Raafat Abdel-Azim1

Bellows assembly

Distensible bellows

Rigid housing

Control Unit

Raafat Abdel-Azim

Control Unit

Bellows assembly

Distensible bellows

Rigid housing

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Raafat Abdel-AzimBS3Driving Gas Flow

Raafat Abdel-Azim

Spill

Exhaust

Safety relief

Driving Gas Flow BS

Insp

Raafat Abdel-AzimBS

Spill

Exhaust

Safety relief

Flow5

Raafat Abdel-AzimBS6

Raafat Abdel-AzimBS

To atmosphere

Exp

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Spill

Scavenging

Overpressure

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Spill Valve

To scavenging system

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Spill Valve

I & beginning E End E (2-4 cmH2O)

Driving gas

Patient’s gas

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Traditional circle system

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Traditional circle system

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Components of the Ventilator

• Driving Gas Supply• Injector (some)• Controls (F, V, T, P)• Alarms• Safety-Relief Valve• Bellows Assembly• Exhaust Valve• Spill Valve• Ventilator Hose Connection

preset 65-80 cmH2O adjustable

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Injector

Driving gas

Air

Air

Some ventilators use a device called an injector (Venturi mechanism) to increase the flow of driving gas. As the gas flow meets restriction, its lateral pressure drops (Bernoulli principle). Air will be entrained when the lateral pressure drops below atmospheric. The end result is an increase in the total gas flow leaving the outlet of the injector but no increase in consumption of driving gas

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Control of Parameters of Ventilation

• VT

• VM

• f (frequency= rate)

• I:E ratio

• IFR (Insp. Flow Rate)

• Maximum Working Pressure

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Narkomed - Drager

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Fabius - Drager

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Cisero - Drager

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Dameca

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 = 360 1 1000 0.5 2.5 1:5

30 0.5 500 1 2 1:2

Raafat Abdel-AzimTime (sec)

0.500

100

200

300

400

500

600

1.5 2.5 3.5 4.5 5.51 2 3 4 5 6

60 L/min30 L/min

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Inspiratory Waveforms

Raafat Abdel-Azim

30

30

60

60

0

Constant Decelerating Accelerating Sinusoidal

TI TI TI TI

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The Ventilation Cycle

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Paw

0 t

PmaxPplat

IF E

ZEEP

IPPV

20

I EP

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• 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 T27

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Inspiratory Phase

During the IF period:Paw depends on:

• The airway resistance (R)

• The total thoracic compliance (C) (V/P)

R C28

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PP PP

P

P

P

PP

PP

P

P

P

Resistance

Flow Rate: FRP29

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Paw

0 t

20

N R F R F

Resistance

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Compliance

C

C

P PVolume

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Compliance

PVolume: VP

P

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During I 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).

Pause

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Paw

0 t

20

C R

Secretions

Bronchospasm

Kinked ETT

EB intubation

CW rigidity

Pulmonary edema

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Compliance = V/P

• Dynamic compliance:

= VT/(PIP-PEEP) L/cmH2O

• Static compliance:

= VT/(Pplat-PEEP) L/cmH2O

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Raafat Abdel-Azim

During the expiratory phase

VT

FRC

FRC

EI

FRCIA contents•Pulmonary edema•ARDS

PEEPPaO2 PaO2

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Goals of Pulmonary Ventilation

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• To provide adequate minute alveolar ventilation

• and to side effects

necessary to maintain the desired PaCO2

PPV ITP

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Adequate Ventilation

• PaCO2 of 40 mmHg = 5.3% of 760 mmHg40/760 = 0.053

• Normal resting VCO2= 200 ml/min= 0.2 L/min

• This requires VM of 3.8 L0.2/ ? = 0.0530.2/ 0.053 =

• Add dead space

Goals

3.8 L/min

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• VD phys: ventilated but not perfused• N = 1 ml/pound 150 ml in a 70 Kg (154 pound) adult• = 0.15 x 10 (f) =

• Required VM = 3.8 + 1.5 =

• A larger VM is required for patients who have VD or VCO2

1.5 L/min

5.3 L

Goals, Adequate Ventilation

For VCO2 For VD

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• 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

Goals, Adequate Ventilation

V1 x P1aCO2 = V2 x P2aCO2

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Side effects

Goals

ITP

VR

EDV

CO

PVR RVAPPV

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Goals, Side Effects

PPV

ITP PA > PAP VVD

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All these effects mean Paw

Therefore, a goal of PPV is to mean Paw while maintaining adequate ventilation and oxygenation

