Mechanical ventilation is a cornerstone in the treatment of respiratory failure in critically ill patients. Over the last decades the field of mechanical ventilation has developed from the use of monotonous methods of mechanical ventilation that dictate the breathing volume and frequency towards methods that support breathing while allowing the patient the freedom to attain a natural and variable breathing pattern. Another shift of paradigm is the increasing application of non-invasive ventilation, liberating the upper airways by avoiding endotracheal intubation. The present article will share the experiences of introducing neural control of mechanical ventilation.
by Dr Christer Sinderby and Dr Jennifer Becket al  as well as Agostoni et al  published new work on trans-oesophageal measurements of electrical activity of the human diaphragm opening up a new avenue on how to monitor neural breathing efforts.
In 1999, we and coworkers  introduced the first method that allowed assisted breathing in critically ill patients to be controlled by their own diaphragm electrical activity. As the diaphragm electrical activity results from neural respiratory output signals coming from the brain, transferred via the phrenic nerves, it represents the neural effort to breathe and is regulated by input from multiple respiratory reflexes feeding back to the respiratory centres.
Neurally Adjusted Ventilatory Assist (NAVA)Monitoring electrical activityNon-invasive ventilationOvercoming the challenge of upper airway interference
Another challenge of non-invasive ventilation is that removing the endotracheal tube exposes the ventilatory assist delivery to interference from the upper airways. The upper airways constitute a crossroad between air and food passages controlled via a complex neural network with the aim of coordinating respiration with e.g. swallowing, speech, cough and vomiting such that no food or other substances aiming for the oesophagus enters the trachea, that no gastric content leaves the oesophagus, and that no air enters into the stomach. Apart from issues of leaks and interface, one common complication of non-invasive ventilation is air passing into the stomach, so-called gastric insufflation, which could lead to severe complications in newborns. It is suggested that gastric insufflation is caused by high pressures of assist but it is likely that it is also linked to poor synchrony of assist. As the act of swallowing occurs during a period of apnea, when the glottis is closed, and is typically followed by an expiration, no diaphragmatic activity occurs during swallowing. Hence, no assist delivery can occur during NAVA since the presence of diaphragm electrical activity is required for the breath to be delivered.
In contrast, a mode of ventilatory assist that terminates assist based on either pneumatic signals or by time criteria can also continue to deliver assist during swallowing. Since the upper oesophageal sphincter pressure (protecting reflux) is relaxed, little pressure should be required for air to enter into the food passage. An important feature of the upper airways is that they are synchronised to breathing and that the glottis and the upper airway passage widens during every inspiration. Delivery of assist in synchrony with the neural inspiration should thus result in a less resistive load. During exhalation, the glottis and upper airways can narrow and restrict flow, a function believed to be linked to maintained lung recruitment. Hence, asynchronous delivery of assist during the exhalation phase is unlikely to be favourable in terms of efficient assist delivery. The most efficient non-invasive assist delivery should hence occur when inspiratory muscles are active, such that the lowest possible pressure is required to inflate the lungs with the least interference of the upper airways. Cough is a complex three step activity initiated by 1) an inspiration, which when interrupted is followed by 2) an expiratory pressure generation against a closed glottis, and 3) opening of the glottis. The diaphragm is only active during the first two steps such that coughing is probably facilitated by NAVA, whereas a mode using a pneumatic controller or timer to terminate assist probably also delivers assist during the 3rd step and interferes with coughing. Another factor during non-invasive ventilation is that the patient actually has the ability to speak. Speech is a voluntary activity associated with rather arrhythmic contractions of the diaphragm. Due to its close neural integration, NAVA is not likely to interfere with speech, whereas a conventional mode using a pneumatic controller or timer to terminate assist introduces problems with timing of assist in synchrony with speech.
In summary, new developments in mechanical ventilation introducing neural control of assist delivery improve the quality of monitoring and assist delivery and open up great opportunities for extending ventilatory care in critically ill patients into previously untouched areas.
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Dr. Christer Sinderby, PhD1,2 and
Dr. Jennifer Beck, PhD1,3
6th floor, room 611
209 Victoria Street
Canada. M5B 1T8
30 Bond St
Canada M5B 1W8
Tel. +1 416 880-7507
3 Department of Pediatrics
University of Toronto
Tel. +1 416 880 3664