|Year : 2014 | Volume
| Issue : 2 | Page : 57-62
Recent innovations in mechanical ventilator support
Manu Chopra, Vasu Vardhan, Deepak Chopra
Department of Pulmonary Medicine, Military Hospital, Namkum, Ranchi, Jharkhand, India
|Date of Web Publication||23-Jun-2014|
Military Hospital, Namkum, Ranchi - 834 010, Jharkhand
Source of Support: None, Conflict of Interest: None
Mechanical ventilation as a means to provide basic lifesaving ventilatory support has grown leaps and bounds in the recent years. The basic modes of ventilation have seen a sea change and in addition other innovative techniques have been developed to prevent lung injury, ease of weaning and improve patient comfort. These modes and techniques though easily available are not adequately utilized for benefits of patient usually due to lack of knowledge about them. This article reviews some of these newer modes and innovations in mechanical ventilatory support.
Keywords: Adaptive support ventilation, airway pressure-release ventilation, continuous mandatory ventilation, high-frequency oscillatory ventilation, neurally adjusted ventilator assistance, positive end-expiratory pressure, pressure support ventilation, proportional assist ventilation, synchronized intermittent mandatory ventilation
|How to cite this article:|
Chopra M, Vardhan V, Chopra D. Recent innovations in mechanical ventilator support. J Assoc Chest Physicians 2014;2:57-62
| Introduction|| |
Mechanical ventilation is the process of using devices to support the total or partial transport of O 2 and CO 2 between the environment and the pulmonary capillary bed. The desired effect of mechanical ventilation is to maintain adequate levels of pO 2 and pCO 2 in arterial blood while also unloading the inspiratory muscles. This basic function of mechanical ventilators is too mechanical causing multiple side-effects such as volutrauma, barotrauma, etc., The conventional teaching stresses on three basic modes of ventilation, that is, continuous mandatory ventilation, synchronized intermittent mandatory ventilation and pressure support ventilation (PSV). Though the modes of mechanical ventilation are one of the most important aspects of the usage of mechanical ventilation, but today's mechanical ventilation has progressed far beyond these basic modes of ventilation. The mode selection is generally based on clinician familiarity and institutional preferences since there is a paucity of evidence indicating that the mode affects clinical outcome. Technologic advances and computerized control of mechanical ventilators have made it possible to deliver ventilatory assistance in new modes. Driving these innovations is the desire to prevent ventilator induced lung injury (VILI), improve patient comfort, and liberate the patient from mechanical ventilation as soon as possible. However, evidence of their benefit is scant. These newer modes are rarely used barring a few centers, probably due to lack of understanding of these modes. Moreover, there are other adjuncts to these basic or newer modes which help in improvement of ventilation, weaning from ventilators and reduce ventilator associated lung injuries. Herein, these recent innovations of mechanical ventilator support would be reviewed [Table 1].
|Table 1: Classification of recent innovations in mechanical ventilatory support |
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| Innovative strategies for lung protection|| |
The diseased lung on mechanical ventilator is too sensitive to small changes in pressure or volume. Moreover, standard protocol based settings of pressure and volume are too mechanical with their own hazards and complications. The recent advances in mechanical ventilation have led to introduction of newer strategies, which are lung protective. The innovative strategies to reduce VILI are:
Airway pressure-release ventilation and biphasic positive airway pressure ventilation
Airway pressure-release ventilation (APRV) (also known as biphasic ventilation [Avea], bilevel ventilation [Puritan Bennet], bilevel positive airway pressure [Drager], Bi Vent [Siemens] and Duo PAP [Hamilton]) is a time-cycled, pressure-targeted form of ventilatory support. ,, Herein, the ventilator maintains a constant pressure (set-point) even in the face of spontaneous breaths. APRV (Stock, et al., 1987) and biphasic positive airway pressure ventilation (BiPAP) (Baum, et al., 1989) are conceptually the same modes are conceptually the same, the main difference being that the time spent in low pressure (T-low) is <1.5 s for APRV whereas in BiPAP two levels of pressure T-high and T-low are set using an active exhalation valve and the patient is free to breathe spontaneously at either pressure level [Figure 1].
