22.Xirouchaki N, Kondili E, Vaporidi K, Xirouchakis G, Klimathianaki M, Gavriilidis G, et al. Although they are popularly used and are well-understood, often they fail to match the patient based requirements. Closed-loop control of mechanical ventilation: Description and classification of targeting schemes. Positive outcome of average volume-assured pressure support mode of a Respironics V60 Ventilator in acute exacerbation of chronic obstructive pulmonary disease: A case report. Patient-ventilator interactions during volume-support ventilation: Asynchrony and tidal volume instability - a report of three cases.
Adaptive support ventilation: An appropriate mechanical ventilation strategy for acute respiratory distress syndrome?
Weaning automation with adaptive support ventilation: A randomized controlled trial in cardiothoracic surgery patients.
Proportional assist ventilation (PAV): A significant advance or a futile struggle between logic and practice?
Proportional assist ventilation with load-adjustable gain factors in critically ill patients: Comparison with pressure support.
Proportional assist ventilation reduces the work of breathing during exercise at moderate altitude. A crossover analysis of mandatory minute ventilation compared to synchronized intermittent mandatory ventilation in neonates.
The impact of spontaneous ventilation on distribution of lung aeration in patients with acute respiratory distress syndrome: Airway pressure release ventilation versus pressure support ventilation.
Airway pressure release ventilation: An alternative mode of mechanical ventilation in acute respiratory distress syndrome. Clinical review: Update on neurally adjusted ventilatory assist-report of a round-table conference. Over the years, many small modifications in ventilators have been incorporated to improve patient outcome. The ventilators of newer generation respond to patient's demands by additional feedback systems. In this review, we discuss the popular newer modes of ventilation that have been accepted in to clinical practice. Various intensive care units over the world have found these modes to improve patient ventilator synchrony, decrease ventilator days, and improve patient safety.Key ideas in understanding mechanical ventilationThe basic understanding into mechanical aspects of ventilation comes from understanding the physics of flow and pressure involved. Before we elaborate the rationale for newer modes of ventilation understanding the key concepts in ventilation are critical. The right hand side of the equation can be expanded to show that elastic recoil pressure is the product of elastance time's volume, while resistive pressure is the product of resistance and flow.
The same are either provided or overcome by the ventilatorClick here to viewThe other aspect of modifications targets "the ventilatory variables". These are as follows:Trigger variable This variable is what helps the ventilator to initiate a breath. The more sensitive is ventilator in sensing the lesser is likely to be patient work of initiation of breathing. Holding, inspiratory flow constant during inspiration causes volume and pressure to rise linearly. In contrast in pressure control ventilation [Figure 4], airway pressure is held constant during inspiration.
This causes inspiratory flow to fall exponentially from its peak value toward 0 as volume rises exponentially.

A ventilator setting appropriate for one point of time may not be optimal with patient deterioration or improvement. The newer modes target to make alterations with the changing lung and take feedback from patient parameters, thus completing the feedback loop and are "Closed loop" type. This has lead to name "Dual control" ventilation .[9],[10] These adjustments can occur in a single initiated breath or in subsequent breaths and thus form the basis of classification of these modes. It makes the ventilator to switch from pressure control mode to volume control if a minimum set tidal volume is not achieved.
In patients where the lung compliance decreases this mode provides safety against increasing blood CO2 levels by maintaining minimal minute ventilation. One must understand if the set pressure limit is too high it will not only cause unwanted barotrauma to lung but also would generate higher volume than minimal set tidal volume, thus negating the volume guarantee. Set flow rates must not be very low; in situations where minimal volume is not met it would cause a delayed switch from pressure control to volume control and would lead to unwanted prolongation of inspiratory time. Evidence in favor of this mode has shown that it leads to decreased work of breathing and lower intrinsic PEEP.
Figure 5: Ventilatory graphics in VAPS (Volume assured pressure support mode) modeClick here to view[12],[13] Dual control in subsequent breathsThe ventilator takes feedback from output parameters of initial breaths and makes automatic adjustments to set targets in the subsequent breaths.
Volume support is a useful mode for weaning as once the patient begins to achieve a desired tidal volume it automatically decreases the pressure support, thus gradually loading the respiratory muscles to take on work of breathing. The basic caution prior to use of this mode is that patient must have spontaneous breathing activity else the ventilator will automatically shift to backup mode.
Also if the set tidal volume is too large, the ventilator will raise the pressure support to achieve it and lead to problems like barotrauma, hemodynamic compromise, and intrinsic PEEP. The control variable is "pressure" and is capable of delivering both pressure control or pressure support breaths. The operating principle is based on pressure controlled synchronized intermittent mandatory ventilation with automatic adjustments to set pressure level and respiratory rate on the basis of measured lung mechanics in the previous breaths.
The target ventilation (tidal volume and respiratory rate) corresponds to the best combination from energy standpoint estimated by automatic measurements of lung compliance and resistance. It can be used as partial to full support mode Acute respiratory distress syndrome (ARDS), asthma, and weaning.
The major limitation if this mode is inability to recognize dead space ventilation or shunts to make adjustments to ventilation. Auto-PEEP may become problem in chronic obstructive pulmonary disease (COPD) patients needing longer expiratory times which are currently unaccounted in current protocols of automatic adjustments.Proportional assist ventilation (PAV)PAV is a promising newer mode with advantage of improving ventilator patient synchrony. It is a form of synchronized partial ventilator assistance with a unique feature that ventilator assists in proportion to patients instantaneous effort. PAV unloads respiratory muscles and unaltered respiratory pattern allows synchrony between ventilator and patient's neural ventilator drive.
The therapist sets the percentage of assistance to be delivered and other parameters are adjusted automatically by patients "air hunger." As the ventilator on take on only a part of breathing work this mode is unlikely to cause respiratory muscle atrophy in chronically ventilated patients. Parameters like I:E ration and inspiratory time are completely patient controlled and further add to successful patient synchronization. The potential disadvantage noted it that if the patient worsens or improves, the proportion of assistance may needed to be reset according to newer clinical situation.
Recent trials with noninvasive PAV have also shown promising results for the above indications.Mandatory minute ventilation (MMV)It is a modification of pressure support ventilation. The ventilator takes feedback to alter both respiratory rate and the level of pressure support to achieve set minimum minute ventilation. The operator sets minimum minute ventilation required adjudging the present patient clinical state.

