Basic Sciences - Respiratory Physiology Flashcards by Thomas Eraifej

SI unit for airway pressure

cm H2O

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SI unit for partial pressure of gas

kPa

(kilo Pascal)

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Definition of cm H2O / mmHg

Equivalent pressure exerted by a column of fluid of a given height, being acted on by gravity

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kPa equivalent to 1 Atmosphere at sea level

101

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cm H2O equivalent to 1 Atmosphere at sea level

1030

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m H2O equivalent to 1 Atmosphere at sea level

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mmHg equivalent to 1 Atmosphere at sea level

760

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Approximate oxygen requirement increase during exercise

Up to 10x increased

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Alveolar surface area for gas exchange in adults

80-90 metres squared

Approx half singles tennis court

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Approx tidal volume in adults

6-8 ml/kg

Approx 500ml

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Anatomical dead space approximate volume

150 ml

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Alveolar ventilation approximate volume

350 ml

(Tidal volume - Anatomical dead space)

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Minute ventilation calculation

Minute ventilation = Tidal volume x RR

Eg. 500 x 14 = 7000 ml/min

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Alveolar minute ventilation calculation

Alveolar minute ventilation = Alveolar ventilation x RR

Eg. 350 x 14 = 4900 ml/min

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Spirometry trace and volume terms

Tidal volume
Inspiratory reserve volume
Expiratory reserve volume
Vital capacity
Total lung capacity
Residual volume
Functional residual volume

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Approximate inspiratory reserve volume for young fit male

3 L

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Approximate expiratory reserve volume for young fit male

1.5 L

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Approximate residual volume for young fit male

1 L

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Approximate total lung capacity volume for young fit male

6 L

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Approximate vital capacity volume for young fit male

5 L

Approximate functional residual capacity volume for young fit male

2.5 L

Functional residual capacity definition

Quantity of gas in the lungs at end of normal expiration

Why functional residual capacity is important in anaesthetics

Provides oxygen reservoir during apnoea

FRC influences distribution of ventilation within lung by determining starting position of each lung area on the compliance curve

Factors which reduce functional residual capacity

Reduced muscle tone:
- General anaesthesia

Increased intrathoracic pressure:
- Supine from standing
- Obesity
- Pregnancy

Negative pressure generated during inspiration in alveoli

Around 2 to 3 cm H2O below atmospheric pressure

What generates positive pressure during expiration in alveoli

Elastic recoil of lungs and chest wall

Dominant muscle contributor during quiet breathing

Diaphragm > intercostal muscles

How does Intermittent Positive Pressure Ventilation generate respiration

Increases pressure at mouth to create pressure gradient

Approximate pressures usually required during IPPV to deliver same tidal volume as normal breathing

Higher pressures needed Around 10 to 15 cm H2O

Undesirable effect of IPPV on circulation

- Increase in mean intrathoracic pressure - Reduces venous return to heart - Fall in cardiac output Similar effect to Valsalva Manoeuvre

Location of respiratory centres

Medulla and Pons

Description of respiratory centres

Groups of neurons which act together to control respiration

Factors which impact respiratory centres

Central chemoreceptors (increase in CO2) Peripheral chemoreceptors (decrease in O2) Cortex (volume control) Stretch receptors (muscle activity)

Most important factor in controlling respiration and reference range

Arterial PaCO2 Usually kept between 5.1 and 5.5

CO2 response curve

As arterial PaCO2 rises, Minute ventilation increases Lower end of curve flattens as automatic firing of respiratory centre maintains minimum minute ventilation

Effect of opioids and GA on CO2 response curve

Depress CNS and make chemoreceptors less sensitive to PaCO2 Curve shifts to right and becomes less steep

Two factors which need to be overcome for gas to reach the alveoli

Resistance (obstructive airway disease) Compliance (restrictive lung disease)

Compliance curve

Initial part of curve - high pressures needed to inflate alveoli from collapsed state Once inflated lungs become compliant until chest wall reaches limit of expansion

Forces involved in expiration

Passive process usually Energy used to overcome compliance in inspiration is stored as potential energy in the elastic tissues This energy used to generate pressure gradient for expiration

Distribution of ventilation in the lungs in normal patient breathing spontaneously at rest

Apices more expanded than bases Therefore greater compliance at bases and mid zone of lungs Bases and mid zones receive greater ventilation

Distribution of perfusion in the lungs in normal patient breathing spontaneously at rest

Due to gravity, perfusion pressure reduces by 1 cmH2O for every cm in height above heart level, and increases by 1 cmH2O for every cm below heart level

Ventilation / Perfusion matching in lungs (V/Q)

Both ventilation and perfusion are greater towards bases of lungs Therefore ventilation and perfusion are well matched

Definition of shunt

Area of lung becomes occluded (airway closure or secretions) No ventilation to this area of lung

Effect of a lung shunt

Unoxygenated blood mixes with blood from ventilated blood to give arterial hypoxaemia Increasing inspired O2 will not correct hypoxaemia as area not ventilated Need to clear obstruction

Definition of alveolar dead space

Perfusion ceases due to occlusion (thrombus or air or drop in cardiac output) No perfusion to ventilated area

Effect of alveolar dead space

Drop in end tidal CO2 Gas not involved in respiratory exchange dilutes CO2 concentration from rest of lung gas

Treatment of incomplete alveolar obstruction

Increase FiO2 as some ventilation occurs so aim to increase O2 in alveoli for gas exchange

How does general anaesthesia produce a degree of V/Q mismatch

- Fall in FRC - Lung moves down compliance curve - Lung bases have less ventilation - Perfusion distribution unchanged as gravity influenced Results in V/Q mismatch