Airway pressure-release ventilation is a bilevel form of ventilation with sudden short releases in pressure to rapidly reduce functional residual capacity and allow for ventilation. In other words, this is a pressure-controlled breath with a very prolonged inspiratory time and a short expiratory time in which spontaneous ventilation is possible at any point ("pressure-controlled intermittent mandatory ventilation [PC-IMV]" in the current nomenclature). It can work both in spontaneous breathing and apneic patients. As inflation phase is prolonged APRV is something like inverse ratio ventilation (IRV), with the advantage of avoiding paralysis [Figure 2].
The advantages of increasing inspiratory time are however enjoyed by APRV/BiPAP. Specifically, the long inflation phase recruits more slowly filling alveoli and raises mean airway pressure without increasing applied positive end-expiratory pressure (PEEP) (although intrinsic PEEP can develop with short deflation periods). The additional spontaneous efforts during inflation may enhance both recruitment and cardiac filling when compared with other controlled forms of support. APRV/BiPAP helps in good gas exchange often with lower maximal airway pressures than used with control ventilation. Thus, these modes are useful in acute lung injury/acute respiratory distress syndrome (ARDS) allowing IRV with or without spontaneous breathing (less need for sedation or paralysis), improving patient-ventilator synchrony if spontaneous breathing is present, improving oxygenation by stabilizing collapsed alveoli and allowing patients to breath spontaneously, while continuing lung recruitment. Though the studies have not shown any difference in outcome, this truly is a revolutionary lung protective strategy.  However, a caution should be exercised in using these modes in patients with obstructive lung diseases and patients with inappropriately increased respiratory drive.
Adaptive support ventilation
This mode of ventilation was described in 1994 by Laubscher et al. and is available in Hamilton Galileo ventilator, Hamilton Medicals. Adaptive support ventilation (ASV) is an assist-control, pressure-targeted, time-cycled mode of ventilation that automatically sets the frequency - tidal volume pattern according to respiratory system mechanics in order to minimize ventilator work.  It is a form of mandatory minute ventilation implemented with adaptive pressure-control (APC). Mandatory minute ventilation is a mode that allows the operator to preset target minute ventilation, and the ventilator then supplies mandatory breaths, either volume- or pressure-controlled, if the patient's spontaneous breaths generate lower minute ventilation.
Adaptive support ventilation automatically selects the appropriate tidal volume and frequency for mandatory breaths and the appropriate tidal volume for spontaneous breaths on the basis of the respiratory system mechanics and target minute alveolar ventilation. Conceptually, this minimal ventilator work may translate into minimal stretching forces on the lungs, which may, in turn, reduce VILI. Importantly, the delivered "minimal work" tidal volume with ASV may sometimes be higher than the 6 mL/kg recommended by the ARDS Network trial. In addition, the PEEP settings are not part of the ASV algorithm and must therefore be set by the clinician. The machine selects a tidal volume and frequency that the patient's brain would presumably select if the patient were not connected to a ventilator. This pattern is assumed to encourage the patient to generate spontaneous breaths. The ventilator calculates the normal required minute ventilation based on the patient's ideal weight and estimated dead space volume (i.e. 2.2 mL/kg). This calculation represents 100% of minute ventilation. The clinician at the bedside sets a target percent of minute ventilation that the ventilator will support - higher than 100% if the patient has increased requirements due, e.g. to sepsis or increased dead space, or <100% during weaning. The ventilator initially delivers test breaths, in which it measures the expiratory time constant for the respiratory system and then uses this along with the estimated dead space and normal minute ventilation to calculate an optimal breathing frequency in terms of mechanical work. ASV has been shown to supply reasonable ventilatory support in a variety of patients as a sole mode of ventilation, from initial support to weaning. Two trials suggest that ASV may decrease time on mechanical ventilation. , However, in another trial,  compared with a standard protocol; ASV led to fewer ventilator adjustments but achieved similar postsurgical weaning outcomes. The effect of this mode on the death rate has not been examined.  This mode may lead to possible respiratory muscle atrophy and longer expiratory times in patients with obstructive airway disease.
