Newer Modes of Mechanical Ventilation

Recently, modes of mechanical ventilation that synchronise not only the timing, but also the level of assist to the patient's own effort, have been introduced

"Microprocessor based technology has led to the development of newer modes, aimed at improving ventilatory support" - Dr Deepak Govil Consultant Intensivist Critical Care Artemis Health Institute New Delhi

Mechanical ventilation, originally used for resuscitation, consisted of simple, manually-driven pump devices. With the advent of positive pressure ventilators, mechanical ventilation has become an integral part of the care of patients with acute respiratory failure. The crudest form delivers fixed volumes or pressures with fixed rates and breath durations, while more advanced modes allow the patient to trigger the ventilator with their own effort, which can then be cycled off when certain criteria have been met, attempting to mimic natural breathing.

The primary goals of mechanical ventilation is to 'buy time' to give patient a chance to recover from some catastrophe by providing adequate gas exchange and unloading the respiratory muscles without causing any further damage to respiratory muscles or lung parenchyma.

The early re-institution of spontaneous breathing in mechanically-ventilated critically ill patients becomes an important therapeutic option to avoid the various complications of controlled mechanical ventilation and facilitate the weaning process. In spontaneously breathing patients, it is also important to achieve patient-ventilator synchrony, that is, the ventilator needs to cycle-on in unison with the onset of the patient's effort. The level of assist delivered should correspond to the patient's demand and most importantly, the ventilator breath should be cycled-off at the end of the patient's inspiratory effort.

Co-ordination between spontaneous breathing and mechanical assistance may not be achieved always and a poor interaction between patient and machine may cause either excessive or insufficient ventilatory support or dys-synchrony. Microprocessor-based technology has led to the development of newer modes aimed at improving ventilatory support.

ASV

Adaptive Support Ventilation (ASV) is a pressure controlled closed-loop system which allows adaptation of assistance during all phases of mechanical ventilation, from control ventilation to weaning. The main principle of functioning is based on Otis's formula which is stored in microprocessor of ventilator. Using this formula, the ventilator can calculate an 'ideal' respiratory pattern (tidal volume and respiratory rate), which needs the smallest total energy expenditure, providing specific minute ventilation, a calculated dead space, which depends on body weight and an expiratory time constant. Minimum minute ventilation is the only specific setting that must be chosen by the clinician. It is based on patient's body weight.

When starting ventilation in ASV, the ventilator provides three pressure control time cycle inspirations and calculates respiratory mechanics. Expiratory time constant is estimated from the tidal volume curve during each expiration. Then, using Otis' formula, a target respiratory rate is calculated. Target tidal volume is computed from minimum minute ventilation and target respiratory rate. Thereafter, target values are calculated cycle-by-cycle. Depending on patient's spontaneous respiratory rate, ASV can works as Pressure Control Ventilation (PCV), if there is no spontaneous breathing, as pressure control SIMV, when patient's respiratory rate is smaller than target, or as Pressure Support Ventilation (PSV), if patient's respiratory rate is greater. Pressure level is then adapted to attain the target tidal volume (within limits imposed by pressure alarms). Cycling off criteria is flow-based in the case of assisted ventilation or time-based for mandatory inspiration.

With ASV, the patient has a high degree of breathing freedom within each breath, since the breath delivery by ventilator is by controlling the inspiratory pressure and never by forcing a given inspiratory flow. With ASV, the user-set minimum minute ventilation is always guaranteed, with a theoretically optimal ventilatory pattern, while the patient always maintains the freedom of increasing his minute ventilation above the user-set target.

Several studies have shown that ASV can provide safe and effective ventilation in patients with normal lungs, restrictive or obstructive diseases. Other benefits include decrease of patient's respiratory efforts, stability of alveolar ventilation without operator's intervention and safety during weaning in selected situations. However, many important unsolved questions remain to be answered for ASV like how minimum ventilation must be set and how ASV must be adapted faced with changes in breathing demand and how weaning must be handled?

New Modes

Recently, modes of mechanical ventilation that synchronise not only the timing, but also the level of assist to the patient's own effort have been introduced, such as Proportional Assist Ventilation (PAV) and Neurally Adjusted Ventilatory Assist (NAVA). PAV and NAVA have been designed with the goal of improving patient-ventilator interaction by matching the ventilator support with the neural output of the respiratory centres. With PAV, the support is continuously re-adjusted in proportion to the predicted inspiratory effort. NAVA is a mode in which the assistance is delivered in proportion to the electrical activity of the diaphragm, assessed by means of an esophageal electrode.

