Modes of mechanical ventilation
Modes of mechanical ventilation refer to the various mechanical ventilator strategies employed to deliver a breath in patients that require mechanical ventilation. The mode refers to the method of respiratory support. In general, mode selection is based on clinician familiarity and institutional preferences, since there is a paucity of evidence indicating that the mode affects clinical outcome. The most frequently used forms of volume-limited mechanical ventilation are intermittent mandatory ventilation and continuous mandatory ventilation.
Terminology
There have been substantial discussion of nomenclature of mechanical ventilation in the past, particularly about the terminology regarding modes of mechanical ventilation. The confusion is still daunting but serious attempts are under way to bring clarity to the issue. Most recently, ISO has updated related standards to include terminology on modes of ventilation.Taxonomy for mechanical ventilation
The taxonomy is a logical classification system based on 10 maxims of ventilator design:How modes are classified
Step 1: Identify the primary breath control variable. If inspiration starts with a preset inspiratory pressure, or if pressure is proportional to inspiratory effort, then the control variable is pressure. If inspiration starts with a preset tidal volume and inspiratory flow, then the control variable is volume. If neither is true, the control variable is time.Step 2: Identify the breath sequence. Determine whether trigger and cycle events are patient or machine determined. Then, use this information to determine the breath sequence.
Step 3: Identify the targeting schemes for the primary breaths and secondary breaths.
Example mode classification is given below
Mode Name: A/C Volume Control :- Inspiratory volume and flow are preset, so the control variable is volume.
- Every breath is volume cycled, which is a form of machine cycling. Any breath for which inspiration is machine cycled is classified as a mandatory breath. Hence, the breath sequence is continuous mandatory ventilation.
- The operator sets all the parameters of the volume and flow waveforms so the targeting scheme is set-point. Thus, the mode is classified as volume control continuous mandatory ventilation with set-point targeting.
- The operator sets the tidal volume but not the inspiratory flow. Because setting volume alone is a necessary but not sufficient criterion for volume control, the control variable is pressure.
- Spontaneous breaths are allowed between mandatory breaths so the breath sequence is IMV.
- The ventilator adjusts inspiratory pressure between breaths to achieve an average preset tidal volume, so the targeting scheme is adaptive. The mode tag is PC-IMVa,s.
Descriptions of common modes
Mechanical ventilation machines are available with both invasive modes and non-invasive modes. Invasive has to do with the insertion of medical devices or tubes internal to the patient, while non-invasive is completely external to the patient, as for example in using a tightly fitting mask or other device that covers the patient's nose and mouth.Assist mode, control mode, and assist-control mode
A basic distinction in mechanical ventilation is whether each breath is initiated by the patient or by the machine. Dynamic hybrids of the two are also possible, and control mode without assist is now mostly obsolete.Airway pressure release ventilation
Airway pressure release ventilation is a time-cycled alternant between two levels of positive airway pressure, with the main time on the high level and a brief expiratory release to facilitate ventilation.Airway pressure release ventilation is a very versatile mode of ventilation. Like other modes, it needs to be well understood to use it effectively. The exhalation time is shortened to usually less than one second to maintain alveoli inflation. In the basic sense, this is a continuous pressure with a brief release.
Different perceptions of this mode may exist around the globe. While 'APRV' is common to users in North America, a very similar mode, biphasic positive airway pressure, was introduced in Europe. The term APRV has also been used in American journals where, from the ventilation characteristics, BIPAP would have been perfectly good terminology. But BiPAP is a trademark for a noninvasive ventilation mode in a specific ventilator.
Other manufacturers have followed with their own brand names. Although similar in modality, these terms describe how a mode is intended to inflate the lung, rather than defining the characteristics of synchronization or the way spontaneous breathing efforts are supported.
Intermittent mandatory ventilation has not always had the synchronized feature, so the division of modes were understood to be SIMV vs IMV. Since the American Association for Respiratory Care established a nomenclature of mechanical ventilation the "synchronized" part of the title has been dropped and now there is only IMV.
Mandatory minute ventilation
Mandatory minute ventilation allows spontaneous breathing with automatic adjustments of mandatory ventilation to the meet the patient's preset minimum minute volume requirement. If the patient maintains the minute volume settings for VT x f, no mandatory breaths are delivered.If the patient's minute volume is insufficient, mandatory delivery of the preset tidal volume will occur until the minute volume is achieved. The method for monitoring whether or not the patient is meeting the required minute ventilation differs by ventilator brand and model, but, in general, there is a window of monitored time, and a smaller window checked against the larger window to decide whether a mechanical breath is needed to maintain the minute ventilation.
