Applied physiology – II:Respiration, oxygen therapy

Molnár Zsolt

AITI

Anatomy, physiology - the missing link…

Upper airway

• The nose:• Clears

• Heats (32-36)

• humidifies (90%)

The larynx

• Which is the narrowest part?

The larynx

• Which is the narrowest part?• Cricoid and acute surgery

The larynx

• Which is the narrowest part?• Cricoid and acute surgery• Epiglottis• Tracheostomy

Anatomy - thorax

• Breathing• Inspiration: active

• Expiration: passive

• End expiratory pause

• Intrapleural pressure:• Normal value: ±2-3 cmH2O

• Coughing, sneezing: > 60 cmH2O

• Peak inspiratory flow (PIF)• PIF at rest ~ 20-30 l/min

Gas exchange

• Function of breathing• Oxigenation

• CO2-elimination

• Acute respiratory failure• Type I: hypoxic

• Type II: hypercapnic

• Mixed or global

Alveolar oxygenation

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PvO2=40 mmHg

PAO2

PaO2~100 mmHg

120mmHg

PAO2=FiO2 x [(PB-PH2O) – PaCO2/R]

PA-aO2 20 mmHg

PiO2~ 150 mmHg

Venous admixture

Molnár ‘99

PvO2=40 mmHg

120

PaO2 = (120+40)/2 = 80 mmHg PA-aO2 = 40 mmHg

• Normal lungs:– CC in ERV– FRC>CC

• ALI/ARDS:– CC in VT

– FRC<CC

VT

FRCERV

RVCC CC

Closing capacity (CC)

Atelectasis and venous admixture

Molnár ‘99

PvO2=40 mmHg

PaO2 = (120+40)/2 = 80 mmHg

120mmHg

O2

Molnár ‘99

PvO2=40 mmHg

180Hgmm

O2

PaO2 = (120+40)/2 = 80 mmHginstead

PaO2 = (180+40)/2 = 120 mmHg

Atelectasis and venous admixture

• „Iso-shunt” diagramNunn JF. Appl. Resp Physiol., 1993

Degree of venous admixture

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100

200

300

400

PaO

2 Hgm

m

0 5% 10%

15%

20%

25%

30%

50%

FiO2

0,2 0,6 1,0

Oxygen therapy

O2 therapy - indications

Molnár ‘99

– Respiratory distress (resp. rate>24/min or laboured breathing)

– Asthmatic attack

– Hypotension (RRsyst < 100 mmHg)

– Signs of abnormal heart function

– Metabolikc acidosis (act HCO3 < 18 mmol/l)

– Suspected AMI

– Severe trauma and/or severe blood loss

– Sepsis

– Altered level of consciousness

– Drug overdose with confusion

– Smoke, CO, toxic gas inhalation

– Complications during labour

– Transport of the critically ill

– Every postoperative condition

Variable performance devices

Features

Molnár ‘99

• Breathing cycle• Inspiration – expiration – end expiratory pause

• Peak inspiratory flow (PIF): • At rest ~ 20-30 l/min

• Forced inspiration >60 l/min

• Variable performance devices• Fresh gas flow < PIF

• Performance depends on patient’s breathing pattern

• Types• Nasal specs - Face mask – Mask with reservoire balloon

O2-rotameter

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• 3 O2 ports/bed

• Flow:0-16 L/min

Nasal specs

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• FiO2 ~ 30%

• Flow: 2-6 L/min

• Comfortable, cheap

• Dries nasal mucous tissues

Face mask

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• Increases dead space

• Flow: 5-10 L/min

• FiO2 ~ 50%

• Humidification unsolved

Mask with a reservoire

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• Flow: 5-15 L/min

• Balloon

• FiO2 ~ 80%

• Humidification unsolved

Fix performance devices

Oxygen therapy

Features

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• Independent from patient’s breathing pattern• Reason:

• High fresh gas flow > PIF

• Types• Venturi-masks

• Anaesthetic breathing curcuits: Mapleson-systems

• Respirators

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• Bernoulli’s principle– „…increase in the speed of the fluid occurs

simultaneously with a decrease in pressure or a decrease in the fluid's potential energy”

Daniel Bernoulli

1700-1782

1738

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Giovanni Battista Venturi

• Venturi’s principle and injector– „…fluid velocity must increase through the

constriction to satisfy the equation of continuity, the gain in kinetic energy is balanced by a drop in pressure or a pressure gradient force”

1746-1822

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Giovanni Battista Venturi

100*8 + X*21 = (8+X)*50 4 = 0.3X

13 = XFresh gas flow = 13 + 8 = 21 LPM

8 LPM 50% O2

• Venturi’s principle and injector– „…fluid velocity must increase through the

constriction to satisfy the equation of continuity, the gain in kinetic energy is balanced by a drop in pressure or a pressure gradient force”

1746-1822

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From Venturi to Vinturi

• 21st century:

1746-1822

Venturi’s injector + humidification

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• 20-50% FiO2

• 60-30 L/min

• Bernoulli effect

• Humidification• Warm water container

• Heating wire

O2

Air

Side effects of oxygen therapy

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• Insignificant comparing to the benefits • Claustrophoby• Dry mucous membranes• Respiratory depression (COPD)• Hyperoxia

Monitoring

Pulsoximetry

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Pletismographand

Oximeter

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• Continuous

• Doesn’t replace blood gas tests

The pulsoximeter

• Reliability• SaO2 ~ 70-100% (inaccuracy < 5%)

• SpO2 > 94% ~ SaO2>90%Van de Louw A et al. Intensive Care Med 2001; 27: 1606

• Reaction time• 5-8s

• Desaturation reaction time: – Ear probes: 7.2-19.8

– Finger probes: 19.5-35.1

– On toes: 41-72.6 Bishop ML. Anesthessiol Review 1994; 256: 1017Molnár ‘99

The pulsoximeter

Motto

First move in the care of a critically ill:

Give oxygen!