Modes of mechanical ventilation

MODES OF MECHANICAL VENTILATION, NEGATIVE PRESSURE VENTILATION, POSITIVE PRESSURE VENTILATION, INDICATION OF MECHANICAL VENTILATION

1. DR. SHASHIKANT SHARMA MD ANAESTHESIOLOGY & CRITICAL CARE Modes of Mechanical Ventilation

2. NEGATIVE AND POSITIVE PRESSURE VENTILATION Every ventilator must generate an inspiratory flow in order to deliver a tidal volume. Since gas flow requires a pressure gradient, a mechanical ventilator must produce a pressure gradient b/w airway opening and alveoli in order to produce inspiratory flow and volume delivery. At end-exhalation and prior to the beginning of inspiration, pressures at the airway opening and the alveoli are both equal to atmospheric pressure. Since these two pressures are equal at this point, there is no pressure gradient and therefore no flow. Since a pressure gradient is needed to generate gas flow and volume mechanical ventilators achieve this condition by creating either a negative or positive pressure gradient.

3. Negative Pressure Ventilation Negative pressure ventilation creates a transairway pressure gradient by decreasing alveolar pressures to a level below i . e . below atmospheric pressure. Unless airway obstruction is present, negative pressure ventilation does not require an artificial airway. Two classical devices that provide negative pressure ventilation are the “iron lung” and the chest cuirass or chest shell.

4. Encloses the patient’s body except for the head and neck in a tank, and air in it is evacuated to produce a negative pressure around the chest cage. This negative pressure surrounding the chest and underlying alveoli results in chest wall and alveolar expansion. Tidal volume delivered to patient is directly related to negative pressure gradient . For example, a more negative pressure applied to chest wall will yield a larger tidal volume. Since negative pressure ventilation does not require tracheal intubation, this noninvasive method of ventilation has been used extensively and successfully to support chronic ventilatory failure. Disadvantages: Poor patient access Potential for a decreased cardiac output known as “tank shock” Iron Lungs

5. Iron Lungs

6. Chest Cuirass The chest cuirass or chest shell is a form of negative pressure ventilation that was intended to alleviate the problems of patient access and tank shock associated with iron lungs. This shell device covers only the patient’s chest and leaves the arms and lower body exposed. To overcome the problem of air leakage, individually designed cuirass “respirators” minimize air leaks, and they have been used successfully to ventilate patients with chest wall diseases such as scoliosis. Because of the availability of positive pressure ventilators, chest cuirass ventilators are seldom used in an acute care facility.

7. Chest Cuirass

8. Positive Pressure Ventilation Positive pressure ventilation is achieved by applying positive pressure i.e. a pressure greater than atmospheric pressure at the airway opening. Increasing the pressure at the airway opening produces a trans-airway pressure gradient that generates an inspiratory flow. This flow, in turn, results in the delivery of a tidal volume. Therefore, tidal volume is directly related to the transairway pressure gradient. All other factors being held constant, increasing the positive pressure being applied to the lungs will result in a larger tidal volume being delivered.

9. OPERATING MODES OF MECHANICAL VENTILATION A ventilator mode can be defined as a set of operating characteristics that control how the ventilator functions. Regardless of which operating mode is selected, it should achieve four main goals: Provide adequate ventilation and oxygenation Avoid ventilator-induced lung injury Provide patient-ventilator synchrony Allow successful weaning from mechanical ventilation. There are at least 23 modes of ventilation available in different ventilators. Two or more of these modes are often used together to achieve certain desired effects.

10. 1. SPONTANEOUS Spontaneous setting on the ventilator is not an actual mode since the frequency and tidal volume during spontaneous breathing are determined by the patient. The ventilator simply provides the flow and supplemental oxygen. Even though the spontaneous mode is not a direct ventilator function, the role of the ventilator during. Spontaneous mode is to provide: Inspiratory flow to the patient in a timely manner Flow adequate to fulfill a patient inspiratory demand Adjunctive modes such as PEEP to complement Patient’s spontaneous breathing effort.

