly improve the matching between demand and supply in states of decreased cardiac output, such as cardiac failure or cardiogenic shock (Aubier, 1985).
Disadvantages of mandatory ventilation:
- It may be less comfortable for the patient.
- The timing of mandatory breaths may be either excessive or insufficient in matching the patient's respiratory drive. In such a situation, if the patirnt is even slightly awake, they may become dyssynchronous with the ventilator. The increased effort expended on "fighting" the ventilator is counterproductive when the main objective of mechanical ventilation is to reduce respiratory effort.
- It may lead to higher sedation requirements.
- Mandatory ventilation is most effective in the absence of patient respiratory effort, and in a number of ways increased patient respiratory effort tends to interfere with mandatory ventilation. The need to suppress patient effort leads to increased sedation requirements, with all the attendant complications (haemodynamic instability, prolongation of recovery, increased susceptibility to delirium, sedative agent toxicity etc).
- It may lead to deconditioning.
- The persistent use of time-triggered ventilation can result in the deconditioning of respiratory drive and respiratory muscles. Prolonged reliance on mandatory ventilation can alter diaphragm function (Gayan-Ramirez et al, 2002) and result in a progressive decrease of respiratory effort tolerance, contributing to failed ventilator weaning.
Advantages of spontaneous ventilation
- It is more comfortable and thus it permits sedation to be reduced; which is a step in ventilator weaning. A spontaneous mode is viewed as one of the necessary first steps towards extubation.
- It allows the patient to have control over their minute volume, which prevents dyssynchrony due to unsatisfied respiratory drive. This in return could actually decrease the work of breathing, as much effort may be wasted in fighting the ventilator.
- It is more comfortable because of a decreased work of breathing in triggering, increased control over ventilation and also because the triggering occurs with less delay, such that the initiation of a breath by the patient is rapidly translated into a mechanical breath.
- There is better distribution of ventilation. Because the diaphragm is contracting spontaneously instead of being bullied passively by the ventilator, the bases of the lungs will inflate more completely. Evidence for this comes from the distant past. Froese & Vryan (1974) anaesthetised their colleagues and measured the movement sof their diaphragms. “The passive diaphragm was displaced preferentially in nondependent zones where abdominal pressure is least”, they found; whereas dependent zones developed collapse. Neither could PEEP re-inflate those collapsed regions, nor could larger tidal volumes restore the FRC of the spontaneously breathing patient.
- There is better haemodynamic stability. Because they spend some of their inspiratory time actively inhaling, these patients have reduced intrathoracic pressure and therefore increased venous return. The effect is that some of the adverse haemodynamic effects of mechanical ventilation are ameliorated. Downs et al (1977) found that mechanically ventilated patients had much better cardiac output when allowed some spontaneous breaths (by about 1L/min).
Disadvantages of spontaneous ventilation
- It may be too sensitive, giving rise to auto-triggering (where non-respiratory influences on circuit flow trigger mechanical breaths; for example, the cardiac pulsation, fluid in the circuit, secretions in the airway, etc). In these cases, a pressure trigger may be of greater value, but ultimately the only way to overcome this possibility is to ventilate with a mandatory mode, taking triggering out of the equation altogether
- It does not guarantee a minute volume, which means it is unsuitable for patients with a diminished or unreliable respiratory drive. If you want to control the respiratory rate, this is the wrong choice. Tidal volume is occasionally under your control with some of the modes (eg. the volume support mode on the SERVO-i) and you have some indirect control over it by manipulation of the cycling variable, but it is unreliable and ultimately making changes to these variables decreases some of the advantages of a spontaneous mode, i.e. where the patient gets to choose when they exhale.
The work of breathing due to triggering may be non-trivial. Apart from triggering, the work of breathing may also be increased because of other ventilator settings. Using older ventilators, Marini et al (1985) demonstrated that with maximally sensitive settings the patients were still responsible for up to 30-50% of the inspiratory workload, and that under what they described as “unfavourable” conditions, “the subjects inspiratory work of breathing substantially exceeded the energy needed by the ventilator to inflate the passive thorax.” Given that the objective of ventilation is usually to decrease the work of breathing, this sort of thing must be viewed as counterproductive.
Syncronised intermittent mandatory ventilation (SIMV)
In an effort to reduce the uncomfortable mandatory-ness of mandatory ventilation while retaining the comfortable security of time-triggering, manufacturers included a means of synchronising mandatory breaths with the patient's effort. Apparently the SERVO 900B was among the first machines to offer such a thing in the 1980s. By 1990, it was well known - enough to be included in this article on the history of mechanical ventilation by Young & Sykes (1990).
In essence, this is still a form of time-triggering. You set a rate and a volume on the ventilator. As such, there are timed breaths. However, there is a window of opportunity around each timed breath. During these windows, a patient effort will be converted to one of those mandatory breaths.
The theory behind this is fairly straightforward. The normal mandatory modes are guilty of neglecting patient breathing efforts, and spontaneous modes are insufficiently supportive of poor or inconsistent efforts. SIMV was supposed to bridge between the two. Back in the day, weaning on a mandatory mode was done by gradually decreasing the ventilator rate and allowing the patient to take over. SIMV allowed synchronised breaths up to a certain rate