Autor: Dean R. Hess, PhD, RRT

Associate Professor of Anesthesia, Harvard Medical School

Assistant Director of Respiratory Care, Massachusetts General Hospital

Boston, MA USA

   

NEW MODELS OF MECHANICAL VENTILATION

 

Introduction

Positive pressure ventilation is described by trigger, control, and cycle variables. The trigger initiates a breath. The control variable remains constant throughout inspiration regardless of changes in respiratory system impedance. Inspiration ends when the cycle is reached. The relationship between the various possible breath types and conditional variables is the mode of ventilation. A mode can include pressure and volume controlled breaths and can be sophisticated enough to switch from one control variable to the other. With each generation of ventilators, new modes and other features become available. The purpose of this paper is to describe the technical aspects of new modes and related features of mechanical ventilators that recently have become available.

 

The Ventilator Trigger

Patient triggering is usually pressure-tirggered or flow-triggered. Pressure triggering requires sufficient patient inspiratory effort to cause airway pressure to fall from the set end-expiratory level to a threshold level (sensitivity) set by the clinician. With flow triggering, breath initiation is based on a flow change in the ventilator circuit beyond some pre-determined threshold. From the available evidence, the following recommendations can be made. 1) The trigger on current generation ventilators is superior to that which existed in the past. Auto-triggering can be problematic secondary to the artifact (e.g., cardiac oscillations) when the trigger is very sensitive. 2) There is no clear superiority of flow triggering and pressure triggering. The choice of trigger type should be based on patient response, using the trigger type that produces the best patient comfort. 3) Patient difficulty triggering the ventilator is usually due to pathophysiology (e.g., auto-PEEP) rather than the trigger type or sensitivity.

               

Pressure Ventilation

Pressure controlled ventilation

With pressure-controlled ventilation (PCV), the ventilator applies a set pressure to the airway for a set inspiratory time. Pressure-controlled breaths can be either patient-triggered or ventilator-triggered. Tidal volume during PCV is determined a number of variables: the pressure control setting, airways resistance, respiratory system compliance, auto-PEEP, and patient effort. Inspiratory time also affects the tidal volume if the flow does not decrease to zero. Inspiratory flow is fixed with volume-controlled ventilation (VCV), whereas flow with PCV is variable. Because of this, PCV may be desirable to VCV if the patient is triggering the ventilator with a strong respiratory drive. PCV has been advocated by some authorities as a lung protective strategy and to improve patient-ventilator synchrony. However, it has recently been shown that, with lung protective ventilation, the work of breathing is greater with pressure-controlled ventilation (compared to volume-controlled ventilation). More important, with pressure controlled ventilation, volume and trans-pulmonary pressure limitation is not assured if the patient makes vigorous inspiratory efforts.

 

Pressure-controlled inverse-ratio ventilation

Early reports of improved oxygenation with pressure-controlled inverse-ratio ventilation (PCIRV) generated considerable enthusiasm for this method. Following the initial enthusiasm for this approach, a subsequent controlled studies reported no benefit or marginal benefit for the use of PCIRV. Based on the available evidence, there seems to be no clear role for PCIRV in the management of patients with ARDS. The likelihood of an improvement in oxygenation using inverse ratio ventilation is small and the risk of auto-PEEP and hemodynamic compromise is great.

 

Pressure support ventilation

Pressure support ventilation (PSV) assists inspiratory muscles during invasive and noninvasive ventilation. PSV is patient triggered and primarily flow triggered. Secondary cycling mechanisms with PSV are pressure and time. Thus, PSV cycles to the expiratory phase when the flow decelerates to a ventilator-determined level, when the pressure rises to a ventilator-determined level, or the inspiratory time reaches a ventilator-determined limit. Although PSV is often considered a simple mode of ventilation, in reality it can be quite complex: 1) the ventilator must recognize the patient’s inspiratory effort, which depends on the trigger sensitivity of the ventilator and the presence of auto-PEEP. 2) The ventilator must deliver an appropriate flow at the onset of inspiration. A flow that is too high can produce a pressure overshoot, whereas a flow that is too low can produce patient flow starvation and dyssynchrony. 3) The ventilator must appropriately cycle to the expiratory phase without the need for active exhalation by the patient.

 

Like PCV, the flow deceleration during PSV is largely a function of the resistance and compliance of the respiratory system. The flow at which the ventilator cycles can either a fixed absolute flow, a flow based on the peak inspiratory flow, or a flow based on peak inspiratory flow and elapsed inspiratory time. Several studies have reported dyssynchrony with PSV in subjects having airflow obstruction (e.g., COPD). With airflow obstruction, the inspiratory flow decelerates slowly during PSV, the flow necessary to cycle may not be reached, and this stimulates active exhalation to pressure cycle the breath. This problem increases with higher levels of PSV and with higher levels of airflow obstruction. Several approaches can be used to solve this problem. 1) PCV can be used, with the inspiratory time set short enough so that the patient does not contract the expiratory muscles to terminate inspiration. 2) On some newer generation ventilators, the clinician can adjust the termination flow at which the ventilator cycles.

