Spontaneous Modes Versus Control Modes

As we have described in Chapter 3, in a spontaneous mode, the patient will initiate a breath from the ventilator and then the ventilator will “support” the patient. It can help eliminate some of the work that the patient is doing to breath by the application of additional pressure to overcome how much the patient is working. Work of breathing can be assessed subjectively by looking at how fast they are breathing and if they are using accessory muscles to breathe.

A health care worker adjusts a tube for a ventilator.
A health care worker calibrates a ventilator on the USNS Mercy.

The key to this process is that the patient must have an intact neurological drive to breathe. They trigger their own breaths and have their diaphragm involved in pulling the breath into the lung and then exhaling. Patients also decide the frequency of each breath completely on their own. The ventilator will sense the patient-initiated efforts with a flow or pressure trigger (see Chapter 3) and deliver a set amount of air to augment the patient’s own breath. If the patient’s internal respiratory rate is not adequate to manage their CO2 and oxygen levels, they do not have a fully intact drive to breathe and may need a control mode or they will deteriorate.

Normal physiologic breaths are not all uniform. Most people take a variety of breaths every minute, including frequent sigh breaths, or have variability in their respiratory rate. Control modes do not allow for any of these variations, and the size of the breath or frequency only changes when the clinician changes the settings. This inflexibility can quickly lead to asynchrony discomfort for the patient. In spontaneous modes, however, the diaphragm is active in triggering, expanding the lungs and relaxing for exhalation once the breath is the desired volume. These variables are controlled by the neurotransmitters in the brain and can change from breath to breath if needed.

Spontaneous modes of ventilation allow for a return of some of the normal physiologic breathing processes. The chemoreceptors of the brain are active at triggering the initiation of breaths and the diaphragm is triggered to contract. This approach maintains some work in the respiratory muscles and decreases the risk of muscle wasting.

The risk of muscle wasting—or muscle atrophy—is key to the necessity of spontaneous modes.  Spontaneous modes of ventilation are beneficial as it helps maintain involvement of respiratory muscles such as the diaphragm. Remember Chapter 1? We discussed the key involvement of the diaphragm in creating the negative pressure that pulls the breath into the lungs. The diaphragm is the most important muscle involved in a person spontaneously breathing. Like all muscles, if not used, the diaphragm starts to weaken.

Control modes of ventilation do not allow the diaphragm to be fully involved in breathing and can cause the diaphragm to atrophy and weaken. This muscle weakening and loss can contribute to patients having difficulty taking over their breathing and being strong enough to breathe on their own. The quicker a health care practitioner can allow a patient to start using their own respiratory muscles to breathe, the better. A fast turnaround will decrease the muscle atrophy that can occur. Spontaneous modes are the number one way that diaphragm use can be maintained and patients will have an easier time taking over their breathing needs without the support of a ventilator.

Though unable to fully eliminate the need for mechanical ventilation, the creation of spontaneous modes of ventilation was key in helping address these concerns.

Key Concept

Spontaneous modes are key to minimize damage from positive pressure ventilation, by reducing asynchrony and mimicking a more natural pattern of breathing the patient can control. Spontaneous modes are also instrumental in maintaining respiratory muscle strength, allowing patients to get off the ventilators easier.

Summary of Control vs. Spontaneous Modes

The following table summarizes the key differences between control modes and spontaneous modes:

Control Modes Spontaneous modes
Drive to breathe No drive to breathe needed Needs to be breathing spontaneously
Oxygen Can help give additional oxygen Can help give additional oxygen
Elevated CO2 levels Used to fix abnormal gases Can only augment normal breathing; not indicated for very high CO2 levels
Sedation level Usually requires more sedation Much more comfortable, less sedation
Work of Breathing Patient is sedated and ventilator completely overtakes the muscles—they are not actively working Can give extra support to unload the work of breathing, but the muscles  still work a little
Maintaining muscle strength The diaphragm is not involved in the creation or delivery of the breath The diaphragm must contract and be involved. Helping maintain muscle strength and decrease muscle wasting
Level of Invasiveness More invasive, less physiologic More physiologic; spontaneous modes are used during the transition off of ventilation (weaning)
A pervasive thought in critical care exists that “resting” patients in a control mode is essential to their recovery and is ideal for the first few days of ventilation once the patient has failed enough to require mechanical ventilation. However, more and more research is pointing to the fact that this is not the case. Putting a patient on a control mode often requires large amounts of sedation and in some cases paralytics. Every day that the diaphragm is not working directly contributes to muscle loss and atrophy. Allowing patients to maintain some work in the breathing process has a large impact on decreasing ventilation days and decreasing length of stay in the hospital. As more and more research is being done, control modes are used for increasingly short durations, and spontaneous modes are becoming the standard of care earlier and earlier in the ventilatory process.

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