High-frequency oscillatory ventilation
Described by Emerson (1952) and clinically developed in the early 1970s by Lunkenheimer, the goal of this mode is to minimize lung injury. It is PC-IMV with a set-point control scheme. In conventional PC-IMV relatively small spontaneous breaths are superimposed on relatively large mandatory breaths whereas, in high-frequency oscillatory ventilation (HFOV) - very small mandatory breaths (oscillations) are superimposed on top of spontaneous breaths. HFOV uses very high breathing frequencies (120-900 breaths/min in the adult) coupled with very small tidal volumes (usually less than anatomic dead space and often <1 mL/kg at the alveolar level) to provide gas exchange in the lungs.  Gas transport under these seemingly unphysiologic conditions may involve such mechanisms as Taylor dispersion, coaxial flows, and augmented diffusion.  High-frequency ventilation can be supplied by either jets or oscillators. Jets inject high-frequency pulses of gas into the airways. Oscillators literally vibrate a fresh bias flow of gas delivered at the tip of the endotracheal tube. Due to this, HFOV has sometimes been referred to as "CPAP with a wiggle". The putative advantages of HFOV are two-fold. First, the very small alveolar tidal pressure swings minimize cyclical overdistention and derecruitment. Second, a high mean airway pressure can also prevent derecruitment. Interestingly, mean pressures used during HFOV are often reported to exceed the 30-35 cm H 2 O threshold employed during conventional ventilation. This tolerance of a higher mean pressure with HFOV may be explained by a better maintained alveolar structure with a slowly applied (albeit vibrating) constant pressure as opposed to cyclical brief tidal pressures. Clinical experience with HFOV has been most extensive in the neonatal and pediatric age groups. ,, One randomized trial evaluating HFOV in severe ARDS suggested a trend toward improved survival with HFOV  [Figure 3].
| Automated weaning strategies|| |
Weaning off the patients from ventilators is very labor intensive. A number of attempts have been made to "automate" the weaning process.  The concept behind automated weaning was that significant clinician time could be saved and ventilatory support could be reduced in a timely fashion based on simple ventilator measurements.
Pressure-control ventilation cannot guarantee minimum minute ventilation in changing lung mechanics or patient's effort, or both. To overcome this APC was introduced. APC is known by various names such as, pressure regulated volume control (Servo-i), Autoflow (Drager), adaptive pressure ventilation (Hamilton Galileo), volume control+ (Puritan Bennett) and controlled volume guaranteed (Engstrom). The APC mode delivers pressure-controlled breaths with an adaptive targeting scheme. In pressure-control ventilation, tidal volumes depend on the lung's physiologic mechanics (compliance and resistance) and patient effort. Therefore, the tidal volume varies with changes in lung physiology (i.e. larger or smaller tidal volumes than targeted). To overcome this effect, a machine in APC mode adjusts the inspiratory pressure to deliver the set minimal target tidal volume. If tidal volume increases, the machine decreases the inspiratory pressure, and if tidal volume decreases, the machine increases the inspiratory pressure. This mode is designed to maintain a consistent tidal volume during pressure-control ventilation and to promote inspiratory flow synchrony. It is a means of automatically reducing ventilatory support (i.e. weaning) as the patient's inspiratory effort becomes stronger, as in awakening from anesthesia.
Adaptive support ventilation
Adaptive support ventilation, as explained earlier using controlled breaths initially calculates resistance and compliance as well as the expiratory time constant (resistance × compliance). The clinician sets only desired minute ventilation and the patient's weight (for estimating anatomic dead space). ASV then partitions the frequency - tidal volume pattern in such a way as to minimize ventilator work (pressure × volume) and thus conceptually minimizes applied forces to the lung [Figure 4].
| Optimizing synchrony during interactive breaths|| |
During mechanical ventilation interactive breaths are used to improve comfort and reduce sedation. The straightforward designs, ease of operation, and safety of these innovations make them appropriate to be considered in all patients receiving interactive breaths.