11. 2. POSITIVE END-EXPIRATORY PRESSURE (PEEP) PEEP increases the end-expiratory or baseline airway pressure to a value greater than atmospheric. It is often used to improve the patient’s oxygenation status, especially in hypoxemia that is refractory to high level of FIO2. PEEP is not commonly regarded as a “stand-alone” mode, rather it is applied in conjunction with other ventilator modes. For example, when PEEP is applied to spontaneous breathing patients, the airway pressure is called continuous positive airway pressure (CPAP). Indications for PEEP Intrapulmonary shunt and refractory hypoxemia Decreased FRC & lung compliance Auto-PEEP not responding to adjustments of ventilator settings.

12. Physiology of PEEP PEEP reinflates collapsed alveoli and supports and maintains alveolar inflation during exhalation. Once “recruitment” of these alveoli occurs and is sustained, PEEP decreases the threshold for alveolar opening and facilitates gas diffusion and oxygenation. PEEP increases the alveolar end-expiratory pressure which decreases pressure threshold for alveolar inflation. Re-expansion of the collapsed alveoli improves ventilation and reverses intrapulmonary shunting.

13. Complications of PEEP Decreased Venous Return Barotrauma Increased ICP Alteration of renal function & water metabolism

14. 3. CONTINUOUS POSITIVE AIRWAY PRESSURE (CPAP) CPAP is PEEP applied to the airway of a patient who is breathing spontaneously. Indications for CPAP are essentially the same as for PEEP with additional requirement that the patient must have adequate lung functions that can sustain Eucapnic ventilation documented by PaCO2. In adults, CPAP may be given via a face mask, nasal mask, or endotracheal tube.

15. 4. BILEVEL POSITIVE AIRWAY PRESSURE ( Bipap ) BiPAP allows the clinician to apply independent positive airway pressures to both inspiration and expiration. IPAP provides positive pressure breaths, and it improves ventilation and hypoxemia due to hypoventilation. EPAP is in essence CPAP, and it improves oxygenation by increasing the FRC and reducing intrapulmonary shunting. INDICATIONS Supporting patients with chronic ventilatory failure Restrictive chest wall disease Neuromuscular disease Nocturnal hypoventilation Appears to be of value in preventing intubation of the end-stage COPD patient.

16. Initial Settings Used in one of three modes: 1. Spontaneous: if the patient is breathing spontaneously, the IPAP and EPAP may be initially set at 8 cm H2O and 4 cm H2O, respectively. Pressures are titrated based on needs, generally with a target of 5 to 7 mL /kg. 2. Spontaneous/timed: Used as a backup mechanism and the frequency per min (f/min) is set two to five breaths below the patient’s spontaneous frequency. 3. Timed mode: set IPAP and EPAP as above and the f/min slightly higher than patient’s spontaneous frequency. A BiPAP device can be used as a CPAP device by setting IPAP and EPAP at same level.

17. 5. CONTROLLED MANDATORY VENTILATION (CMV) Also known as continuous mandatory ventilation or control mode, ventilator delivers the preset tidal volume at a time-triggered frequency. Since ventilator controls both the patient’s tidal volume and respiratory frequency, ventilator controls the patient’s minute volume. In the control mode, patient cannot change the ventilator frequency or breath spontaneously. Control mode should only be used when the patient is properly medicated with a combination of sedatives, respiratory depressants, and neuromuscular blockers. Control mode ventilation should not be instituted by decreasing the ventilator’s triggering sensitivity to the point that no amount of patient effort can trigger ventilator into inspiration.

18. Problem with this approach should be obvious since any spontaneous inspiratory effort would be like attempting to inspire through a completely obstructed airway. Regardless of how vigorous the patient’s inspiratory effort is, no gas flow would be delivered to the patient until the ventilator automatically becomes time-triggered. Indications for Control Mode: Initial stage of mechanical ventilation Tetanus Crushed chest injury Complete rest for patients

19. Complications of Control Mode Since the patient’s spontaneous respiratory drive will have been blunted with sedation and neuromuscular block in the control mode, the patient is totally dependent on the ventilator for ventilation and oxygenation. Rapid disuse atrophy of diaphragm fibers Prolonged mechanical ventilation leads to diaphragmatic oxidative injury, elevated proteolysis, and reduced function of the diaphragm

20. 6. ASSIST/CONTROL (AC) With the assist/control (AC) mode, patient may increase ventilator frequency (assist) in addition to the preset mechanical frequency (control). Each control breath provides the patient with a preset, Ventilator-delivered tidal volume. Each assist breath also results in a preset, ventilator-delivered tidal volume. The assist control mode does not allow the patient to take spontaneous breaths. The mandatory mechanical breaths may be either patient-triggered by the patient’s spontaneous inspiratory efforts (assist) or time-triggered by a preset frequency. If a breath is patient-triggered, it is referred to as an assisted breath; if a breath is time-triggered, the breath is referred to as a control breath.

