Continuing Education Activity

Adaptive support ventilation (ASV) is a type of mechanical ventilation, which is a relatively newer mode of closed-loop ventilation. Other modes of mechanical ventilation include proportional assist ventilation, neurally adjusted ventilatory assistance, and knowledge-based systems. Many of the contemporary modes of ventilation are known to result in barotrauma, volutrauma, ineffective weaning, or prolonged weaning, higher hospital costs, and demanding one-to-one and vigilant, critical care staff. This activity reviews the ventilatory mode of adaptive support ventilation and its advancements in ventilating patients in critical care. This article demonstrates the advancements in mechanical ventilation and highlights the role of the interprofessional team in improving care for patients with ASV mode.

Objectives:

• Describe the mechanism and principles of mechanical ventilation in the mode of adaptive support ventilation.
• Review the latest clinical studies and the stance of adaptive support ventilation in the present day.
• Identify the conditions intensivists might use or prefer ASV over other modes of ventilation.
• Explain the advantages of ASV over other forms of ventilation.

Introduction

Adaptive support ventilation (ASV) is a type of mechanical ventilation which is a relatively newer mode of closed-loop ventilation. The feature was introduced in the Galileo ventilator (Hamilton Medical, 1994). Hewlett first described it in 1977 as a form of mandatory minute ventilation (MMV) with adaptive pressure control.[1] The invention is credited to Dr. Fleur T Tehrani, who used a modified Otis equation.

ASV is also called the “no mode” or “integrated mode” or the “three in one way” because of its highly adaptive characteristic to alter its ventilatory settings, which are not found in other closed-loop modes of ventilation. Other modes include proportional assist ventilation (PAV), neurally adjusted ventilatory assistance (NAVA), and knowledge-based systems (KBS).

Indications

One of the key benefits of adaptive support ventilation (ASV) is the earlier recognition of spontaneous breathing.[2] There have been several studies showing faster weaning times, fewer manipulations with ASV, and fewer alarms when compared to conventional modes of ventilation.[3] Based on the status of the patient, the switch occurs from pressure control ventilation for passive breathers to adaptive pressure support ventilation for those initiating spontaneous breathing. The period of weaning is seen to be reduced in COPD patients with the incorporation of ASV in various randomized controlled trials. And contrasting to other modes of ventilation, we have the leeway to plug into this mode of ventilation, beginning from intubation to extubation.

Various weaning protocols have been tried in the mode of ASV for better outcomes after cardiac surgery – namely, in the case of post-elective coronary artery bypass grafting. In a randomized controlled trial comparing the efficacy of weaning under ASV, the protocol using decremental target minute ventilation served a better outcome and earlier extubation compared to that of maintaining a constant target minute ventilation.[4] This also enables a better quality of life for patients who were provided ASV with the former mode, thus contributing to a lower cost of hospitalization.

Contraindications

One of the main contraindications for the use of adaptive support ventilation (ASV) is in obese patients, post-major cardiothoracic surgery low on functional residual capacity (FRC) who are prone to derecruitment. Those with restrictive lung disease tend to be provided with low tidal volumes.[5] The other disadvantage of this mode of ventilation is that it is available only on Hamilton ventilators and not available on any other mode of ventilators. The other issue with this mode is that you are unable to set tidal volume or respiratory rate, in certain conditions such as hypercapnic respiratory failure, you may want to change those parameters on the ventilators.

Equipment

ASV is a type of closed-loop control mechanical ventilation that uses the 'inter-breath control method' – i.e., to maintain a respiratory setting between breaths, and not change it during the breath cycle. The respiratory variables are continuously obtained by the microprocessor and assessed based on positive and negative feedback. Then it is used to adjust the ventilator settings automatically to accommodate the physiological and individualized variations for ventilator support.[5]

The ventilator allows us to set the maximum ventilator plateau pressure and desired minute ventilation, based on the ideal body weight (IBW). Gathering the user's input and the collected respiratory mechanic's data from the ventilator monitoring system (resistance, compliance, and auto-PEEP), it selects a target ventilatory pattern. ASV can be used in both acute and chronically ventilated patients, and as a strategy for the initiation, maintenance, or weaning.

Ventilator Settings:

1. Height of the patient – Used to calculate the IBW of a patient;
2. Weight - Calculation of dead space by 2.2 ml/Kg
3. Gender
4. Percentage of the minimum volume of V
   1. Normal – 100%
   2. Asthma – 90%
   3. ARDS – 120%
   4. Others – 110%
   5. If T > 101.3 F – add 20%
   6. Altitude – 5% per 500m above sea level
5. Trigger – flow rate of 2 L/min
6. Expiratory trigger sensitivity – to start with 25% to 40% in COPD
7. Tube resistance compensation – to 100%
8. High-pressure alarm
9. PEEP
10. FiO2

Personnel

Adaptive support ventilation (ASV) is highly impactful in the context of the respiratory therapist shortage in intensive care units. ASV allows automation to take over the controls rather than to depend on human resources for minute setting changes. ASV is also associated with lesser alarms, and manual setting changes are evident in many studies of the past.[6][7][8] 

When the patient begins the attempts to spontaneous breaths, there occurs an automatic decrease of PS. Such subtle reductions in ventilator demand are often missed by non-automated and human-operated modes of ventilation, thus taking more time for weaning. ASV  allows a reduced requirement for skilled human resources in the care of the ventilated patient.

Preparation

Operating Principles

Based on the IBW (according to the Radford nomogram), the provider must set the target minute ventilation, PEEP, FiO, and maximum pressure alarm (P). With these provided variables, ASV selects the respiratory rate (RR), inspiration-expiration ratio (I: E ratio), and tidal volume (V).