Endotracheal tube resistance compensation
The endotracheal tube provides a significant resistance to flow during both inspiration and expiration. During the inspiratory phase, this means that pressure build-up in the airways "lags" behind the pressure build-up in the ventilator circuitry. Thus, the "square" wave of pressure in the circuitry provided by a pressure-targeted breath is distorted in the airways to a slower rise of pressure. This may create significant initial flow dyssynchrony in patients with vigorous inspiratory efforts. During expiration, a similar gradient between airway pressures and set circuit PEEP can develop. Endotracheal tube resistance compensation (automatic airway compensation or automatic tube compensation), initially provides an inspiratory pressure higher than the set pressure-target. As inspiration proceeds, this delivered pressure then tapers to the set inspiratory pressure-target. This compensation mechanism also can operate in expiration with an initial expiratory airway pressure below the set PEEP that then rises to the set PEEP. It can be considered in virtually all patients receiving assisted/supported pressure-targeted breaths, - especially those with vigorous inspiratory efforts. 
Pressure-targeted inspiratory pressure slope adjusters
Newer ventilators allow to adjust the rate of rise of pressure during inspiration (slope adjusters), and clinical studies have suggested that slope adjustment could significantly enhance flow synchrony in many patients.  The rate of rise depends on patients demands, rapid in vigorously demanding and slower in less vigorously breathing patients. This can be easily done using circuit pressure graph and adjusting the slope to create a "smooth square wave" appearance to the circuit pressure profile.
Proportional assist ventilation
Patients who have normal respiratory drive but who have difficulty sustaining adequate spontaneous ventilation are often subjected to PSV, in which the ventilator generates a constant pressure throughout inspiration regardless of the intensity of the patient's effort. Described by Younes and available in Puritan Bennet, in proportional assist ventilation (PAV), the pressure applied is a function of patient effort: The greater the inspiratory effort, the greater the increase in applied pressure. The operator sets the percentage of support to be delivered by the ventilator. The ventilator intermittently measures the compliance and resistance of the patient's respiratory system and the instantaneous patient-generated flow and volume, and on the basis of these it delivers a proportional amount of inspiratory pressure. In PAV all breaths are spontaneous. The patient controls the timing and size of the breath. There are no present pressures, flow, or volume goals, but safety limits on the volume and pressure delivered can be set. The sensed patient effort is boosted according to a proportion of the measured work of breathing set by the clinician. The greater the patient effort, the greater the delivered pressure, flow, and volume. In theory, PAV should reduce the work of breathing, improve synchrony, automatically adapt to changing patient lung mechanics and effort, decrease the need for ventilator intervention and manipulation, decrease the need for sedation, and improve sleep. , It is contraindicated in respiratory depression (bradypnea), large air leaks (e.g. bronchopleural fistulas) and should be used with caution in patients with severe hyperinflation (in which the patient may still be exhaling but the ventilator doesn't recognize it) and in patients with high ventilatory drives (in which the ventilator overestimates respiratory system mechanics).
Neurally adjusted ventilatory assistance
Neurally adjusted ventilatory assistance (NAVA) utilizes a diaphragmatic electromyographic (EMG) signal to trigger and cycle ventilatory assistance. The EMG sensor is positioned in the esophagus at the level of the diaphragm. Triggering of the ventilator breath is thus virtually simultaneous with the onset of phrenic nerve excitation of the inspiratory muscles. Moreover, breath cycling becomes tightly linked to the cessation of inspiratory muscle contraction. Small clinical studies have demonstrated improved trigger and cycle synchrony with NAVA,  but data demonstrating improved outcomes (e.g. duration of mechanical ventilation, sedation needs) are lacking. Another concern with NAVA is the expense associated with the EMG sensor [Figure 5].
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]