21. ADVANTAGES: Patient’s work of breathing requirement in the AC is very small when the triggering sensitivity is set appropriately and the ventilator supplies an inspiratory flow that meets or exceeds the patient’s inspiratory flow demand. if the patient has an appropriate ventilatory drive, this mode allows patient to control the frequency and therefore the minute volume required to normalize patient’s PaCO2.

22. 7. INTERMITTENT MANDATORY VENTILATION (IMV) IMV is a mode in which ventilator delivers control (mandatory) breaths and allows the patient to breathe spontaneously at any tidal volume the patient is capable of in between the mandatory breaths. Primary complication associated with IMV was random chance for breath stacking. This occurs when the patient is taking a spontaneous breath and ventilator delivers a time-triggered mandatory breath at the same time. If this occurs, patient’s lung volume and airway pressure could increase significantly. Setting appropriate high pressure limits will reduce the risk of barotrauma in the event of breath stacking. As long as the breath stacking only occurs Occasionally, IMV mode is an acceptable mode of ventilation with few complications. The sophistication of ventilator technology has progressed to the point that no new adult ventilators offer the IMV mode. Rather, all ventilators currently available have been designed to provide SIMV.

23. 8. SYNCHRONIZED INTERMITTENT MANDATORY VENTILATION (SIMV) Mode in which ventilator delivers either assisted breaths to the patient at beginning of a spontaneous breath or time-triggered mandatory breaths. mandatory breaths are synchronized with the patient’s spontaneous breathing efforts so as to avoid breath stacking. The SIMV mandatory breaths may be either time-triggered or patient-triggered. triggering mechanism is determined by whether or not the patient makes a spontaneous inspiratory effort just prior to the delivery of a time-triggered breath. Synchronization Window: time interval just prior to time triggering in which ventilator is responsive to the patient’s spontaneous inspiratory effort is commonly referred to as the synchronization window.

24. SIMV Spontaneous Breath-Triggering Mechanism In between mandatory breaths, SIMV permits patient to breathe spontaneously to any tidal volume the patient desires. Gas source for spontaneous breathing in the SIMV mode is typically supplied by a demand valve. Demand valve is always patient-triggered, either by pressure or flow depending on the ventilator. It is important to understand that spontaneous breaths taken by patient in SIMV mode are truly spontaneous. ventilator provides humidified gas at selected FIO2, but spontaneous frequency and spontaneous tidal volume are totally dependent on the patient’s breathing effort.

25. Advantages of SIMV Mode Maintains respiratory muscle strength/avoids muscle atrophy Reduces ventilation to perfusion mismatch Decreases mean airway pressure Facilitates weaning. The primary disadvantage associated with SIMV is desire to wean the patient too rapidly, leading first to a high work of Spontaneous breathing and ultimately to muscle fatigue and weaning failure. Without PSV, best practice is to decrease SIMV mandatory frequency slowly and monitor patient closely for signs of fatigue.

26. 9. MANDATORY MINUTE VENTILATION (MMV) Also called minimum minute ventilation, is a feature of some ventilators that provides a predetermined minute ventilation when the patient’s spontaneous breathing effort becomes inadequate. It is especially useful in preventing hypoventilation and respiratory acidosis in the final stages of weaning with SIMV when the patient’s spontaneous breathing is assuming a significant portion of the total minute volume.

27. 10. PRESSURE SUPPORT VENTILATION (PSV) PSV is used to lower the work of spontaneous breathing and augment a patient’s spontaneous tidal volume. When PSV is used with SIMV, it significantly lowers the oxygen consumption requirement presumably due to the reduced work of breathing. PSV applies a preset pressure plateau to the patient’s airway for the duration of a spontaneous breath. Pressure-supported breaths are considered spontaneous because they are patient-triggered: Tidal volume varies with patient’s inspiratory flow demand Inspiration lasts only for as long as the patient actively inspires Inspiration is terminated when patient’s inspiratory flow demand decreases to a preset minimal value.