MMV forms a reliable weaning mode, [25] as the set value can be gradually decreased loading the respiratory muscles. In patients with apneic episodes or central drive pathologies, MMV sets safety by providing a set value ventilation as mandatory ventilation.
A maximal respiratory rate set too high can cause significant increase in ventilator frequency during ventilator automatic adjustment leading to "rapid shallow" breathing. The ventilator cycles between two set pressures: P-High and P-Low (high pressure and low pressure, respectively). The likelihood of ventilator-patient asynchrony is minimal due to the fact that the patient can breathe irrespective of ventilator cycle and these breaths can be aided by the ventilator. The release phase (expiratory phase) brings down mean airway pressure and plays significant role in maintaining normocarbia. The mode has dual functionality; in presence of spontaneous breathing the patient can breathe in any phase of respiratory cycle with supported breaths thus bringing down needs of sedation. In absence of spontaneous breathing activity, the bi-level pressure acts as time-cycled inverse ratio ventilation.
The therapist sets P high ,(high pressure in CPAP) P low ,(low pressure in CPAP) T high (time for P high )and T low (Time for P low ). Initial P high should be set to plateau pressure or the upper inflection point and P low should be set at lower inflection point on volume-pressure curve.
APRV should be used with caution in hypovolemic as increased intrathoracic pressure further lower venous return.
APRV should be avoided in patients with obstructive lung diseases as it can cause air trapping or rupture of bullae in COPD. APRV has been shown to improve oxygenation in patients with ARDS simultaneously decreasing need of sedation or paralysis. Unlike all other modes available that use pressure, flow or volume sensing to initiate a breath, NAVA uses diaphragmatic electromyogram to detect inspiratory effort of patient.
This mode over scores all other modes in synchronization of patients efforts to ventilator by having least possible delay among the two. Like PAV, NAVA assists in accordance to measured strength of diaphragmatic contractions and most of parameters like inspiratory time, I:E ratio are patient controlled. Leaks in the circuit can cause false initiation of breaths in conventional modes leading to severe ventilator asynchrony; NAVA, however, is completely unaffected by this phenomena as breath initiation is completely independent of physical properties of the circuit. Setting up a NAVA ventilator is straightforward and gain over diaphragmatic electrographic potential is the only single input required. A practical limitation with NAVA is placement of esophageal catheter with electrodes and its validity of longer duration of ventilation. The system mimics clinical approach of adjusting ventilator assistance depending on patient's respiratory pattern and literature based weaning protocols. Ventilator takes feedback from monitored respiratory rate, tidal volume, and end-tidal CO2.
Once the patient remains stable on a setting, it tries to gradually wean off to maintain optimal respiratory variables, which are predefined as "Patient comfort zone." Trials have shown that it not only reduces weaning failures but also significantly hastens weaning duration.
Classical modes are used in most intensive care setups; however, understanding these newer modes may help to chart out patient-based ventilatory strategies and improve outcomes.

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