Modes and Terms of Mechanical Ventilation Explained
Overview of Modes of Mechanical Ventilation
The modes of mechanical ventilation are important for clinicians who work with these patients to understand. An iron lung is an example of negative pressure ventilation. Most modern mechanical ventilators are positive pressure ventilation.
A ventilator is a device used to support, assist or control respiration (inclusive of the weaning period) through the application of positive pressure to the airway when delivered via an artificial airway, specifically oral/nasal endotracheal or tracheostomy tube. Ventilation and lung expansion devices that deliver positive pressure to the airway (for example: CPAP, Bipap, bi-level, IPPB and PEEP) via non-invasive means (for example: nasal prongs, nasal mask, full face mask, total mask, etc.) are not considered ventilators unless positive pressure is delivered via an artificial airway (oral/nasal endotracheal tube or tracheostomy tube).
In mechanical ventilation there are two primary control variables; volume control and pressure control.
Pressure-cycled ventilators: Gas is allowed to flow into the lungs until a present airway pressure limit is reached, at which time a valve opens allowing exhalation to ensue. The volume delivered by the ventilator varies with changes in airway resistance, lung compliance, and integrity of the ventilatory circuit.
Volume-cycled ventilators: Gas flows to the patient until a preset volume is delivered to the ventilator circuit, even if this entails a very high airway pressure.
Individuals require mechanical ventilation for different reasons. It is used for individuals with respiratory failure who are unable to breathe on their own. The indications for mechanical ventilation include airway protection, treatment of hypoxemic respiratory failure (low blood oxygen), treatment of hypercapnic respiratory failure (elevated carbon dioxide in the blood), or treatment of a combined hypoxic and hypercapnic respiratory failure. Other indications include decreased level of consciousness with inability to protect teh airway, massive hemoptysis, severe angioedema or airway compromise such as burns, cardiac arrest and shock. On some occasions, patients are also intubated and placed on mechanical ventilation for surgical procedures.
Regardless of the mode of mechanical ventilation, the guiding principals will be to provide lung protection (prevent overdistension), provide adequate gas exchange, unload the respiratory muscles, improving patient comfort and synchrony.
Modes of Mechanical Ventilation
Below is a brief overview of the different modes of mechanical ventilation. There are many different modes of ventilation that vary minimally between each other. We will focus on the common modes of mechanical ventilation and their clinical use. The mode of ventilaiton includes controlled mechanical ventilation, assist control, synchronized intermittent mandatory ventilation, pressure support, volume support and continuous positive airway pressure.
Controlled Mechanical Ventilation (CMV)
One mode of mechanical ventilation is controlled mechanical ventilation (CMV). In controlled mechanical ventilation, the ventilator provides a mechanical breath on a preset timing. Patient respiratory efforts are ignored. This is generally uncomfortable for children and adults who are conscious and is usually only used in a fully sedated patient and receiving a neuromuscular blocking agent. It may also be used in infants who often quickly adapt their breathing pattern to the ventilator timing.
Another mode of mechanical ventilation is assist control. In assist control, the operator can set either a controlled volume or controlled pressure. A minimum number of preset mandatory breaths are delivered by the ventilator. The patient may trigger additional machine assisted breaths above the set rate. If patient does not trigger the ventilator with sufficient inspiratory effort, the ventilator automatically delivers the preset volume/pressure. The sensitivity control setting can be changed to make it easier or harder for the patient to initiate a breath.
Assist control is most often used when mechanical ventilation is first initiated for the patient because this mode provides full ventilatory support, keeping the patient’s work of breathing low.
An example of ventilator settings is assist control of 12 (set rate) and volume control of 600. When the patient takes a deep enough breath to trigger the ventilator, the patient is provided with the preset volume of 600. If the patient does not take enough breaths on their own (spontaneous breaths), then the ventilator will provide the breath. In this case, if the patient does not take any spontaneous breaths, the ventilator would provide 12 breaths per minute.
