Tools
Average Volume-Assured Pressure Support
Introduction
Respiratory failure is one of the most important and common causes of patient admissions to the intensive care unit. It has a plethora of causes and, depending on the duration of the condition can be broadly categorized as either acute or chronic respiratory failure. Treatment of the underlying cause and supportive measures involving the utilization of ventilatory support form the fundamental principles in the management of respiratory failure.[1][2][3]
In the last few years, non-invasive ventilation has gained increased prominence in the management of a variety of conditions causing acute as well as chronic respiratory failure. As studies demonstrating its benefits have materialized, its usage has become more widespread and widely accepted. Contrary to invasive ventilation, which uses an endotracheal or tracheostomy tube, non-invasive ventilation makes use of nasal masks, oronasal masks, total face masks, mouthpieces, or helmets for respiratory assistance. Without causing any of the complications of invasive ventilation, non-invasive ventilation ultimately achieves adequate gaseous exchange, correcting hypoxemia and/or hypercapnia. Different modalities of non-invasive ventilation exist, with continuous positive airway pressure (CPAP) and bilevel positive airway pressure (BiPAP) being the most commonly used modes. Average volume-assured pressure support (AVAPS) is a relatively newer modality of non-invasive ventilation that integrates the characteristics of both volume and pressure-controlled non-invasive ventilation.[4][5][6][7]
This review article focuses on AVAPS, its functioning, clinical significance, complications, and interprofessional communication to enhance health care outcomes.
Function
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Function
When the Respironics V60 ventilator debuted in 2009, it was equipped with an intelligent newer modality of pressure-control ventilation called the average volume-assured pressure support (AVAPS). This mode doesn’t contain an inspiratory positive airway pressure (IPAP) setting. Instead, it has been replaced with a target tidal volume (VT) setting.
Rather than having one fixed IPAP setting, the AVAPS mode can set a range of values for the IPAP, a maximum and a minimum IPAP. The pressure support is no longer fixed as the IPAP changes by itself within the set range. The ventilator does this based on the targeted tidal volume, a pre-set value. It uses a feedback loop to either increase or decrease the inspiratory pressure from breath to breath in order to ensure the pre-set tidal volume is delivered. Patient-ventilator dyssynchronization is prevented as the inspiratory pressure changes smoothly, thus improving patient comfort.
The tidal volume also varies with each breath, but the machine ensures that the average targeted tidal volume over the course of one minute is achieved.
Although there is a set range for the IPAP, the AVAPS has to select an initial inspiratory pressure for the first breath. It uses either one of the following three algorithms to calculate this value.
- IPAP min
- VT/60 ml/cmH2O + EPAP
- 8 cm/H2O + EPAP cmH2O
The calculation which produces the highest value will be the initial IPAP setting. AVAPS automatically calculates, selects, and then institutes the highest value as the initial IPAP setting.
In AVAPS, the expiratory positive airway pressure (EPAP) is fixed; however, the auto-titration mode of non-invasive ventilation, 'average volume-assured pressure support-auto-titrating EPAP'(AVAPS-AE), regulates EPAP as well.
The ventilatory settings in AVAPS mode are typically predetermined, based on the patient’s condition and his clinical assessment, and are then manually set on the ventilator. The target tidal volume is set to 8 ml/kg of ideal weight and adjusted based on the patient’s pathology. The maximal IPAP value is generally fixed at 20-25 cm H20, and the minimal IPAP value equals EPAP + 4 cm H20. The value of the minimal inspiratory pressure is no less than 8 cmH2O and is commonly higher. The respiratory rate is set at 2-3 BPM below the resting respiratory rate.