Goals

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Modes of Pulmonary Ventilation

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Paw

0 t

SB

-ve

+ve

Anesthesia

ICU: demand valve WOB CPAP

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Paw

0 t

CPAP

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Paw

0 t

IPPVAnesthesia (or sedation) + MRFew days muscle atrophy

ZEEP

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Paw

0 tPEEP

IPPV + PEEP

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Assisted Ventilation (A)Paw

0

A

t

S

Patient f and timingHazard: hypoventilation

If or No S

No A

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Assisted Ventilation (A)Paw

0

CPAP

A

t

Patient f and timingHazard: hypoventilation

If or No S

No A

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Paw

0

A+CProvides a minimum f below which C

Assist-Control (AC)

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• Conscious patients are more comfortable on A & AC > C

• Muscle atrophy is still a problem

• Most AV don’t provide A or AC

AC

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IMV

0

Paw

St

Preset VT and f

SB is allowedMuscle atrophy is less likely

IMV PaCO2 < apneic threshold no SB IMV = IPPV

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Paw

0 tTriggering

window

Mandatory

S

Synch. Mandatory

NO

Preset VT and fSIMV

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Paw

t

Pressure-limited ventilation (PLV) (PCV)

Pmax

Pplat

0

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Ventilator Classification

• Pressure versus Volume Ventilators

• Single-Circuit versus Double-Circuit Ventilators

• Bellows Assembly

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P vs V Ventilators

P Ventilators V Ventilators

Changes in R & C change

VT P

Commonly used in

Neonates Barotrauma Compressible V

ICU

Most AV

Other names P generators

P-limited

P-stable

Flow generators

V-stable

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ICU

Anesthesia

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Bellows assembly

Distensible bellows

Rigid housing

Driving gas BS to patient

E

I

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Drager (End-insp.)

Ohmeda (End-exp.)

VT

VT

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NA Drager bellows Ohmeda bellows

E Expands partially Fully expands

I Empties completely Partially empties

Spill valve External (visible) Hidden in housing

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Ascending bellows Descending bellows

E

AttachmentMovement

EEPVT adjustment

Disconnection or leak

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Interaction between the Ventilator and Breathing Circuits

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Preset / delivered VT difference due to:

1. Effect of circuit compliance

for all V ventilators: both single circuit and double circuit

2. Effect of FGFR

only for double circuit ventilators

Interaction

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Effect of Circuit ComplianceInteraction

PIP Delivered VT

Circuit compliance = Compressible volume (gas compression, tubing distension)

CC= the V of gas that must be injected into a closed circuit to cause a unit in the circuit P

It is related to the volume of the circuit PV

Ventilator tubing V C

Anesthesia 6 L 9 ml/cmH2O

ICU (1.5 ml/cmH2O)66

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• Preset – Del VT difference CC and PIP

Interaction, Circuit Compliance

Del VT = Preset VT – [(PIP - PEEP) x CC](i.e, VT loss = P x CC)

Ventilator tubing

CC

(ml/cmH2O)

PIP

(cmH2O)

V Loss

(ml)

Anesthesia 9 20

30

180

270

ICU 1.5 20

30

30

45

Example:

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Volume loss is specially important in:

• Patients with PIP

ICU patients

• Patients with small VT

Neonates PLV

Interaction, Circuit Compliance

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Effect of Fresh Gas Flow Rate(from anesthesia machine)

Interaction

FGFR Delivered VT

V

FGF from AM

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Volume added = TI x FGFR

TI depends on

I:E I fraction of the respiratory cycle f

TI = 60/f x I fraction

Example: I:E = 1:2, f = 20, FGFR = 6 L/min

TI = 60/20 x 1/3 = 1 s

FGFR = 6000 ml/min = 100 ml/s

V added = 1 x 100 = 100 ml

At a certain I:E, f TI V added

Interaction, FGFR

I:E I fr

1:1 ½

1:2 1/3

1:3 ¼

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Interaction, FGFR

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Volume added is specially important:

When making large changes in FGFR

O2 flush 55 L/min (don’t use during MV)

When delivering small VT

For a neonate, changing FGFR from 1 to 5 L/min double VT

Interaction, FGFR

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Delivered VT = Preset VT - V lost (compression) + V added (An. machine)

Interaction

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Fresh Gas Decoupling (FGD)

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Circle system with piston ventilator and fresh gas decoupling (FGD)(Dräger Narkomed 6000)

Fresh Gas Decoupling (FGD)

During the I phase the FG coming from the AWS through the fresh gas inlet is diverted into a separate reservoir by a “decoupling valve” (located between the fresh gas source & the circle BS). In the case of the NM 6000 series, the reservoir bag serves as an accumulator for FG storage until the E phase begins.