Synchronized Intermittent Mechanical Ventilation (SIMV)
Synchronized intermittent mechanical ventilation (SIMV) is another mode of mechanical ventilation where the operator can set either a controlled pressure or controlled volume. Each mandatory breath in SIMV will deliver the identical set parameters (set pressure or volume) every specified number of seconds. Thus a set respiratory rate of 12 results in a 5 second cycle time (Respiratory rate of 12 divided by 60 seconds equals 5).
The main feature of SIMV is to allow the user to take spontaneous breaths in between the set mechanical breaths. If no spontaneous effort is detected, the ventilator delivers a mechanical breath within a cycle time. Within the cycle time, the ventilator waits for the patient to initiate a breath using either a pressure or flow sensor. When the ventilator senses the first patient breathing attempt within the cycle, it delivers the preset ventilator breath. If the patient fails to initiate a breath, the ventilator delivers a mechanical breath at the end of the breath cycle. Additional spontaneous breaths after the first one within the breath cycle do not trigger another SIMV breath. However, SIMV may be combined with pressure support (see below).
SIMV is frequently employed as a method of decreasing ventilatory support (weaning) by turning down the rate, which requires the patient to take additional breaths beyond the SIMV triggered breath. This could help prevent muscle atrophy.
Pressure Support Ventilation (PSV)
In pressure support ventilation, a fixed amount of pressure (set by the clinician) augments each breath during the inspiratory phase of ventilation. The tidal volume is variable and depends on the patient’s effort and lung elasticity.
Pressure support ventilation was developed as a method to decrease the work of breathing in-between ventilator mandated breaths by providing an elevated pressure triggered by spontaneous breathing that “supports” ventilation during inspiration. Thus, for example, SIMV might be combined with PSV so that additional breaths beyond the SIMV programmed breaths are supported. However, while the SIMV mandated breaths have a preset volume or peak pressure, the PSV breaths are designed to cut short when the inspiratory flow reaches a percentage of the peak inspiratory flow (e.g. 10-25%). The peak pressure set for the PSV breaths is usually a lower pressure than that set for the full ventilator mandated breath. PSV can be also be used as an independent mode.
In pressure support ventilation, the patient has control over the rate, inspiratory and inspiratory flow rate. The tidal volume is determined by the level of PSV, patient effort and pulmonary mechanics.
Volume Support Ventilation (VS)
In volume support ventilation the ventilator delivers a supported breath to help the patient reach a set tidal volume. This mode is dependent on the patient’s effort. The ventilator varies the inspiratory pressure level with each breath to achieve the target volume. Volume support ventilation can be used as an independent mode or in combination with mandatory breath types.
This mode is not as common but is sometimes used to wean patients off anesthesia.
Continuous Positive Airway Pressure (CPAP)
Continuous Positive Airway Pressure (CPAP) is similar to PEEP (Positive End Expiratory Pressure) relating to its purpose to provide end expiratory pressure. A continuous level of elevated pressure is provided through the patient circuit when in continuous positive airway pressure mode. No cycling of ventilator pressures occurs and the patient must initiate all breaths. In addition, no additional pressure above the CPAP pressure is provided during those breaths. CPAP may be used invasively through an endotracheal tube or tracheostomy or non-invasively with a face mask or nasal prongs.
CPAP can be used as an independent mode, or in combination with mandatory breath type modes. When used independently, there is no set tidal volume or rate. This is a useful mode when weaning patients off mechanical ventilation.
Terms of Mechanical Ventilation
Positive End Expiratory Pressure (PEEP)
Positive End Expiratory Pressure (PEEP) is a technique in which airway pressure greater than atmospheric pressure is achieved at the end of exhalation by the introduction of a mechanical impedance to exhalation. In patients on mechanical ventilation, PEEP is one of the key parameters that can be adjusted depending on the patient’s oxygenation needs, and is typically in the range of 0 to 15 cmH2O. PEEP set by the clinician is also known as extrinsic PEEP, or ePEEP, to distinguish it from the pressure than can arise with air trapping.
