Principles of Mechanical Ventilation in ICU

Name: Principles of Mechanical Ventilation in ICU
Uploaded: 2017-08-27T04:46:09+00:00
Duration: PTM40S7
Channel: Constance Owens
Description: Principles of Mechanical Ventilation in ICU

Principles of Mechanical Ventilation in ICU
Raafat Abdel Azim

Treatment of Respiratory Failure
ECMO PEEP ETI + MV ECCO2R NPPV O2 IVFV CFAV Drugs Secretions TTT of the cause

Oxygen Supplementation
Aim: PAO2  PaO2 > 60 mmHg (60:100) If < 60  abrupt  of saturation & content If > 100  no more benefit Not > 50% > 24h Potential complication: O2  PaCO2

Methods of O2 Supplementation (O2 Devices)
Flow Rate? Patient’s IFR? AIR (21% O2)

O2 Devices Classification
Delivered O2 % High O2 (up to 100%) Controlled O2 (set %) Flow Capacity High Flow Low Flow

O2 Devices Nasal Cannula Low flow 0.5 – 5 L/min
Maximal tracheal FIO2 0.4 – 0.5 (cannot be precisely controlled, VE) FR  No  in FIO2 Drying and irritating effect Low flow, low O2

Air-Entrainment Face Masks (Venturi Masks)
O2 Devices Air-Entrainment Face Masks (Venturi Masks) High FR FIO2 precisely controlled (0.24 – 0.5) by changing jet nozzle adjusting FR Most useful in COPD patients (titratable) Air O2 High flow, controlled O2

Aerosol Face Masks Large side holes, large bore tubing, a nebulizer
O2 Devices Aerosol Face Masks Large side holes, large bore tubing, a nebulizer Flow matching can be evaluated by observing the aerosol mist Moderate flow, variable O2

O2 Devices Reservoir Face Masks FR is adjusted so that the reservoir bag remains distended High flow, high O2

Resuscitation Bag-Mask-Valve Unit
O2 Devices Resuscitation Bag-Mask-Valve Unit High flow, high O2 Mask held firmly over the face  air entrainment High flow > 15 L/min Bag need not be compressed to supply O2

ABG PaO2? SaO2? PaCO2 > 6 mmHg (in 30 min) = significant retention

NPPV Standard Ventilator or or NPPV ventilator PS Volume cycled
Better tolerated Less effective in mouth breathers and edentulous patients Nasal mask or or Face mask NPPV ventilator PS Volume cycled Patient triggered

Not recommended unless the patient is:
NPPV Not recommended unless the patient is: Alert, oriented & cooperative Not having: Swallowing dysfunction Difficulty clearing secretions Hypotension Uncontrolled arrhythmias Acute cardiac ischemia Acute GI hemorrhage

May not be desirable with:
NPPV May not be desirable with:  levels of ventilatory requirements ( C requires  P)  ability to adequately clear secretions (especially with face mask) Careful observation and monitoring Possible G distension & aspiration risk

Titrate P, V & FIO2  PaO2 & PaCO2
NPPV Settings Specialized Unit Standard Ventilator PS mode 8-12 cmH2O IPAP AC mode 10 ml/kg PEEP or EPAP Titrate P, V & FIO2  PaO2 & PaCO2

ETI and MV ETI when? PaO2 < 60 (FIO2 > 0.5) PaCO2 + pH
Respiratory muscle fatigue Loss of protective upper airway reflexes Ineffective cough + secretions Level of consciousness

MV O2 in air Time Mode How? f % (FIO2) Volume VT Others PEEP Seconds %
I:E Time Mode I E How? O2 in air MV f % (FIO2) Volume VT Alarms & Limits Wave form Flow Trig. sensitivity Others PEEP

Mechanism of action of the ventilator (Mode of operation): 4 phases
Inspiratory phase Cycling from E to I Cycling from I to E Expiratory phase

Respiratory cycle Two phases: I & E Two phase transitions:
From I to E = expiratory cycling Between E and I = inspiratory cycling Inspiration can itself sometimes have two phases: an active ‘flow’ (TI flow) phase during which gas is being delivered to the patient an end-inspiratory pause (TI pause ) The total duration of inspiration is made of the sum of these two: TI = TI flow + TI pause

