mechanical ventilation in anaesthesia

1. Mechanical
Ventilation
Dr. Serge M. Tshijuke,
MD, Mmed (Anaesthesia & Critical Care)

2. Physiology
 Main fx of lungs:
 Gas exchange: O₂ in and CO₂ out
 Exchange take place where?
 Dead spaces;
 Air enters lungs following negative intra-thoracic pressure
(inspiration) or by application of PAP
 Pressures to overcome resistances
 Gas convection vs diffusion

3. Physiology
 Oxygen diffusion path: alveolus ⇢ alveolar epithelium ⇢
capillary endothelium ⇢ capillary blood (<0.3μm)
 CO₂ diffuse from pulm arterial blood to alveolar gas
 As for O₂, blood leaving alveolus will have same partial
pressure of CO₂ as the alveolar gas

4. Physiology
 O₂ diffusion is determined by:
 Surface area for diffusion
 Time red cell is in contact with alveolus
 Partial pressure of O₂ between alveolar gas and capillary blood
 Thickness of this barrier
 There’s lung compliance/expansion if PAP is > alveolar
pressure

5. Introduction to ventilation
 Mechanical ventilation aims to assist or replace spontaneous
breathing
 To maintain gas exchange functions (oxygenation/CO₂
elimination)
 Failure to ventilate, oxygenate or both needs mech vent.
 Ventilator: Automatic machine designed to provide all or part of
the work the body must produce to move gas into and out of the
lungs

6. Categories of mechanical ventilation
 Two types
 Invasive
 Non-invasive
 2 approaches:
 Application of positive pressure
 Application of negative pressure

7. Types of ventilators
2 broad types
 Conventional ventilators
 Breathing patterns approximate those produced by a normal spontaeous
breath
 Enough Vt to clear the anatomical dead space
 Respiratory rate in the range of normal rates
 Gas transport in the airway by convection, in alveoli by molecular
diffusion
 Used in ICU, patient transport, home care, theater, and for all ages of
patients

8.  High Frequency Ventilators
 Produce high frequency and low amplitude breaths
 Vt smaller than the anatomic dead space
 E.g. dogs do not sweat, they regulate their temperature by panting
(shallow, fast breaths)
 Used in conditions where lungs don’t expand properly: ARDS,
neonates with immature lungs, injured lungs of adults, air leaking
lungs

9. Types HFV
 High frequency jet ventilators (HFJV)
 Directs a high frequency pulsed jet of gas into the trachea from a
thin tube within the ETT or tracheostomy tube
 High frequency oscillatory ventilators (HFOV)
 Uses piston arragement that moves back and forth rapidly to
oscillate gas in the breathing circuit and airway
 In both methods the exchange is achieved by enhancing
mixing and diffusion in the airways

10. Indications
 Physiologic changes(deterioration of lung parenchyma)
 Diseases (ARDS, head trauma, heart failure, cardiogenic
shock, neuro-mx conditions, acute severe asthma…)
 Medication/surgical procedures (post-anaesthesia recovery,
drug overdose)

11. Indications
 Regardless of diagnosis, 6 major pathophysiological factors ⇒
of oxygenation, ventilation failure or both
 ↑ airway resistance
 Changes in lung compliance
 Hypoventilation
 v/Q mismatch
 Intrapulmonary shunting
 Diffusion defect

12. Ventilator
 1st volume-controlled vent Engström 100, 1951
 Ventilators can provide full support, partial support (assisting)
or zero support
 3 types of vent by source of energy
 Pneumatically powered: Bird Mark7, Monaghan 225/SIMV,
Percussionaire IPV and VDR
 Electrically powered: Puritain Bennett 540, CareFusion LTV 1150
 Combined: Hamilton-C2, Viasys AVEA, Puritain Bennett 840

13. Control variables in vent
 Vent controls 4 primary variables during inspiration
 Volume controller
 Volume is used as signal to control the volume delivered
 It allows pressure to vary with changes in resistance and
compliance while volume remains constant

14. Pressure controller
 Pressure above baseline (positive) or below (negative)
 Positive pressure vent applies pressure inside the chest to expand it;
need a tight-fitting mask or artificial airway to apply pressure > atmosp
pressure in lungs, thus expansion
 Neg pressure vent apply subatmosp pressure outside of chest to
inflate lungs; neg pressure causes chest wall to expand

15. Flow controller
 Flow signal is used to control its output
 It allows pressure to vary with changes in compliance and
resistance while controlling flow
 Flow ≠ volume
Volume(L) = Flow(L/sec) x Inspiratory time (sec)

16. Time controller
 Control of inspiratory and expiratory time
 It allows pressure and volume to vary with changes in
compliance and resistance