28. A pressure-supported breath is therefore patient-triggered, pressure-limited, and flow-cycled. It is pressure-limited because the maximum airway pressure cannot exceed preset pressure support level. It is flow-cycled because a pressure-supported breath cycles to expiration when the flow reaches a minimal level. Indications for PSV Mode: Pressure support is typically used in the SIMV mode to facilitate weaning in a difficult-to-wean patient. In this application, pressure support: Increases the patient’s spontaneous tidal volume Decreases the patient’s spontaneous frequency Decreases the work of breathing.

29. 11. ADAPTIVE SUPPORT VENTILATION (ASV) Dual control mode that provides a mandatory minute ventilation. Ventilator measures the dynamic compliance and expiratory time constant to adjust mechanical tidal volume and frequency for a target minute ventilation. Once the target minute ventilation is set, ventilator uses test breaths to measure system compliance, airway resistance, and any intrinsic PEEP. Following determination of these variables, the ventilator selects and provides the frequency, inspiratory time, I:E ratio, and high pressure limit for mandatory and spontaneous breaths.

30. If there is no spontaneous triggering effort, the ventilator determines and provides mandatory frequency, tidal volume, and high pressure limit needed to deliver preselected tidal volume, inspiratory time, and I:E ratio. As the patient begins to trigger the ventilator, the number of mandatory breaths decreases and pressure support level increases until a calculated tidal volume is able to provide adequate alveolar volume.

31. 12.PROPORTIONAL ASSIST VENTILATION (PAV) With PAV, there is no target flow, volume, or pressure during mechanical ventilation. Pressure used to provide the pressure support is variable and is in proportion to the patient’s pulmonary characteristics ( elastance and airflow resistance) and demand (volume or flow). PAV is set to overcome 80% of the elastance and airflow resistance. PAV may be flow assist (FA) or volume assist (VA). In FA, the applied pressure is provided to meet the patient’s inspiratory flow demand. FA reduces the inspiratory effort needed to overcome airflow resistance. VA occurs when PAV provides the pressure to meet the patient’s volume requirement. VA reduces the inspiratory effort needed to overcome systemic elastance such as restrictive lung defects

32. PAV improves ventilation and reduces neuromuscular drive and work of breathing in ventilator-dependent patients with COPD. When PAV is used with CPAP, the reduction of inspiratory muscle work reaches values close to those found in normal subjects.

33. 13. VOLUME-ASSURED PRESSURE SUPPORT (VAPS) VAPS incorporates inspiratory PSV with conventional volume-assisted cycles (VAV). This combination provides an optimal inspiratory flow during assisted/controlled cycles, reducing the patient’s work of breathing commonly seen during VAV. Unlike typical PSV, VAPS assures stable tidal volume in patients with irregular breathing patterns. If the delivered volume equals preset volume, breath is considered a pressure support breath. Since pressure support breaths are dependent on the patient effort, delivered volume may be larger than preset volume. It is essential to set pressure support level that provides a volume that is lower than the preset volume. On the other hand, if delivered volume falls short of preset volume, ventilator switches from a pressure-limited breath to a volume-limited breath. This results in a longer inspiratory time until the preset volume is delivered. Since VAPS may prolong the inspiratory time automatically, patients with airflow obstruction should be monitored closely in order to prevent air trapping and other undesirable cardiovascular effects associated with prolonged inspiratory time.

34. 14. PRESSURE-REGULATED VOLUME CONTROL (PRVC) Similar modes to PRVC in subsequent ventilators are known as adaptive pressure control (Servo-I), AutoFlow , adaptive pressure ventilation, volume control, volume targeted pressure control and pressure controlled volume guaranteed. PRVC is used primarily to achieve volume support while keeping PIP at a lowest level possible. This is achieved by altering the peak flow and inspiratory time in response to changing airway or compliance characteristic.