 

The flow at the onset of the inspiratory phase is determined by rise time - the time required for the ventilator to reach the PSV level at the onset of inspiration. Newer generation of ventilators allow adjustments of the rise time during PSV. The rise time is adjusted to patient comfort and ventilator graphics may be useful to guide this setting. In patients with a strong respiratory drive, a rapid rise time may decrease the work of breathing and the patient’s sensation of dyspnea. However, patient comfort may be compromised using rise times that are either to low or too high. Moreover, a high inspiratory flow at the onset of inspiration is not necessarily beneficial for several reasons. First, if the flow is higher at the onset of inspiration, the inspiratory phase may be prematurely terminated if the ventilator cycles to the expiratory phase at a flow that is a fraction of the peak inspiratory flow. Second, the existence of a flow-related inspiratory terminating reflex in the airway has recently been described. Activation of this reflex due to a higher inspiratory flow causes shortening of neural inspiration, which could result in brief, shallow inspiratory efforts.

Another issue with PSV is the presence of leaks in the system (e.g., bronchopleural fistula, cuffless airway, mask leak with noninvasive ventilation). If the leak exceeds the termination flow at which the ventilator cycles, either active exhalation will occur to terminate inspiration or a prolonged inspiratory time will be applied. With a leak, either PCV or a ventilator that allows an adjustable termination flow should be used.

 

Proportional Assist Ventilation

Proportional assist ventilation (PAV) was designed to increase or decrease airway pressure in proportion to patient effort, which should improve patient-ventilator synchrony. This is accomplished by a positive feedback control that amplifies airway pressure proportionally to inspiratory flow and volume, where respiratory elastance and resistance are the feedback signal gains. Unlike other modes of ventilatory support, which deliver a preset tidal volume or inspiratory pressure at the airway, with proportional assist ventilation the amount of support changes with patient effort, assisting ventilation with a uniform proportionality between ventilator and patient. The advantage of a proportional ventilatory support lies in its ability to track changes in ventilatory effort. To the extent that inspiratory effort is a reflection of ventilatory demand, this form of support may result in a more physiologic breathing pattern.

 

Tube Compensation

Tube compensation (TC) is designed to compensate for endotracheal tube resistance via closed loop control of calculated tracheal pressure. The proposed advantages of ATC are to overcome the work-of-breathing imposed by artificial airways, to improve patient/ventilator synchrony as a result of variable inspiratory flow commensurate with demand, and to reduce air-trapping as a result of compensation for imposed expiratory resistance. This system uses the known resistive coefficients of the tracheal tube (tracheostomy or endotracheal) and measurement of instantaneous flow to apply pressure proportional to resistance throughout the total respiratory cycle. Because in vivo tracheal tube resistance tends to be greater than in vitro resistance, incomplete compensation for endotracheal tube resistance may occur. Additionally, kinks or bends in the tube as it traverses the upper airway and accumulation of secretions in the inner lumen will change the tube’s resistive coefficient and result in incomplete compensation.

 

Whether endotracheal tube resistance poses a clinical concern for increased work-of-breathing in adults is controversial. The imposed work-of-breathing through the endotracheal tube is modest at usual minute ventilations for the tube sizes most commonly used for adults. Several recent studies cast doubt on the importance of endotracheal tube resistance during short trials of spontaneous breathing. For example, similar outcomes have been reported when spontaneous breathing trials were conducted with PSV (7 cm H2O) or with a T-piece. Moreover, it has been reported that the work-of-breathing through the endotracheal tube amounted to only about 10% of the total work-of-breathing. The work-for-breathing during a two-hour spontaneous breathing trial with a T-piece may be similar to the work-of-breathing immediately following extubation. Although prolonged spontaneous breathing through an endotracheal tube is not desirable due to the resistance of the tube, this may not be important for short periods of spontaneous breathing to assess extubation readiness.

 

Airway pressure-release ventilation

Airway pressure-release ventilation (APRV) produces alveolar ventilation as an adjunct to continuous positive airway pressure (CPAP). Airway pressure is transiently released to a lower level, after which it is quickly restored to reinflate the lungs. Because the patient is allowed to breathe spontaneously at both levels of CPAP, the need for sedation is potentially decreased, hemodynamics are potentially better, dependent atelectasis may be less, and oxygenation may be better. Tidal volume for the APRV breath depends on lung compliance, airways resistance, the magnitude of the pressure release, the duration of the pressure release, and the magnitude of the patient’s spontaneous breathing efforts. Of concern is the potential for alveolar derecruitment during the release of pressure with APRV.

