Respiratory Support

M6506 Clinical and Healthcare Services Engineering

21/2/2008

Applications

• Mechanical Ventilation– Negative and Positive Pressure Ventilation

• Artificial lung– Bubble and Membrane Oxygenators

Negative Pressure Ventilation

• Used to treat respiratory paralysis, especially when caused by polio

• First demonstration in 1824

• Two types: – Tank– Cuirass

Negative Pressure Ventilation - Principle

Hypobaric ChamberMotor

Iron Lungs

Image Courtesy of Virtual Museum of the Iron Lung, © Richard Hill

Iron Lung, Singapore, 1952

Modern Iron Lung c. 1980

Image Courtesy of Virtual Museum of the Iron Lung, © Richard Hill

Other Negative Pressure Ventilators

Images from A.K. Simonds (ed.), “Non-invasive Respiratory Support”, Arnold, London, 2001

Bodysuit ventilator Cuirass Ventilator (Hayek Oscillator)

Cuirass Ventilator

Negative Pressure Ventilation

• Advantages– Patient can talk

• Disadvantages– Uncomfortable – Pooling of blood in abdomen with tank

ventilator– Not as effective as PPV

Positive Pressure Ventilation

• Invasive – Tracheotomy or tracheal device

• Non-invasive– access via face/nose mask

Positive Pressure Ventilation

Oxygen Supply (optional)

Ventilator

controller

Face mask

PPV Equipment

For more, visit: http://freespace.virgin.net/michael.bowell/ventgall.html

PPV Equipment

For more, visit: http://freespace.virgin.net/michael.bowell/ventgall.html

Positive Pressure Ventilation

• Volume targeted– Ventilator is programmed to deliver a fixed

volume of gas with each breath• More efficient

• Uncomfortable

Positive Pressure Ventilation

• Pressure targeted– Ventilator is programmed to stop delivering gas

when maximum pressure is achieved• More comfortable

• May not deliver enough O2

Other Ventilation Modes

• Continuous Positive Airway Pressure (CPAP)– Positive pressure maintains patency of airway– Used to treat obstructive diseases of the airways

such as tracheal cancer

Critical Care, 1960s

Critical Care, 1980s

Critical Care, 1990s

Communicating on a ventilator

Artificial Lung (Oxygenator)

• Needed when:– Obstructive condition prevents gas entering

lungs– Surgeons are operating on the lungs or heart

• Mechanical pumping can damage lungs

– Condition of lung prevents sufficient gas transfer

Extracorporeal Oxygenation

• Used for relief of heart and lung function in cardiopulmonary bypass and adult and neonatal respiratory failure– Mechanical replacement of heart/lung function– Hypothermic conditions– Duration: ECLS - hours

ECMO/ECCO2R - days / weeks

Modes of Operation

• Extracorporeal Life Support (ECLS)– Heart and Lungs are isolated– Pump and Oxygenator take over their function– Principal means of life support in

cardiopulmonary bypass procedures– Short term - hours

Modes of Operation

• Extracorporeal Membrane Oxygenation (ECMO)– Long term life support (days-months)– Partial bypass– Blood is taken from patient, oxygenated and

returned– Some mechanical ventilation of lungs (very low

frequency

Modes of Operation

• Extracorporeal Carbon Dioxide Removal (ECCO2R)

– Long term life support (days-months)

– Removes all metabolically produced CO2 from blood

– Oxygen is taken up by passive diffusion in lungs– Flow rate is lower than ECMO - less biomaterial

exposure

Short history of Oxygenators1885 - Demonstration of disc oxygenator.