It takes several minutes for the AVAPS to attain the targeted tidal volume. IPAP automatically increases or decreases in synchronization with changes in the patient’s respiratory effort, lung compliance, or extrinsic lung resistance.[8][9][10][11][4][12]
Clinical Significance
AVAPS in Chronic Respiratory Failure
-
Stable hypercapnic chronic obstructive pulmonary disease (COPD) - AVAPS has been proven safe and reliable in the management of stable hypercapnic COPD patients. Besides, it is a more favorable treatment strategy compared to the non-invasive fixed inspiratory pressure support as the patients perceive better sleep efficiency and quality of life.[13][14]
-
Obesity hypoventilation syndrome - Not only is AVAPS as competent as the BPV S/T modality of non-invasive ventilation in improving oxygenation, sleep quality, and health-related quality of life (HRQOL), but it also has the added advantage of decreasing PtcCO2 more efficiently.[15]
-
Kyphoscoliosis - AVAPS has been associated with a significant improvement in arterial blood gases and forced vital capacity of patients with kyphoscoliosis-related chronic respiratory failure. It was well accepted by the patients, resulting in good patient compliance with the treatment, and is thus an effective treatment option.[16]
-
Congenital central hypoventilation syndrome - Different case studies have found the AVAPS to be beneficial in the management of pediatric patients with congenital central hypoventilation syndrome. The evidence has reported successful management of infants with this syndrome through AVAPS. Furthermore, the successful transition from invasive ventilation to non-invasive ventilation via AVAPS was demonstrated in children and teenagers as well, with this syndrome.[17][18][19]
AVAPS in Acute Respiratory Failure[20][21][9][10]
- AVAPS has been successful in the treatment of COPD-associated acute hypercapnic respiratory failure. Significant improvement was seen in arterial blood gases, pH, respiratory rate, and the Glasgow Coma Scale. Nonetheless, it is important to note that the patients with a higher APACHE II score, calculated during ICU admission, had a higher risk of treatment failure.
- Although both AVAPS and BiPAP are reliable in the treatment of acute hypercapnic respiratory failure, AVAPS is associated with a speedy improvement of the patient clinically. The patients have also experienced greater comfort and satisfaction comparatively, and therefore, improved treatment-compliance.
- In patients suffering from acute exacerbation of COPD compounded with hypercapnic encephalopathy(GCS<10), treatment with BiPAP S/T + AVAPS was superior compared to the treatment with BiPAP S/T alone, as evidenced by a more rapid improvement in the arterial blood gases and Glasgow Coma Score. Remember that both therapies are effective, and there was no difference in the duration of hospital stay or the time period that the patient was on non-invasive ventilation.
- AVAPS can be considered as the preferred treatment in patients with acute exacerbation of COPD as well as obstructive sleep apnea in comparison to regular non-invasive ventilation.
- AVAPS has been documented to facilitate successful extubation in Acute respiratory distress syndrome (ARDS), thus demonstrating its potential in saving ventilators and setting them aside for sicker patients at times of need.
Other Issues
Non-invasive positive pressure ventilation (NIPPV) is associated with substantial financial benefits without compromising the quality of patient healthcare. It is associated with a decreased risk of pneumonia, sepsis, and mortality compared to invasive positive pressure ventilation.[4] Data from several hospitals has shown significant per annual savings for each patient following whether they received non-invasive ventilation or not.[22]
Similar complications and contraindications exist for the varying modes of noninvasive positive-pressure ventilation (NIPPV), including AVAPS, and comprises the following:
- Mucosal dryness/plugging
- Nasal congestion
- Sinus/ear pain
- Claustrophobia
- Air leaks
- Fascial erythema/abrasions due to tight fascial mask
- Gastric insufflation
- Treatment failure
- Hypotension (seen in patients with cardiac dysfunction)
- Pneumonia (infrequent)
- Barotrauma (rare)
Contraindications[25]
- Respiratory arrest
- Cardiac arrest/hemodynamic compromise/unstable arrhythmia
- Fascial trauma/surgery/burns
- Blockage of upper-airway
- Impaired cough reflex/swallowing (lack of airway protection)
- Profuse gastrointestinal bleeding
- Severe encephalopathy
Enhancing Healthcare Team Outcomes
AVAPS is a relatively newer modality of non-invasive ventilation. It necessitates good collaboration amongst the interprofessional team of medical professionals to ensure the appropriate selection of candidates for average volume-assured pressure support.
Operating a ventilator is occasionally considered difficult and requires good experience and training to ensure successful therapy without causing any negative harm. To avert the risks associated with it, effective communication and teamwork between the critical care specialist, pulmonologist, internist, critical care nurse, respiratory therapists, and the healthcare workers in the ICU is essential to resuscitate the patient and improve patient outcomes.[26]
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