Intra-thoracic pressures

Pressure gradients within the thorax

Distending pressures

Control of Parameters of Ventilation
VT f (frequency= rate) VM I:E ratio Flow Rate Flow Profile Trigger Sensitivity

IFR (Inspiratory Flow Rate)
VT (ml) f (b/min) Cycle time (s) IFR TI (s) TE (s) I:E L/min L/s ml/s 500 20 60/20 = 3 60 1 1000 0.5 2.5 1:5 30 2 1:2 60 L/min 600 Volume (ml) 30 L/min 500 400 300 200 100 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 Time (sec)

Inspiratory Waveforms
Constant Decelerating Accelerating (ramp) Sinusoidal (reverse ramp) Inspiration 60 Flow (L/min) 30 Expiration TI TI TI TI 30 60

The Ventilation Cycle

IPPV Paw Pmax Pplat 20 Pause IF E t ZEEP I E

F V P T Duration of ventilation cycle (sec) f (60/duration)
I phase (IF period, IP period) E phase VT, VM TI, TE, I:E F V P T

R C Inspiratory Phase During the IF period: Paw depends on:
The airway resistance (R) The total thoracic compliance (C) (V/P) R C

Resistance P P P P P P P P P P P P P P Flow Rate: FRP

Resistance Paw 20 t N R F R F

Raw = (PIP – Pplat) / Flow
Calculation of Airway Resistance (Raw) in a Ventilated Patient Raw = (PIP – Pplat) / Flow Example: If in a given situation, PIP = 40 cm H2O, Pplat = 38 cm H2O, flow = 60 L/min (i.e., 1 L/s), Raw will be: = (40 − 38)/1 = 2 cm H2O/L/s.

Compliance P P Volume C C

Compliance P P Volume: VP

During I pause Pause No gas F into or out of the lungs  Paw depends only on VI & CT Gas redistributes among alveoli This improves gas distribution in the lungs of patients with small AWD (BA, smokers).

C R Paw t EB intubation CW rigidity Pulmonary edema Secretions
Bronchospasm Kinked ETT Paw 20 t C R

Compliance = V/P Dynamic compliance: = VT/(PIP-PEEP) L/cmH2O
Static compliance: = VT/(Pplat-PEEP) L/cmH2O

Time Constants of the Lung
In most diseases, the involvement of the lung is not uniform. Regional differences in C and R occur. Owing to this, alveoli in different parts of the lung behave differently; diseased alveoli take longer to fill and to empty. The rate of filling of an individual lung unit is referred to as its time constant. For a particular lung unit Time Constant = R x C

It takes the equivalent of 5 time constants for the lung to completely fill (or to empty).
In the time afforded by one time constant, 63% of the lung will fill (or empty); two time constants allow 86% of the inspiratory or expiratory phase to be completed; three time constants allow for 95%, and four time constants for 98%. 100 5 98 95 4 86 3 63 2 1

Example: A lung unit with a normal Raw of 1 cm H2O/L/s and a normal compliance of 0.1 L/cm H2O would have a time constant of: = 1 × 0.1 = 0.1 s Five times this is 0.5 s, which would be the time required for this unit to fill or empty satisfactorily. This information comes useful while setting a ventilator’s TI and TE Since diseased air units take longer to fill, deliberately prolonging the TI may enable such units to participate more meaningfully in gas exchange

Goals of Mechanical Ventilation

Provide appropriate O2 supplementation
Assure adequate alveolar VM  work of breathing (WOB)  patient comfort during respiration

necessary to maintain the desired PaCO2
To provide adequate minute alveolar ventilation and to  side effects necessary to maintain the desired PaCO2 PPV   ITP

Adequate Ventilation PaCO2 of 40 mmHg = 5.3% of 760 mmHg
Goals Adequate Ventilation PaCO2 of 40 mmHg = 5.3% of 760 mmHg 40/760 = 0.053 Normal resting VCO2= 200 ml/min= 0.2 L/min This requires VM of 3.8 L 0.2/ ? = 0.053 0.2/ = Add dead space (VD) 3.8 L/min