1916 - Discovery of Heparin, the first safe reversible anticoagulant

1920s & 30s animal experiments demonstrate feasibility of extracorporeal circulation using excised lungs and direct contact devices

1953 - First clinical use of Oxygenator (Gibbon)

1956 - First disposable membrane oxygenator

1971 - Introduction of silicone rubber hollow fibres

1980s - Hollow fibre membrane oxygenators overtake direct contact (bubble) oxygenators for first time

Oxygenators - 1953 to 2004

Univox Oxygenator, 1992

Gibbon’s screen oxygenator, 1953

Early flat platemembraneoxygenator

1956

CPB in Practice

Nursing students observing cardiothoracic surgery, TTSH, 1977 – note perfusionists

perfusionists

Extracorporeal Oxygenation

• Patient is resting– Low metabolic demand– Metabolic demand is reduced further by

hypothermia

– Venous blood (O2 pressure = 40 mmHg) is oxygenated (O2 pressure = 100-300 mmHg)

– CO2 is removed

Venous blood from vena cava

Filter andreservoir

Heatexchanger

Oxygenator

Heart Lung Bypass Circuit

Water in

Water out

Gas in

Gas out

Types of Oxygenators

1. Bubble Oxygenator

• Advantages: Relatively cheap, simple to use

• Disadvantages: Increased risk of thrombosis, poor compatibility, defoamers required. Longer post operative recovery.

Bubble oxygenator

1

1 Oxygen and venous blood enter oxygenator

2 Tiny O2 bubbles mix with

ascending blood stream, Gas exchange occurs

3 Arterialised blood contacts chemical defoamer and exhaust gas is expelled

4 Arterialised blood leaves oxygenator before going on to filters and bubble traps

3

4

2

Venous bloodO2

Types of Oxygenators

2. Membrane Oxygenator

• Advantages: Less damaging to blood than bubble. Can be used for longer periods, with shorter post operative course.

• Disadvantages: Longer set up time. More expensive than bubble.

Membrane Oxygenator - Principle

1

1 Venous blood enters device at gas outlet (counter-current operation)

2 Blood and gas sides separated by membrane permeable only to gas. Gas exchange takes place.

3 Arterialised blood collects in arterial manifold and passes to body via filter to remove any thrombi (blood clots).

2

3

Membrane Oxygenator Geometry

Blood

Gas

Gas

Blood

Blood

Gas

Gas

Blood

a

c

b

d

a: Intraluminal flowb: Extraluminal parallel flowc: Cross flow (perpendicular)d: Cross flow (spiral wound)

Types of Oxygenators

3. Intravascular Oxygenator

• Inserted into vena cava via femoral vein

• Advantages: Non biologic surface contact area minimised.

• Disadvantages: Cannot yet supply all patient’s metabolic O2 needs. Systemic anticoagulant needed.

Performance of membrane oxygenator

• Under-pressure vs. over-pressure– Air Embolisms are bad!

• Boundary layers

• Pore wetting in microporous oxygenators

Performance of membrane oxygenator

• Under-pressure

Membrane ruptures, blood leaks out Leaking blood clots, oxygenator can still function

Performance of membrane oxygenator

• Over-pressure

Membrane ruptures, air enters blood compartment

Air embolus lodges in artery:Stroke, CVA, PVD

Build up of oxygenated layer

Oxygen

Arterialised blood, S=100%

Venous blood, S=0.65

Computational Model - Haemoglobin Saturation

l=0 l=L

65%

100%

– Note that the boundary between saturated and unsaturated is quite sharp!

Computational Model - Oxygen Partial Pressure

l=0 l=L

Pgas

– For pressure the gradient is larger, as can be seen here

Pin

Computational Model - Oxygen Concentration

l=0 l=L

Cin

Cmax

– The difference in concentration between red layer and the wall is mostly due to dissolved O2

Mechanism of pore wetting

Blood plug infiltratesinto pores

Polar Phospholipids coathydrophobic membrane

Ways to prevent pore wetting

• Use homogeneous membranes (currently the only way!

• Develop microporous membranes with an ultra-thin homogeneous layer

Full model of cross flow oxygenator: Advanced Simulation and Design

Gmbh