PAV: Novel Mode

Proportional assist ventilation (PAV) is a novel mode of partial ventilatory support in which the ventilator generates an instantaneous inspiratory pressure in proportion to the instantaneous effort of the patient, that is, the more the patient pulls, the more pressure the machine generates. Thus, with PAV the ventilator amplifies the patient's inspiratory effort without any preselected target volume or pressure. There are two main forces to overcome when breathing in.

Firstly, the resistance of the airways and secondly the breathing circuit and the elasticity of the chest wall and lung. In PAV, the support that is applied at any time point during inspiration is in proportion to the work required to overcome the individual resistive and elastic forces. To offload the resistive work, pressure is applied in proportion to the flow rate, and to offload the elastic work, pressure is applied in proportion to the volume inhaled. To apply PAV correctly, the resistance and elastance need to be known. The degree of support applied, is then set as a percentage between zero percent and 90 per cent. Levels of support above 90 per cent are not applied due to the risk of amplification or 'runaway.' The aim of PAV is to allow the patient to attain whatever ventilation and breathing pattern seems to fit the ventilatory control system. PAV follows the equation of motion, which states that the pressure applied by the respiratory muscles to the system, is used to overcome the elastic and resistive opposing forces. The former is proportional to the volume displacement whereas the latter is proportional to the airflow rate, neglecting inertia.

In theory, PAV should normalise the neuro-ventilatory coupling by making the ventilator an extension of patient's respiratory muscles, while leaving to the patient the entire control of all aspects of breathing. PAV, however, shares a common problem with the conventional partial ventilatory support modes. In mechanically ventilated patients, the respiratory system impedance may change over time. These changes may impair the good matching between ventilator output and patient's ventilatory demand and lead to patient-ventilator asynchrony. The necessity of regular measurements of respiratory system mechanics and adjustments in ventilatory setting, however, imposes a major obstacle to the widespread use of this mode. (See Graph)

To overcome these problems, an ideal ventilator should be able to record the activity of the respiratory neural system, and use that measurement to select a satisfactory tidal volume, but at this time, it is not feasible to record the activity of the respiratory centres in patients.

NAVA: Latest Module

The newly introduced NAVA, with the recording of the electrical activity of the diaphragm, comes close. NAVA provides pressure assistance in proportion to the electrical activity of the diaphragm, thereby ensuring a positive relationship between the ventilator assistance and the patient's spontaneous effort. The conventional ventilators have a pneumatic trigger and they deliver ventilatory assist much after the diaphragmatic activity and this leads to increased effort by the patient and ventilator-patient asynchrony whereas NAVA uses the electrical activity of the diaphragm (EAdi) during spontaneous breathing to trigger, cycle off and adjust the intra-breath assist profile in proportion to the EAdi. This is done with the help of a special electrode, 12 cms long which is placed over an esophageal catheter and inserted like a nasogastric tube. The signals coming from these electrodes undergo a computer based analysis to adjust the ventilatory assist for the delivered breaths. Signals coming from cardiac pulsation and esophagus are filtered and then the remaining signals are amplified into a processing unit. These signals are finally delivered to the ventilator, which finally assists the patient proportional to the strength of the EAdi and a fixed gain constant. A user-adjustable proportionality factor (the gain) is available to set the amount of ventilator assistance for a given level of EAdi. EAdi represents the global activity of the diaphragm and so the intactness of the phrenic nerve is necessary for the reliability and strength of the signal. (see diagram).

The important advantages of NAVA are that it offers better patient-ventilator synchrony, off-loading of inspiratory muscles, better response time and the tight coupling between neural output and ventilator function allows the ventilatory assist to be independent of changes in lung and ribcage elasticity, flow resistance, intrinsic Positive-End-Expiratory Pressure (PEEP), leakage, or abdominal compliance. Due to these properties, NAVA can also be used effectively in delivering non-invasive ventilation as an alternative to endotracheal intubation in hypoxemic respiratory failure.

Drdeepak_govil@yahoo.co.in