MMV is an optimal mode for weaning in neonatal and pediatric populations and has been shown to reduce long-term complications related to mechanical ventilation.
Pressure-regulated volume control
Pressure-regulated volume control is an Assist Controlled Ventilation based mode. Pressure-regulated volume control utilizes pressure-limited, volume-targeted, time-cycled breaths that can be either ventilator- or patient-initiated.The peak inspiratory pressure delivered by the ventilator is varied on a breath-to-breath basis to achieve a target tidal volume that is set by the clinician.
For example, if a target tidal volume of 500 mL is set but the ventilator delivers 600 mL, the next breath will be delivered with a lower inspiratory pressure to achieve a lower tidal volume. Though PRVC is regarded as a hybrid mode because of its tidal-volume settings and pressure-limiting settings fundamentally PRVC is a pressure-control mode with adaptive targeting.
Continuous positive airway pressure
Continuous positive airway pressure is a non-invasive positive pressure mode of respiratory support. CPAP is a continuous pressure applied to keep the alveoli open and not fully deflate. This mechanism for maintaining inflated alveoli helps increase partial pressure of oxygen in arterial blood, an appropriate increase in CPAP increases the PaO2.Automatic positive airway pressure
Automatic positive airway pressure is a form of CPAP that automatically tunes the amount of pressure delivered to the patient to the minimum required to maintain an unobstructed airway on a breath-by-breath basis by measuring the resistance in the patient's breathing.Bilevel positive airway pressure
Bilevel positive airway pressure is a mode used during non-invasive ventilation. First used in 1988 by Professor Benzer in Austria, it delivers a preset inspiratory positive airway pressure and expiratory positive airway pressure. BPAP can be described as a Continuous Positive Airway Pressure system with a time-cycle change of the applied CPAP level.CPAP/APAP, BPAP, and other non-invasive ventilation modes have been shown to be effective management tools for chronic obstructive pulmonary disease, acute respiratory failure, sleep apnea, etc.
Often BPAP is incorrectly referred to as "BiPAP". BiPAP is the name of a portable ventilator manufactured by Respironics Corporation; it is just one of many ventilators that can deliver BPAP.
Medical uses
BPAP has been shown to be useful in reducing mortality and reducing the need for endotracheal intubation when used in people with chronic obstructive pulmonary disease.High-frequency ventilation (Active)
The term active refers to the ventilator's forced expiratory system. In a HFV-A scenario, the ventilator uses pressure to apply an inspiratory breath and then applies an opposite pressure to force an expiratory breath. In high-frequency oscillatory ventilation the oscillation bellows and piston force positive pressure in and apply negative pressure to force an expiration.High-frequency ventilation (Passive)
The term passive refers to the ventilator's non-forced expiratory system. In a HFV-P scenario, the ventilator uses pressure to apply an inspiratory breath and then returns to atmospheric pressure to allow for a passive expiration.This is seen in High-Frequency Jet Ventilation, sometimes abbreviated HFJV. Also categorized under High Frequency Ventilation is High Frequency Percussive Ventilation, sometimes abbreviated HFPV. With HFPV it utilizes an open circuit to deliver its subtidal volumes by way of the patient interface known as the Phasitron.
Volume guarantee
Volume guarantee an additional parameter available in many types of ventilators that allows the ventilator to change its inspiratory pressure setting to achieve a minimum tidal volume. This is utilized most often in neonatal patients who need a pressure controlled mode with a consideration for volume control to minimize volutrauma.Spontaneous breathing and support settings
Positive end-expiratory pressure
Positive end expiratory pressure is pressure applied upon expiration. PEEP is applied using either a valve that is connected to the expiratory port and set manually or a valve managed internally by a mechanical ventilator.PEEP is a pressure that an exhalation has to bypass, in effect causing alveoli to remain open and not fully deflate. This mechanism for maintaining inflated alveoli helps increase partial pressure of oxygen in arterial blood, and an increase in PEEP increases the PaO2.