 150 ml in a 70 Kg (154 pound) adult = 0.15 x 10 (f) =
Goals, Adequate Ventilation N = 2.2 ml/Kg (1 ml/pound)  150 ml in a 70 Kg (154 pound) adult = 0.15 x 10 (f) = Required VM = = A larger VM is required for patients who have VD or VCO2 1.5 L/min 5.3 L For VCO2 For VD VD phys = VD ana + VD alv VD ana = conducting airways = 150 ml in a 70 Kg adult VD alv is created when non-perfused alveoli are ventilated (negligible in health, expands in disease) This 150 ml of VD ana is reduced by ETT and can be cut down to about 60% by tracheostomy

When VCO2 & VD are stable: VM  1/ PaCO2 VM x PaCO2 = constant
Goals, Adequate Ventilation When VCO2 & VD are stable: VM  1/ PaCO2 VM x PaCO2 = constant e.g., PaCO2 = 50 mmHg with VM = 5 L/min  VM to 7 L/min   PaCO2 to 36 mmHg V1 x P1aCO2 = V2 x P2aCO2

VM =VT x f Manipulation of VT has a different effect on the PaCO2 than does altering f. Consider the following: A set VT of 500 ml and f of 10 b/min results in a VM of 500 × 10 = 5,000 ml/min. The same VM can be produced by a VT of 250 ml delivered at f of 20 b/min, i.e., 250 × 20 = 5,000 ml/min. If, however, the VD is taken into consideration, the implications of these two settings are vastly different. Assuming a VD phys of 150 ml, the alveolar ventilation (the effective ventilation or the ventilation that takes part in gas exchange) in the first example would be: (500 – 150) × 10 = 3,500, and in the second example would be: (250 – 150) × 20 = 2,000. PaCO2 is inversely proportional not to all of the VM, but to that part of the ventilation that is independent of VD (i.e., the alveolar ventilation VA)

Goals  Side effects PPV  PVR  RVA  ITP  CO  VR  EDV

Goals,  Side Effects PPV  ITP VD PA > PAP

All these effects  mean Paw
Goals All these effects  mean Paw Therefore, a goal of PPV is to  mean Paw while maintaining adequate ventilation and oxygenation

Effect of IFR on mean Paw
Mean Paw = area under the curve

Modes (examples) Breathing support CPAP Volume controlled IPPV (CMV) A
AC IMV SIMV PSV BIPAP APRV Other modes Volume controlled IPPV (CMV) Pressure controlled (PCV) (PLV) IRV

Volume Controlled Ventilation (Controlled Mode Ventilation) (CMV) (IPPV)
Initial settings: f /min VT 8-10 ml/Kg FIO2 1 I:E 1:2 (1:3 in COPD) Aim pH 7.36 : 7.44 PaO2 60: 100 mmHg PaCO2 36: 44 mmHg Adjust settings (ABG, SpO2 > 92-94%)

IPPV Preset f & VT No patient interaction with ventilator
Advantage: rests muscles of respiration Disadvantages: requires sedation/NMB, potential adverse hemodynamic effects, muscle atrophy

IPPV Paw Pmax Pplat 20 Pause IF E t ZEEP I E

Inspiratory Plateau Pressure (Pplat)
Paw at end of I with no gas flow present It estimates PA at end I Indirect indicator of alveolar distension

I:E Ratio Spontaneous breathing I:E = 1:2
TI determinants with preset V breaths: VT GFR f I pause TE passively determined

I:E Ratio TE too short for exhalation  Auto-PEEP by  TI
Breath stacking Auto-PEEP  Auto-PEEP by  TI GFR  VT  f

During the expiratory phase
VT I E FRC FRC PEEP PaO2 FRC IA contents Pulmonary edema ARDS PaO2

IPPV + PEEP Paw PEEP t

Expansion of collapsed perfused alveoli PaO2 CL FRC
PEEP PEEP, When? PaO2 < 60 mmHg (FIO2 > 0.5) Action Expansion of collapsed perfused alveoli PaO2 CL FRC Prevention of absorption atelectasis Improvement of V/Q  QS/QT