Otis et al. and Mead et al. Equations:

Adaptive support ventilation (ASV) uses the equation by Otis et al.[9] and Mead et al., which was derived in the 1950’- 60’s respectively, which states that for a given set of alveolar ventilation, there is a particular RR that achieves a lower work of breathing (WOB).

General Principles

In ASV, the inspiratory pressure and the respiratory rate (RR) are automatically adjusted by a microprocessor to help minimize the WOB. The microprocessor switches from controlled ventilation (CV) to assisted ventilation (AV). Hence, in general, if the patient’s RR is higher than the target, pressure-controlled ventilation if there is no spontaneous breathing, and a synchronized intermittent mandatory ventilation (SIMV) when the patient’s RR is lower than the target. The rationale is to provide a natural respiratory pattern to the patient and thereby reduce the WOB on the respiratory muscles and therefore stimulate spontaneous breathing. The optimal RR-V combination associated with the least WOB is derived from the Otis equation.[9]

ASV maintains an operator preset minimum minute ventilation. Having MMV, the device administers mandatory volume or pressure-controlled mandatory breaths to maintain the preset minute ventilation. The inspiratory pressure and the mandatory rate are fluctuant according to the patient’s lung mechanics – thus providing different tidal volumes (V) and respiratory rates (RR).[10] 

The automation in ASV has a high potential to select adequate ventilatory patterns based on the condition suffered by the patient – i.e., normal lungs, restrictive or constrictive diseases.[11][12] It takes into account of spontaneous breathing, and it is useful to prevent tachypnea, the development of auto-PEEP, and excessive dead-space ventilation. In comparison to other modes of ventilation, it has been demonstrated to be superior or equally efficacious to SIMV with pressure support (PS).[13][6]

Complications

The goal of adaptive support ventilation (ASV) mode is to ensure adequate alveolar ventilation, minimizing the WOB and transitioning the patient to an optimal ventilatory pattern by deductively preventing barotrauma, volutrauma, and air-trapping.[10] Damage of the lung tissue due to high-volume ventilation occurs with surfactant inactivation and increased microvascular permeability.[14]

Clinical Significance

1. Post-Cardiac Surgery: A randomized control trial of post-operative weaning of patients after planned and uncomplicated non-fast-track coronary artery bypass graft, weaning with ASV was found to take a similar time to tracheal extubation compared to that of standard weaning – SIMV and PS.[15] Whereas in the case of fast-track patients after cardiac surgery, it was seen that there is a reduction in time to tracheal extubation. Rapid tracheal extubation is of implicative clinical importance because it has been shown to reduce ICU length of stay, hospital length of stay, resource utilization, and cost.[16] The time to extubation is determined by various factors such as rewarming, post-operative bleed, and control of hemodynamic problems, which comprise the “window of opportunity.” ASV has also been shown equally efficient to the pressure control (PC) followed by PS [15] for those who have undergone cardiac surgery. Petter et al. conducted a study comparing the ventilator extubation of post-cardiac surgery patients in ASV mode and SIMV + PS mode. It demonstrated equivalent outcomes, although there were fewer alarms and ventilator setting manipulations in the ASV mode.[6] Dongelman et al. demonstrated in an RCT that ASV is safer and more useful even when extubation times were the same in post-coronary artery bypass graft surgery patients when compared to those placed in PC with PS.[15]

2. ASV for Weaning: Patients with chronic respiratory illnesses requiring ventilation can be placed on ASV mode. It has shown benefits as of lesser costs and requirement of respiratory therapists and intensive care personnel.[17]

3. Lung Protective Strategy: ASV provides a better patient-ventilator interaction and lowers WOB compared to SIMV.[18] On the other hand, barotrauma due to high plateau pressure seldom occurs in ASV as it automatically adjusts the airway pressure, thus lowering the tidal volume. In a recent randomized control trial by Dai et al., it was shown that there is no significant difference in mortality in acute respiratory distress syndrome patients placed on either ASV or PC ventilation modes. Moreover, it was understood in animal studies that ASV had induced lower alveolar strain compared to volume-control ventilation, hence lesser lung injury. ASV mode of ventilation controls tidal volume in the range of 6-8 mL/kg of IBW. This implies that patients with ARDS would benefit from opting for the ASV mode of ventilation compared to other modes.[14]

4. Acute Exacerbation of COPD: Non-invasive ventilation (NIV) compared to invasive positive pressure ventilation (IPPV) decreases the risk of ventilator-associated pneumonia (VAP), sepsis, sinusitis and hence reduces the rate of hospitalization and mortality.[19] NIV is the standard for acute exacerbations of COPD (AECOPD). A randomized control trial comparing NIV support of PSV vs. ASV demonstrated that both are viable options without significant differences in mortality.[20] This study by Sehgal et al. also confirmed that there was a lower tracheal intubation rate in patients who were provided ASV.

5. Pediatric Conditions: Proper studies and literature are inadequate for advocating the use of ASV in children. However, there have been reports of positive outcomes from case reports and small observational studies.

Enhancing Healthcare Team Outcomes

Ventilator management is a multidisciplinary effort. When you put a patient on adaptive support ventilation (ASV) mode on a ventilator, it requires the clinical staff to be trained and familiar with this mode of ventilation. At the same time, this is an extremely safe mode of ventilation and weaning. A point to consider is, there is no backup mode while you wean a patient on this mode. There is a risk of hypoventilation and hypercapnia if you have set the patient on the lower percent of minute ventilation support. Familiarity and training regarding this mode of the ventilator are essential among the staff when they are managing the patient on the adaptive mode of the ventilator.