 

A modification of APRV is the situation in which the I:E ratio is not reversed. This is available on some ventilators as PCV+ (called BIPAP in Europe) or Bilevel. Without spontaneous breathing, PCV+ is similar to PCV and APRV is similar to PCIRV. One potential advantage of these modes is that the exhalation valve is active during both the inspiratory and expiratory phase. Prior to the current generation of ventilators, the exhalation valve was active during the expiratory phase, but closed completely during the inspiratory phase. An active exhalation valve during the inspiratory phase will open as necessary to maintain a constant inspiratory pressure. The use of APRV has become fashionable in some trauma centers, but evidence is lacking for improved patient outcomes compared to traditional ventilator modes.

 

One use of PCV+ (or Bilevel) is to provide sighs during PCV or CPAP. With this technique, several periods (2 to 4/min) of elevated airway pressure (25 to 35 cm H2O) is used periodically as a sigh (1 to 3 seconds at the higher pressure level). This approach differs from the sighs that were available in older generation of ventilators in several ways. 1) They are provided more frequently. 2) They are pressure limited. 3) They are applied for a time longer than the typical inspiratory times set on the ventilator. 4) Due to the active exhalation valve, the patient can continue to breathe spontaneously at the higher pressure. Although this strategy is attractive in spontaneously breathing patients prone to develop atelectasis, its benefit to date is anecdotal.

 

Dual Control Modes

Recently developed modes allow the ventilator to control pressure or volume based on a feedback loop (dual control). It is important to appreciate, however, that the ventilator can only pressure or volume - not both at the same time. Dual control within a breath describes a mode where the ventilator switches from pressure control to volume control during the breath. Dual control breath-to-breath is simpler because the ventilator operates in the either PCV or PSV, and the pressure limit increases or decreases to maintain the selected tidal volume.

 

Dual Control Breath-to-breath – Pressure limited flow cycled ventilation

Breath-to-breath dual control is available on several ventilators as Volume Support (VS). Its proposed advantages are to provide the positive attributes of PSV with a constant minute volume. This is closed-loop control of PSV, wherein tidal volume provides feedback control for continuously adjusting the pressure support level. All breaths are patient triggered, pressure limited, and flow cycled. The pressure support level varies breath-to-breath to maintain a constant tidal volume. The maximum pressure change is < 3 cm H2O and can range from 0 cm H2O above PEEP to 5 cm H2O below the high pressure alarm setting. Considerable speculation, but little data, suggests that VS will wean the patient from pressure support as patient effort increases and lung mechanics improve. If the pressure level increases in an attempt to maintain tidal volume in the patient with airflow obstruction, auto-PEEP may result. In cases of hyperpnea, as patient demand increases, ventilator support will decrease. This may be the opposite of the desired response. Additionally, if the minimum tidal volume chosen by the clinician exceeds the patient demand, the patient may remain at that level of support and weaning may be delayed.

 

Dual Control Breath-to-breath – Pressure limited time cycled ventilation

This approach is available as Pressure Regulated Volume Control (PRVC), Auto-Flow (Drager Evita 4), and Volume Control Plus (VC+). This approach provides the positive attributes of PCV with a constant minute volume. This mode is a form of pressure limited, time cycled ventilation that uses tidal volume as a feedback control for continuously adjusting the pressure limit. All breaths are ventilator or patient triggered, pressure limited, and time cycled. The pressure increases or decreases by £ 3 cm H2O per breath to deliver the desired tidal volume. The pressure limit fluctuates between PEEP and 5 cm H2O below the upper pressure alarm setting. The proposed advantage of PRVC is that it maintains the minimum peak pressure that provides a constant set tidal volume and automatic weaning of the pressure as the patient improves. Perhaps the greatest advantage of this mode is the ability of the ventilator to change inspiratory flow to meet patient demand while maintaining a constant minute volume. PRVC and similar modes are attractive with implementing lung protective strategies (such as the ARDSnet protocol), because the tidal volume can me set to 6 mL/kg and the peak pressure can be set to 30 cm H2O. However, only anecdotal support of this approach is currently available.

 

Conclusions

New ventilator modes and related features have become available over the past decade, with the claim that they improve the efficiency and safety of mechanical ventilation. Some also claim that these modes facilitate the weaning process. The decision to apply a particular mode of ventilation, however, should also be based upon an understanding of the underlying physiology. Just because a new mode does what it claims does not mean it will be more useful than existing modes. Unfortunately, there is very few clinical outcomes data upon which to base a decision regarding the choice of ventilator mode. The choice of a particular mode is often based on clinician experience and bias, institutional preferences, and the capabilities of the ventilators available at that institution.

 

References

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