17. Ventilation modes
 A mech vent mode is a specific combination of breathing
pattern, control type, and operational algorithms
 It generates gas flow and volume by creating pos/neg pressure
gradient
 Pressure gradient = trans-airway pressure (PTA)
PTA = airw opening pressure(PAO) – Alv pressure (PALV)

18. Negative pressure ventilation
 It ↓ PALV below PAO (below atmosp pressure), thus creating PTA
gradient
 Unless airway obstruction, neg pressure vent doesn’t need
artificial airway
 Classically, 2 devices to provide neg pressure vent:
 Iron lung
 Chest cuirass or chest shell

19. Iron lung
 It encloses pt’s body in a tank except head and neck
 Air is evacuated to create neg pressure around the chest wall
and underlying alveoli, thus chest and alveolar expansion
 Vt delivered depends on neg press gradient
 Used extensively in chronic ventilatory failure
 Disadvantages:
 poor pt access for routine healthcare
 ↓ cardiac output (tank shock) caused by ↓ venous return

20. Chest cuirass
 Intended to alleviate iron lung issues
 It covers only the chest leaving the limbs exposed
 Disadvantages:
 Difficulty to maintain an airtight seal, thus limited ventilation
 Air leakage issue overcome with individually designed cuirass
 Used in chest wall disease (scoliosis), acute care facility and
home care

21. Positive pressure ventilation
 Achieved by applying positive pressure (> atmosp pressure) at
airw opening
 ↑ PAO ⇒ PTA generating an inspiratory flow
 Vt is directly related to PTA
 Thus, increasing positive pressure applied to lungs ⇒ larger
tidal volume

22. Operating modes
 4 main goals that should be achieved when choosing an
operating mode:
 Provide adequate ventilation and oxygenation
 Avoid ventilator-induced lung injury (VILI)
 Provide patient-ventilator synchrony
 Allow successful weaning from mech vent

23. Operating modes
 >23 vent modes available in different ventilators
 2 or more modes can be used together to achieve desired
effects, e.g.
 Spontaneous + PEEP = CPAP
 SIMV + PSV to reduce work of spontaneous breathing

24. .
1. Spontaneous
2. Positive end-expiratory pressure (PEEP)
3. Continuous positive airway pressure (CPAP)
4. Bilevel positive airway pressure (BiPAP)
5. Controlled mandatory ventilation (CMV)
6. Assist/control (AC)
7. Intermittent mandatory ventilation (IMV)
8. Synchronized intermittent mandatory ventilation (SIMV)
9. Mandatory minute ventilation (MMV)
10. Pressure support ventilation (PSV)
11. Adaptive support ventilation (ASV)
12. Proportional assist ventilation (PAV)
13. Volume-assured pressure support (VAPS)
14. Pressure-regulated volume control (PRVC)
15. Adaptive pressure control (APC)

25. Ventilation modes cont’d
1. Volume ventilation plus (VV+)
2. Pressure-controlled ventilation (PCV)
3. Airway pressure release ventilation (APRV)
4. Biphasic positive airway pressure (Biphasic PAP)
5. Inverse ratio ventilation (IRV)
6. Automatic tube compensation (ATC)
7. Neurally adjusted ventilator assist (NAVA)
8. High-frequency oscillatory ventilation (HFOV)

26. Spontaneous mode
 Not considered as “mode” actually, since frequency and Vt is
determined by pt
 The vent just supplies
 Adequate flow for inspirat in timely manner
 Adjunctive modes like PEEP to support effort
 Apnea ventilation is incorporated in as safety feature
 It deliveries Vt, freq, FiO₂,… to vent the pt

27. Positive end-expiratory pressure (PEEP)
 Increases end-exp or baseline airw pressure above atmosp
pressure
 Not commonly used alone
 Indicated for:
 Intrapulm shunt and refractory hypoxemia
 ↓ FRC and lung compliance
 Auto-PEEP not responding to vent adjustments

28. PEEP
 Complications:
 Reduced venous return and CO
 Barotrauma
 ↑ ICP
 Alterations of renal fx

29. Continuous positive airway pressure (CPAP)
 It PEEP applied to pt breathing spontaneously
 Same indication as PEEP, but adequate lung fx required to
sustain eucapnic ventilation
 In adults, can be given by face mask, nasal mask, ETT
 In neonates, nasal CPAP is method of choice