Pressure support
Pressure support is a spontaneous mode of ventilation also named Pressure Support Ventilation. The patient initiates every breath and the ventilator delivers support with the preset pressure value. With support from the ventilator, the patient also regulates their own respiratory rate and their tidal volume.In Pressure Support, the set inspiratory pressure support level is kept constant and there is a decelerating flow. The patient triggers all breaths. If there is a change in the mechanical properties of the lung/thorax and patient effort, the delivered tidal volume will be affected. The user must then regulate the pressure support level to obtain desired ventilation.
Pressure support improves oxygenation, ventilation and decreases work of breathing.
Also see adaptive support ventilation.
Other ventilation modes and strategies
Flow-controlled ventilation
Flow-controlled ventilation is an entirely dynamic ventilation mode, without pauses, with continuous and stable gas flows during both inspiration and expiration, aiming for linear changes in both volume and pressure. FCV is an invasive ventilation mode but, unlike Volume- and pressure controlled modes, it does not rely on a passive expiration created by collapse of the thoracic wall and elastic recoil of the lungs. A high resistant breathing circuit inhibits a passive expiration and therewith allows to fully control and stabilize the expiration flow. FCV creates an inspiration by generating a stable flow from a set End-expiratory pressure to a set Peak pressure. Then a stable expiratory flow is created by suctioning. This expiratory flow rate is preferably similar to the inspiratory flow, aiming for an I:E ratio of 1:1.0, to minimize energy dissipation in the lungs. FCV is a more efficient ventilation as compared to conventional modes, allows ventilation through even small lumens and results in less applied mechanical power. FCV was invented by Professor Dr. med. Dietmar Enk.Negative pressure ventilation
Negative-pressure ventilation stimulates breathing by periodic application of partial vacuum, applied externally to the patient's torso—specifically, chest and abdomen—to assist the chest to expand, expanding the lungs, resulting in voluntary inhalation through the patient's airway.Various "negative pressure ventilators" have been developed to serve this function—most famously the "Iron lung," a tank in which the patient lays, with only their head exposed to ambient air, while air pressure on the remainder of their body, inside the tank, is varied by pumping, to stimulate chest and lung expansion and contraction. Though not in wide use today, NPVs were the principal forms of hospital and long-term mechanical ventilation in the first half of the 20th century, and remain in limited use today.
Closed loop systems
Adaptive Support Ventilation
Adaptive Support Ventilation is the only commercially available mode that uses optimal targeting. In this positive pressure mode of ventilation, the frequency and tidal volume of breaths of a patient on the ventilator are automatically adjusted and optimized to mimic natural breathing, stimulate spontaneous breathing, and reduce weaning time. In the ASV mode, every breath is synchronized with patient effort if such an effort exists, and otherwise, full mechanical ventilation is provided to the patient.Automatic Tube Compensation
Automatic Tube Compensation is the simplest example of a computer-controlled targeting system on a ventilator. It is a form of servo targeting.The goal of ATC is to support the resistive work of breathing through the artificial airway.
Neurally Adjusted Ventilatory Assist
Neurally Adjusted Ventilatory Assist is adjusted by a computer and is similar to ATC but with more complex requirements for implementation.In terms of patient-ventilator synchrony, NAVA supports both resistive and elastic work of breathing in proportion to the patient's inspiratory effort
Proportional Assist Ventilation
Proportional assist ventilation is another servo targeting based mode in which the ventilator guarantees the percentage of work regardless of changes in pulmonary compliance and resistance.The ventilator varies the tidal volume and pressure based on the patient's work of breathing. The amount it delivers is proportional to the percentage of assistance it is set to give.
PAV, like NAVA, supports both restrictive and elastic work of breathing in proportion to the patient's inspiratory effort.
Liquid ventilation
Liquid ventilation is a technique of mechanical ventilation in which the lungs are insufflated with an oxygenated perfluorochemical liquid rather than an oxygen-containing gas mixture. The use of perfluorochemicals, rather than nitrogen, as the inert carrier of oxygen and carbon dioxide offers a number of theoretical advantages for the treatment of acute lung injury, including:- Reducing surface tension by maintaining a fluid interface with alveoli
- Opening of collapsed alveoli by hydraulic pressure with a lower risk of barotrauma
- Providing a reservoir in which oxygen and carbon dioxide can be exchanged with pulmonary capillary blood
- Functioning as a high-efficiency heat exchanger