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Raafat Abdel-Azim

During the E phase, the decoupling valve opens, allowing the accumulated FG in the reservoir bag to be drawn into the circle system to refill the piston ventilator chamber. Because the ventilator exhaust valve also opens during the E phase, excess FG & exhaled patient gases are vented into the scavenging system

NPR valve = negative pressure relief valve

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• Most FGD systems are designed with either piston-type or descending bellows-type ventilators.

• Because the ventilator piston unit or bellows in either of these type systems refills under slight negative pressure, it allows the accumulated fresh gas from the reservoir to be drawn into the breathing circuit for delivery to the patient during the next ventilator cycle.

• As a result of this design requirement, it is not possible for FGD to be used with conventional ascending bellows ventilators, which refill under slight +ve pressure

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Advantage of FGD

risk of barotrauma & volutrauma

With a traditional circle system, in FGF from the flow meters, or from inappropriate activation of the oxygen flush valve, may contribute directly to VT excessive FG pneumothorax, pneumomediastinum, other serious patient injury, or even death.

Because systems with FGD isolate the patient from FG coming into the system while the ventilator is in I phase, delivered VT tends to remain stable over a broad range of FGF rates, and the potential risk of barotrauma to the patient is greatly reduced.

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Disadvantages of FGD

1. The possibility of entraining room air into the patient gas circuit

• In a FGD system, the ventilator piston unit (or descending bellows) refills under slight -ve pressure. If the total volume of gas contained in the reservoir bag + that returning as exhaled gas from the patient’s lungs is inadequate to refill the ventilator piston unit, -ve patient Paw could develop.

• To prevent this, a negative pressure relief valve is placed in the FGD BS. If BS P < a preset threshold value, then the relief valve opens and ambient air is allowed to enter into the patient gas circuit.

• If this goes undetected, the entrained atmospheric gases could lead to dilution of either or both the inhaled anesthetic agent(s) or an enriched oxygen mixture (lowering an enriched oxygen concentration toward 21%).

• If allowed to continue, this could lead to intraoperative awareness and/or hypoxia.

• High-priority alarms with both audible and visual alerts should notify the user that fresh gas flow is inadequate and room air is being entrained.

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2. If the reservoir bag is removed during mechanical ventilation, or if it develops a significant leak (from poor fit on the bag mount or a bag perforation), room air may enter the breathing circuit as the ventilator piston unit refills during the expiratory phase.

This could also result in dilution of either the inhaled anesthetic agent(s) concentration and/or the enriched oxygen mixture. This type of a disruption could lead to significant pollution of the operating room with anesthetic escaping into the atmosphere.

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Hazards of Ventilators

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• Apnea– Erroneous setting of ventilator selector

switch– Ventilator switched off– Disconnection– Obstruction of inspiratory gas pathway– Ventilator failure

Hazards

V B

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• Hypoventilation– Circuit leak– Incompetent spill valve– Open APL valve in circuit– Driving gas leak– Improper settings– Ventilator limitation at high airway

pressure

Hazards

PL

S

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Hyperventilation & dilution of anesthetic gasesBellows leak

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• High airway pressure

Exhaust obstruction

Obstruction of expiratory gas pathway

Ventilator cycling failure

Improper settings

Oxygen flush during inspiration

Bellows leak

Hazards

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Summary of important pointsSummary of important points

Raafat Abdel-Azim

•Driving gas and patient gas don’t mix•The ventilation cycle is controlled by F, V, P and T•During the I phase, Paw depends on R and CT

•Goals of MV are to provide adequate VM (necessary to maintain

the desired PaCO2) and to side effects•Modern AV provide a range of ventilatory modes•The difference between the delivered VT and the preset VT depends on CC and FGF•FGD decreases the risk of barotrauma and volutrauma•Hazards of MV can be avoided

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Examples of questions to assess the ILOsExamples of questions to assess the ILOs

Raafat Abdel-Azim

•Describe causes and ventilatory management of high airway pressure during inspiratory phase of mechanical ventilation•Discuss goals of pulmonary ventilation under anesthesia•With IPPV: if f (rate) = 10/min, I: E= 1:1 and FGF= 3 LPMWhat will be the volume added to the preset tidal volume?

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Thank you

http://telemed.shams.edu.eg/moodle

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