PEEP PEEP, How? 2.5-5 cmH2O increments until PaO2 > 60 (FIO2 < 0.5) Goal: PEEP with maximum improvement of PaO2 without hazards Hazards COP (VR, PVR, left septal displacement) Barotrauma (Pnx, Pnp, SC emphysema)  abrupt PaO2 & COP

Don’t give PEEP > 15 cmH2O How to avoid  COP IVFV Inotropics
Best PEEP O2 transport Static CL Best PEEP Don’t give PEEP > 15 cmH2O How to avoid  COP IVFV Inotropics PA catheter

Intermittent PEEP (Expiratory Sigh) Paw t Pmax Int PEEP PEEP
Sigh phase t

Auto-PEEP Can be measured on some ventilators
 peak, plateau, and mean Paw Potential harmful physiologic effects

Pressure-limited ventilation
(PLV) (PCV) Paw Pmax Pplat t

PCV Used to limit inflationary pressures Allows setting of TI
Complexity of interacting ventilatory variables

Inverse Ratio Ventilation (IRV)
Paw Improves oxygenation Neonates ARDS 20  Alveolar recruitment by creating auto PEEP I E t No advantage over 1:1 (+PEEP) at f < 15

FIO2 requirements > 0.6 or SaO2 < 90%
IRV Hypoxemic RF Optimize PEEP FIO2 requirements > 0.6 or SaO2 < 90% Consider IRV VC- IRV PC- IRV  IFR I pause I:E with decelerating flows Most effective

No uniformly accepted criteria. Proposed criteria:
Indications of IRV ARDS with severe hypoxemic RF (especially with FIO2 requirements and PEEP) No uniformly accepted criteria. Proposed criteria: VC-IRV: FIO2 > 0.6 or PEEP >10 cmH2O to maintain SaO2 >90% PC-IRV: Above parameters + PIP > 45 cmH2O

Spontaneous Breathing (SB)
Breathing Support Point of Reference: Spontaneous Breathing (SB)

SB Paw +ve t -ve

CPAP Paw CPAP t

CPAP No machine breaths delivered Allows SB at elevated baseline P
Patient controls f & VT

Assisted Ventilation (A)
Paw Patient  f and timing Hazard: hypoventilation A No A t S If  or No S

Assist-Control (AC) Paw A+C Provides a minimum f below which  C

AC Preferred initial mode in most situations Preset VT & minimal f
Additional patient-initiated breaths receive preset VT Advantages:  WOB; allows pt. to modify VM Disadvantages: potential adverse hemodynamic effects or inappropriate hyperventilation

IMV   PaCO2 < apneic threshold  no SB  IMV = IPPV
Preset VT and f Paw SB is allowed Muscle atrophy is less likely S t IMV   PaCO2 < apneic threshold  no SB  IMV = IPPV

Indications of IMV Drug overdose Intermittent heavy sedation
Unstable ventilatory drive Weaning (may be combined with PSV)

SIMV Paw t Preset VT and f Synch. Mandatory Mandatory NO S
t Triggering window

SIMV Preset VT at a preset f
Additional SBs at VT & f determined by patient Often used with PSV Indications: 1ry means of MV if adequate VE is delivered Severe respiratory alkalosis To prevent auto PEEP Weaning

SIMV Potential advantages Better patient-ventilator interaction
Less hemodynamic effects Potential disadvantages Higher WOB > CMV, AC

Pressure Support Ventilation (PSV)
(Inspiratory Pressure Support = IPS) (Assisted Spontaneous Breathing = ASB) Paw PSV Spont. t Trig. Sensitivity

PSV Paw CPAP t

PSV Pressure assist during SB (= ASB)
P assist continues until inspiratory effort  Delivered VT dependent on I effort & R/C of lung/thorax

PSV Potential advantages Indications: Patient comfort WOB < SB
May enhance patient-ventilator synchrony Used with SIMV to support SB Indications: Stable patients receiving long-term MV (WOB) Weaning

PSV Potential disadvantages
Variable VT if pulmonary R/C changes rapidly If sole mode of ventilation, apnea alarm is only backup Gas leak from circuit may interfere with cycling