30. Bilevel positive airway pressure (BiPAP)
 Applies independent positive airw pressure to both inspiration
(IPAP) and expiration (EPAP)
 IPAP applies PP to improves ventilation and hypoxemia due to
hypoventilation
 EPAP ⇔ CPAP, improves oxygenation by ↑ FRC and ↓ intrapulm
shunting
 Indicated in end-stage COPD pts, chronic ventilatory failure,
restrictive chest wall, noninvasive PPV, neuromx disease

31. BiPAP
 Used in 3 modes: spont, spont/timed, timed
 Spont/timed is a backup mechanism, freq/min is 2-5 breaths (<
pt’s spont freq)
 In timed mode, freq/min is set > pt’s
 BiPAP doesn’t directly control volume. To deliver larger
volume, it needs: ↑IPAP, ↓EPAP, ↑compliance, ↓ airflow
resistance
 BiPAP can be used as CPAP by setting IPAP and EPAP at
same level

32. Controlled mandatory ventilation (CMV)
 Also known as continuous mandatory volume or control mode
 Delivers preset Vt and time-triggered freq, so the vent controls
the minute volume
 Requires sedatives, NMBD, respiratory depressants
 Indicated in pts fighting vent, in tetanus and other seizure,
crushed chest injury
 Complications: apnea and hypoxia if disconnected or electrical
failure…, disuse atrophy of diaphragm fibers

33. Assist/control (AC)
 In addition to the mech freq (control), pt may ↑ freq (assist).
Both control and assist breaths are preset.
 AC doesn’t allow spontaneous breaths but pt can trigger
inspiratory efforts(assist)
 Indicated for pts newly placed on mech vent, pts with stable
respiratory drive to trigger the vent for inspiration

34. AC
 Advantages:
 ↓ work of breathing
 Pt has adequate ventilatory drive to control freq, thus the minute
volume
 Complications:
 Resp alkalosis due to alveolar hyperventilation
 If freq > 20-25/min and VT at 10-15ml/kg, this will lead to
hypocapnia and resp alkalosis

35. Intermittent mandatory ventilation (IMV)
 Vent delivers control (mandatory) breaths and allow pt to
breath spontaneously at any VT in between mandatory breaths
 IMV can provide AC or CMV mode
 Complications are few and rare
 Breath stacking when pt is taking spont breath and the vent is
giving a mandatory breath at the same time

36. Synchronized intermittent mandatory
ventilation (SIMV)
 Vent delivers either assisted breaths at beginning of spont
breath or time-triggered mandatory breaths
 Mandatory breaths are synchronized with spont breaths to
avoid breath stacking
 Synchronization window is a time interval when the vent
assists pt’s spontaneous inspiratory effort
 SIMV provides partial ventilatory support by gradually
decreasing the mandatory freq.

37. SIMV
 Advantages:
 Maintain respiratory mx strength/avoid atrophy
 Reduces ventilation to perfusion mismatch
 Decreases mean airw pressure
 Facilitates weaning
 Complication: tendency to wean pt to rapidly, thus leading to
mx fatigue and weaning failure

38. Mandatory minute ventilation (MMV)
 Also known as minimum minute ventilation
 Provide predetermined minute ventilation when spont breath
becomes inadequate
 MMV is an additional fx of SIMV, useful to prevent
hypoventilation and resp acidosis
 Mandatory breaths are volume-cycled
 Pts control their own spont freq and volume

39. Pressure support ventilation (PSV)
 Lower the work of spont breathing, ↑ spont TV
 If used with SIMV, it ↓ O₂ consumption due to ↓ work of
breathing
 Pressure-supported breaths are:
 Triggered by pt, pressure-limited, flow-cycled
 TV varies with inspiratory flow demand
 Pt determines inspiration time and duration
 PSV can be used together with spont breathing

40. PSV
 Indications:
 With SIMV in difficult-to-wean pts
 When pt takes spont breaths
 Pressure support is not active during mandatory breaths

41. High frequency oscillatory ventilation (HFOV)
 Gives small volume at high freq.
 Minimizes lung injury while mechanically ventilating
 Primary settings of HFOV vent are:
 Airw pressure amplitude (delta P or power)
 Frequency, mean airw pressure, bias flow
 Percent inspiration and FiO₂
 Gives 180-900 breaths/min
 Requires sedation

42. HFOV
 Increase ventilation by:
 ↓ oscillation freq (in tradional vent, we increase freq)
 Increasing amplitude of oscillations
 Increase the inspiration time or the bias flow (intentional cuff leak)
 Increase oxygenation by:
 Increase mean airw pressure
 Increase FiO₂

43. Extracorporeal membrane oxygenation
(ECMO)
 Used where it is difficult or near impossible to maintain O₂by
conventional means (CPAP, PEEP)
 Blood is oxygenated outside the body thru membrane
oxygenator