35. Automode combines PRVC and volume support. This mode alters between time-cycled and flow-cycled breaths depending on the degree of patient effort. If there is no spontaneous triggering effort for a time period (i.e., apnea for 12, 8, and 5 sec in adult, pediatric, and neonatal modes, respectively), ventilator provides PRVC and breaths are time- triggered.Delivered volume is preset with a variable PIP up to the high pressure limit. When the patient has two consecutive breaths that trigger the mechanical breaths, automode switches to volume support in which all breaths become patient triggered, pressure-limited, and flow-cycled.

36. 15. ADAPTIVE PRESSURE CONTROL It combines functions of volume ventilation (stable TV) with functions of pressure ventilation via variable flow. It is a PC breath that uses variable inflation pressures to deliver a minimum targeted TV. Inflation pressure is variable. As Patient’s inspiratory effort increases inflation pressure is reduced. This is a concern because ventilator can not distinguish between improved pulmonary compliance and increased patient effort . Increasing patient’s breathing effort due to hypoxia or pain may potentially create a greater work of breathing, due to decreasing inflation pressure.

37. 16. VOLUME VENTILATION PLUS (VV+) It is an option that combines two different mode: volume control plus and volume support. Volume Control Plus Used to deliver mandatory breaths during AC and SIMV modes of ventilation. Intended to provide a higher level of synchrony than standard volume control ventilation. Clinician sets target TV and Ti. Ventilator delivers a single test breath using standard volume and decelerating flow and plateau to determine relative compliance. Target pressures for subsequent breaths are adjusted accordingly to compensate for any TV differences. Flow is adjusted automatically to reduce likelihood of inadequate flow or aggressive flow demand. Active spontaneous breaths are allowed during the inspiratory phase of a mandatory breath by way of a pressure control style of breath and use of an active exhalation valve. Excessive pressure caused by breathing or coughing is vented, thus maintaining synchrony.

38. Volume Support (VS) Provide a control TV & increased patient comfort. Weaning from anesthesia is a common application for VS. Clinician sets target TV but not inspiratory time or mandatory frequency . Ventilator delivers a single spontaneous pressure support type of breath and uses variable pressure support levels to provide target TV. During weaning or awakening from anesthesia, patient assumes a higher spontaneous TV and ventilator decreases pressure support level accordingly. When spontaneously TV decreases, ventilator increases PS level automatically to maintain the target TV. During VS, the ventilator frequency and MV are determined by triggering effort of the patient. Inspiratory time is determined by patient respiratory demand.

39. 17. PRESSURE-CONTROLLED VENTILATION (PCV) Pressure-controlled breaths are time-triggered by a preset frequency. Once inspiration begins, a pressure plateau is created and maintained for a preset inspiratory time. Pressure-controlled breaths are therefore time-triggered, pressure-limited & time-cycled. PCV is usually indicated for patients with severe ARDS who require extremely high PIP during mechanical ventilation in a VC mode. As a result of these high airway pressures, incidence of barotraumas is more Likely. Advantage of switching these patients from the conventional VC ventilation to PC is that a lower PIP can be used and maintained while providing oxygenation and ventilation.

40. 18. AIRWAY PRESSURE RELEASE VENTILATION (APRV) APRV has two CPAP or pressure levels- high pressure (Phigh or Pinsp ) and low pressure (Plow or PEEP) & patient is allowed to breathe spontaneously without restriction at high or low pressure levels. When high pressure level is dropped to low pressure level, it simulates a mechanical exhalation. Likewise, when low pressure level is raised to high pressure level, it simulates an inspiratory mechanical breath. Patient spends most of the time at high pressure level with less than 1.5 sec at the low pressure level.

41. Since APRV mode is pressure-limited, for a given pressure gradient, patient’s TV will vary directly with changes in lung compliance and inversely with changes in airway resistance. For this reason, exhaled TV should be closely monitored to prevent hyperinflation. Patient-ventilator dyssynchrony may result when pressure release occurs during spontaneous inspiration, or when pressure increase occurs during spontaneous expiration. Primary indication for this mode is similar to that of pressure control, namely, as an alternative to conventional VC ventilation for patients with significantly decreased lung compliance such as patients with ARDS.

42. 19. BIPHASIC POSITIVE AIRWAY PRESSURE Mode that has two baseline pressure levels ( Pinp . & PEEP) and it allows spontaneous breathing at any point in mechanical ventilation cycle. Similar to APRV with one exception. In APRV, the patient spends most of the time at high pressure level. While in Biphasic PAP, Patient spends more time at low pressure level.

