Introduction
Most patients admitted to the cardiothoracic critical care unit will require a form of mechanical ventilation at some point during their journey. However, the spectrum of indications is wide and ranges from patients on a ‘fast-track’ concept, who are only being ventilated for a very short period of time directly after surgery, to very complex patients with severe cardiovascular and respiratory failure due to their underlying disease or the impact of major cardiothoracic surgery. Therefore a sound knowledge of the various techniques of invasive and non-invasive mechanical ventilatory support is essential to develop the appropriate strategy for the individual patient and their current clinical state.
Technical Aspects
The basic principle of modern intensive care ventilators is a positive pressure gradient, which creates gas flow into the lungs during inspiration. Expiration is usually passive due to fall of the pressure gradient to an expiratory level (PEEP). The gas flow is delivered either invasively through an endotracheal tube or a tracheostomy cannula, or non-invasively via a tight fitting facemask or similar device.
Traditionally ventilation modes have been described as either volume or pressure controlled. Modern ventilators however provide a variety of different sophisticated modes, some of which combine characteristics of both, and are controlled by the so-called ‘phase variables’ triggering, limiting and cycling.
Triggering Phase
The triggering variable sets the start of inspiratory gas flow and can be time, pressure, flow or volume. Thus the trigger can be controlled by the ventilator, the patient, or a combination of both. When time is the trigger variable, the breath is considered mandatory.
Limiting Phase
The limiting variable defines the length or size of inspiratory gas flow. It can also be set to a certain pressure, volume, flow or time. The type of waveform produced for pressure, flow and volume defines a breath delivery. Depending on the chosen limiting variable the ventilation pattern may be one of the three following types:
Volume controlled:
A constant volume and constant flow waveform; the pressure waveform will vary depending on lung characteristics.
Pressure controlled:
The pressure waveform has a preselected specific pattern but volume and flow will vary depending on lung characteristics.
Time controlled:
Pressure, volume and flow waveforms all vary depending on lung characteristics.
Cycling Phase
The cycling variable starts the expiration phase by ending inspiratory gas flow. This may also be pressure, volume, flow or time. For example the ventilator may cycle to the expiratory phase after a certain time (i.e. preset frequency for mandatory ventilation), when the pressure reaches a certain set maximum during inspiration, or cycle to the expiratory phase when flow falls to a set level (commonly 5 l/min) or percentage (commonly 25%) of peak flow (patient trigger).
By combining various settings to the different phase variables, modern ventilators can create a large number of ventilation patterns, which can be further individualised to specific clinical situations. These are all based on three types of breath:
A mandatory breath is one in which the ventilator does all the work of breathing and controls the transition between phases of the breath.
An assisted breath is one where the patient begins or ‘triggers’ inspiration but the ventilator controls the inspiratory phase and the cycling of inspiration to expiration.
A spontaneous breath is one in which the patient controls the transition between all breathing phases.
Ventilation Modes
Nomenclature of modes of ventilation has become confusingly complex, with different names being used for similar modes by different manufacturers. We want to review the most common terms and explain the technical principles of the different ventilator modes.
In principle, there are two different techniques of ventilation: the mandatory technique and the spontaneous technique. In mandatory modes, ventilation is fully or partially controlled by the respirator. Spontaneous modes allow either independent breathing of the patient or respirator assisted spontaneous breathing.
Three main subgroups can be classified:
Mandatory ventilation – volume controlled modes
– pressure controlled modes
Spontaneous ventilation – spontaneous/assisted modes.
However, this classification is not static and an overlap with spontaneous/assisted ventilator modes is possible, as the elaborated mandatory techniques can provide assisted or augmented ventilation as well. This is an important option, especially during weaning from mechanical ventilation. Even though modern ventilators have factory presets, to assist application under common conditions, a profound understanding of the control variables is necessary to adjust mechanical ventilation to individual patient needs.
Variables of Mechanical Ventilation
Inspiration
The inspiration phase is preceded by a triggering phase during which the ventilator is ‘waiting’ for a signal to start the inspiration. There are two trigger options:
Patient triggered:
Via flow or pressure trigger, the ventilator identifies the inspiration effort of the patient after which a mandatory breath is delivered. An adjustable trigger threshold guarantees individual patient sensitivity. If no patient effort is detected during a set time window, a mandatory inspiration can be triggered (time trigger) to guarantee a certain ventilation frequency.
Respirator triggered:
A mandatory breath is triggered by set parameters such as inspiratory time, frequency and I:E (inspiration:expiration) ratio. The timing is obligatory and the patient has no influence.