Apnea Ventilation Paw IPPV Apnea alarm CPAP 15 s t Apnea time 15-60 s

Biphasic Intermittent Positive Airway Pressure
Paw BIPAP (PCV+) P & T can be independently set Spont. P2 PCV P1 Spont. t T high T low

SB superimposed on standard PCV

E valve PCV: closed BIPAP: controlled

BIPAP Auto Flow Auto flow  application of the "open breathing system" even to Volume Controlled ventilation modes Can be used + any Volume oriented mode like IPPV SIMV MMV

Mode Contribution of SB IPPV-BIPAP No SB SB only at  P level SIMV-BIPAP Continuous SB, both P levels are equal CPAP Continuous SB at 2 P levels Genuine BIPAP

SB possible at all times (open breathing system)  Patient comfort
BIPAP SB possible at all times (open breathing system)  Patient comfort Patient is never locked out No fighting against the ventilator Can cough and clear his airways at any time  Sedation/MR required Improved SB Proph. & ttt of atelectasis No barotrauma or CVS Full ventilatory support, No switching between modes is required

Airway Pressure Release Ventilation (APRV)
SB on 2 CPAP levels P high  PAO2 E P low  CO2

MV is achieved by  instead of  Paw If no SB, APRV = PC-IRV
 Barotrauma ( Pp – Pmean)  CVS Indications: (not clear) Mild ALI Alveolar hypoventilation states with minimal airflow obstruction

Bilevel Positive Airway Pressure (BiPAP) System
A home care device  PSV to augment patient ventilation A non-invasive alternative to traditional management in non life support applications P 2 levels of P P Cycling between the 2 levels is in response to patient F If the patient fails to initiate P change  a timed phase

Useful in (home care): Obstructive sleep apnea COPD
BiPAP Useful in (home care): Obstructive sleep apnea COPD Musculoskeletal disorders

BIPAP = PCV + SB at all times
APRV = similar + extended times at higher Ps BiPAP system = a non continuous form of breathing support

Advantages of different modes
Rests muscles of respiration CMV Patient determines amount of ventilatory support WOB AC Improved patient-ventilator interaction Interference with normal CV function SIMV Patient comfort PSV Allows limitation of PIP Control of I:E PCV

Disadvantages of different modes
No patient-ventilator interaction Requires sedation/NMB Muscle atrophy Potential adverse hemodynamic effects CMV May lead to inappropriate hyperventilation AC WOB compared to AC SIMV Apnea alarm is only backup Variable effect on patient tolerance PSV Potential hyper- or hypoventilation with R/C changes PCV

High Frequency Ventilation (HFV)
What is it? VT (1-3 ml/kg) , f Types: Applied to chest wall: HF body surface oscillations Applied at air openings: HFPPV b/min (60-100) HFJV b/min ( ) HFO b/min ( )

Advantages: Raw and CL don’t affect efficacy of ventilation
HFV Advantages: Raw and CL don’t affect efficacy of ventilation Paw  no  COP, no barotrauma Reflex suppression of SB  no need for sedatives/MR

Bronchoscopy, upper AW procedures ARDS
HFV Indications BP fistula Bronchoscopy, upper AW procedures ARDS Patients at  risk for barotrauma (stiff L + Paw) Patients who cannot be intubated ICP Shock Thoracic surgery (e.g., descending A. Aneurysm) Lithotripsy

Permissive Hypercapnia
Acceptance of  PaCO2, e.g.,  VT to  peak Paw Contraindicated with  ICP Consider in severe asthma and ARDS

Pediatric Considerations
Infants (< 5 kg) Time-cycled, PLV PIP initiated at 18–20 cm H2O Adjust to adequate chest movement or exhaled VT 10–15 mL/kg Low level of PEEP (2–4 cm H2O) to prevent alveolar collapse

Children SIMV mode VT 10 mL/kg Flow rate adjusted to yield desired TI Infants 0.6–0.7 secs Toddlers 0.8 secs Older 0.9–1.0 secs f <18-20 /min PEEP 2-4 cm H2O

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