43. 20. INVERSE RATIO VENTILATION Ratio of inspiratory time (I time) to expiratory time (E time) is known as the I:E ratio. In conventional mechanical ventilation, the I-time is traditionally lower than E-time so that I:E ratio ranges from about 1:1.5 to 1:3. This resembles normal I:E ratio during spontaneous breathing, and it is considered physiologically beneficial to normal Cardiopulmonary function. Physiology of IRV IRV improves oxygenation by: Reduction of intrapulmonary shunting Improvement of V/Q matching Decrease of dead space ventilation

44. Two notable changes are observed during IRV are: A. Increase of Mean Airway Pressure: To achieve same degree of ventilation and oxygenation, IRV requires a lower peak airway pressure and PEEP, but a higher mean airway pressure ( mPaw ) than conventional mechanical ventilation. Increase in mPaw during IRV helps to reduce shunting and improve oxygenation in ARDS patients. B. Addition of Auto-PEEP: Since IRV provides a longer I-time and shorter E-time, breath stacking with an increase of end-expiratory pressure is likely when there is not enough time for complete expiration. Presence of auto-PEEP during IRV may help to reduce shunting and improve oxygenation in ARDS patients.

45. Adverse Effects of IRV Increase in mPaw and presence of auto-PEEP both contribute to increase of mean alveolar pressure and volume. So Incidence of barotrauma may be as high as that obtained by conventional ventilation with high levels of PEEP. Higher rate of transvascular fluid flow or flooding induced by an increased alveolar pressure. Patients receiving IRV are often agitated. They may require sedation and neuromuscular blocking agents to facilitate ventilation.

46. ` Pressure Control-IRV (PC-IRV) Since IRV may increase mPaw, create auto-PEEP, and increase incidence of barotrauma. it is sometimes used in conjunction with PC-ventilation due to its pressure-limiting capability. By using pressure control, peak airway pressure may be kept at a safe level. This strategy helps to minimize pressure induced lung injuries. When an inverse I:E ratio is used with pressure-controlled ventilation, it is called PC-IRV.

47. Several studies compare the outcomes of ARDS patients before and after implementation of PC-IRV. Changes that may occur when positive pressure ventilation with PEEP (PPV + PEEP) is switched over to PC-IRV mode of ventilation are:

48. 21. AUTOMATIC TUBE COMPENSATION This tubing compensation can be applied in all ventilation modes. ATC offsets and compensates for airflow resistance imposed by artificial airway. It allows the patient to have a breathing pattern as if breathing spontaneously without an artificial airway. With ATC, pressure delivered by ventilator to compensate for airflow resistance is active during inspiration and expiration. Example: when airway diameter decreases or flow demand increases, pressure is raised to overcome a higher airflow resistance or increased flow demand.

49. 22. NEURALLY ADJUSTED VENTILATORY ASSIST Patient’s electrical activity of diaphragm is used to guide the optimal functions of the ventilator. Neural controls of respiration originated in the patient’s respiratory center are sent to diaphragm via phrenic nerves. In turn, bipolar electrodes are used to pick up electrical activity. Electrodes are mounted on a disposable catheter and positioned in esophagus at level of diaphragm. NAVA is available for adults, children, and neonates. Ability to wean these patients rapidly reduces or eliminates incidence of disuse atrophy of the diaphragm.

50. Use of NAVA Management & weaning of mechanically ventilated patients with spinal cord injury. Head injury COPD H/O of ventilator dependency.

51. 23. HIGH-FREQUENCY OSCILLATORY VENTILATION Delivers extremely small volumes at high frequency. Its main application is to minimize development of lung injury while providing mechanical ventilation. It delivers a constant flow and its piston pump oscillates at frequencies ranging from 3 Hz to 15 Hz (180 breaths/min to 900 breaths/min). Adult patients are sedated to prevent deep spontaneous breathing, as this will trigger alarms and affect ventilator performance.

52. Ventilation can be increased by: Decreasing the oscillation frequency Increased by increasing amplitude of oscillation Increasing inspiratory time Increasing flow (with an intentional cuff leak) Oxygenation to patient can be increased by: Increasing mean airway pressure Increasing the FIO2.

53. THANK YOU