Expiration
The variable that ends inspiratory gas flow can be either flow or time:
Flow cycled:
Dependent on the inspiratory flow of the patient, the expiratory period is triggered as soon as the inspiratory flow has reached a selected part of the maximum inspiratory flow.
Time cycled:
Inspiratory time is defined and expiration begins as soon as a set inspiratory time is terminated.
Principles of Mechanical Ventilation
Volume Control Ventilation
The parameter remaining constant during this ventilation technique is the tidal volume, which is delivered by a constant inspiratory flow. The frequency of the mandatory breaths and the tidal volume can be adjusted, producing a certain minute volume. Inspiratory pressure is dependent on patient conditions (for example lung mechanics). As patient physiologies vary, it is important to adjust pressure limits when using volume control ventilation.
Special Feature PRVC and Autoflow®
Closed loop/servo control modes utilise feedback monitoring systems within the ventilator to assess breath delivery, compare and contrast the actual breath delivered with the set target parameters, and then adjust flow and pressure to match these set parameters more closely. The pressure regulated volume controlled ventilation (PRVC) combines a constant tidal volume with the minimum necessary airway pressure. The inspiratory pressure level is adapted to lung condition with every mandatory breath. Autoflow® provides additionally the possibility of patient triggering during the whole breathing cycle.
Pressure Control Ventilation
Two parameters are remaining constant during pressure control ventilation: the lower pressure level (i.e. PEEP) and the upper pressure level (i.e. inspiratory pressure). Tidal volume results from patient conditions, i.e. inspiratory effort, as well as lung mechanics and especially from the difference between the two pressure levels. The inspiration time determines the inspiration period and the upper pressure level is constant during the whole inspiration time whereas the decelerating gas flow and tidal volumes are dependent variables
PEEP (Positive End Expiratory Pressure)
A positive airway pressure level during the expiration period can be achieved by either flow resistance or threshold resistance. Flow resistors work as an expiratory retard device. Pressure across the resistor is regulated by flow. Threshold resistors allow continuity of expiratory flow until pressure within the breathing cycle has reached PEEP level (according to threshold value). If possible, ventilators with threshold resistance technique should be preferred as they reduce resistance during the expiration period and decrease the risk of barotrauma.
Intrinsic PEEP (or auto-PEEP) is defined as the fixed recoil pressure of the respiratory system at end-expiration. It is caused by airway obstruction, or incomplete exhalation, which generates flow resistance.
The application of PEEP may improve ventilation conditions of the patient and optimise oxygenation and decarboxylation in different ways, including:
increase of the functional residual capacity (FRC) with improvement of the alveolar oxygen reservoir,
recruitment of collapsed alveoli, which improves lung compliance and reduces pulmonary shunting,
redistribution of lung oedema from the alveoli to the interstitium.
However, application of PEEP might also increase dead space, increase the risk of barotrauma and potentially reduce cardiac output. These factors have to be taken into account when finding the ‘optimum PEEP’ for an individual patient.
Which Mode for Which Patient?
There is a spectrum of indications for mechanical ventilation in patients admitted to cardiothoracic critical care. Therefore different ventilation strategies and ventilation modes need to be applied according to the indication but also according to the clinical phase.
Most patients after cardiac surgery are admitted to the ICU sedated and mechanically ventilated to allow an initial stabilisation phase. Once the patient’s clinical situation is deemed stable with regard to cardiovascular and respiratory stability, bleeding and coagulopathy, a structured process of weaning from mechanical ventilation should be started as early as possible in order to keep the ventilation phase as short as possible. As soon as sedation reduction has begun, there should be a protocol based repetitive spontaneous breathing trial, to evaluate muscle weakness and breathing control of the patient. In general, after routine surgery, breathing control and adequate muscle strength return quickly.
However, a proportion of patients experience a complex ICU journey with prolonged phases of mechanical ventilation, and require differentiated strategies. If sedation can be decreased, breathing control returns potentially unstable. Breathing muscles may be weak due to critical illness myopathy and neuropathy. Thus, mixed ventilation modes are required, which enable the patient to trigger a mandatory breath or in the next step to take spontaneous breaths. Breathing effort is shared between equipment and patient. By the time breathing control improves, but muscle weakness is still present, a supported spontaneous breathing mode might be adequate. After a long phase of critical illness with ventilator support, a phase of non-invasive ventilation (NIV) might be required to stabilise the patient’s spontaneous breathing and prevent further respiratory failure needing invasive mechanical ventilation.
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