High levels of PEEP may cause barotrauma, increased intracranial pressure and decreased cardiac output.
Fraction of inspired air (Fi02)
The fraction of oxygen in inspired gas. For example, the FiO2 of ambient air is 0.21; the oxygen concentration of ambient air is 21%. In patients on mechanical ventilation, the FiO2 is one of the key parameters that can be adjusted depending on the patient’s oxygenation needs, and is typically in the range of 0.30 (oxygen concentration of 30%) to 1.0 (oxygen concentration of 100%).
The lowest amount of FiO2 to achieve oxygenation should be used.
Frequence (f) or Respiratory Rate (RR)
The respiratory rate (RR) may be set on the ventilator. It is the set number of breaths per minute. When determining the patient’s actual respiratory rate, this must include the patient’s spontaneous breaths in addition to the set mechanical breaths. Hypoventilation may cause respiratory acidosis; hyperventilation may cause respiratory alkalosis.
Tidal Volume (TV)
Tidal volume is the lung volume representing the normal volume of air displaced between normal inhalation and exhalation when extra effort is not applied. In a healthy, young human adult,tidal volume is approximately 500 mL per inspiration or 7 mL/kg of body mass. Tidal volume can be set on a ventilator to deliver a set amount of volume, delivered in millimeters.
The acceptable range of tidal volume on mechanical ventilation is set between 6-8ml/kg (based on height and birth gender) to prevent barotrauma.
Peak Inspiratory Pressure (PIP)
Peak Inspiratory Pressure (PIP) is the highest level of pressure applied to the lungs during inhalation. During mechanical ventilation the number reflects a positive pressure in centimeters of water pressure (cmH2O). The PIP is displayed on most ventilators.
It is best to target a peak inspiratory pressure of less than 35 cm H2O. An elevated PIP and normal Pplat is indicative of increased airway resistance. An elevated PIP and elevated Pplat is indicative of abnormal compliance. Determining whether the patient has a resistance problem or a compliance problem can assist in the differential diagnosis of respiratory failure.
Plateau Pressure (Pplat) is the pressure that remains in the alveoli during the plateau phase, when flow through the airway has stopped, or with a breath-hold. To calculate plateau pressure, the clinician can push the “inspiratory hold” button on the ventilator. This measures airway pressure at the end of inspiration when flow through the airway has finished. When flow has stopped, the amount of resistive work is zero. The plateau pressure is effectively the pressure at the alveoli with each mechanical breath, and reflects the compliance in the airways. Normal plateau pressure is below 30cm H2O, and higher pressure can generate barotrauma. To prevent lung injury, the Pplat should be maintained at < 30 cm H2O.
Peak Inspiratory Flow
Peak inspiratory Flow or flow rate is the rate that the tidal volume is delivered, expressed in liters per minute. A normal setting for flow is between 50 and 60 L per minute. Higher flow rates may be required for higher ventilator demands. Changing the inspiratory flow indirectly affects the I:E ratio.
Minute ventilation (VĖ, Vė, or MV ) is the total sum of volume the patient receives in one minute, including both spontaneous and mechanical breaths. It is calculated as the tidal volume multiplied by the respiratory rate (TV x RR), and expressed in liters per minute (L/min). Most healthy adults have a baseline minute ventilation of 4-6 L/min, but critically ill patients, such as those attempting to compensate for a metabolic acidosis, may require a minute ventilation of 12-15 L/min, or even higher, to meet their demands.
Inspiratory time to expiratory time ratio. Normal is 1:2 to 1:4 or 5.
Speed in liters per minute at which the ventilator delivers breathes.
Change in volume divided by change in pressure. In respiratory physiology, total compliance is a mix of lung and chest wall compliance as these two factors cannot be separated in a live patient.
It is important for clinicians working with patients on mechanical ventilation to know the basic modes of ventilation. Mechanical ventilation is used in intensive care, long term and home care settings to assist patients who require respiratory support.