Respiratory Physiology

Question 1

How big is the area in the lung for alveolar gas exchange?

Answer 1

80-90m2

Question 2

What is the average tidal volume and what is it made up of?

Answer 2

500ml

Anatomical dead space 150ml
Alveolar ventilation 350ml

Question 3

What is the formula for minute volume and alveolar minute volume?

Answer 3

TV x RR =MV

500ml x 14 = 7000ml

AV x RR = AMV

350ml x 14 = 4900ml

Question 4

Label a spirometry trace.

Answer 4

VT Tidal volume (500 mL)

IRV Inspiratory reserve volume (3000 mL)

ERV Expiratory reserve volume (1500 mL)

RV Residual volume (1000 mL)

TLC Total lung capacity (6000 mL)

VC Vital capacity (5000 mL)

FRC Functional residual capacity (2500 mL)

Question 5

Define FRC and explain its significance in anaesthesia?

Answer 5

It is the quantity of gas in the lungs at the end of a normal expiration.

1. During apnoea it provides the reservoir in the lungs from which oxygen can be taken to maintain arterial oxygenation – the greater the oxygen reservoir, the longer the time before hypoxaemia develops
2. FRC has a major influence on the distribution of ventilation within the lung by determining where the starting position of each area of the lung is on the compliance curve

Question 6

What factors influence your FRC?

Answer 6

Gravity (aka being in the supine position)

Obesity

Pregnancy

Anything which may splint the diaphragm.

General anaesthesia.

Question 7

In which structure does gas exchange first occur and at which generation is this?

Answer 7

In the respiratory bronchioles around generation 16.

Question 8

What causes the flow of gas during spontaneous ventilation?

Answer 8

Diaphragm moves down and the external intercostals contract moving the rib cage up and out; resulting in the lungs expanding this creates a -ve pressure in the lungs causing gas to flow in. (~2-3cmH20)

Flow of gas resulting from respiratory effort only occurs down to division 16.

Below this, gases move passively by diffusion along partial pressure gradients, between the alveoli and higher airways.

Question 9

What nerve supplies the diaphragm and what are its roots?

Answer 9

Phrenic nerve C3, C4, C5

Question 10

What effect does IPPV have on cardiac output?

Answer 10

Due to increased intra-thoracic pressure, there is reduced venous return resulting in a reduced SV.

Question 11

Where is the respiratory centre located?

Answer 11

Medulla and Pons

Question 12

What are the influences on respiratory rate and depth?

Answer 12

CO2 central chemoreceptors (most important factor) aiming to keep atrerial CO2 between 5.1-5.5kPa

Peripheral O2 chemoreceptors

Voluntary control (cortex)

Stretch receptors in muscles

Question 13

Draw/describe the shape of the compliance curve?

Answer 13

Sigmoid shaped
Pressure on X axis
Volume on Y axis

Lots of pressure needed to initially inflate alveoli.

Middle zone of peak compliance

Once mostly inflated less compliant

Question 14

What force is responsible for expiration

Answer 14

Elastic recoil

Question 15

How does the compliance vary throughout the lung in someone in an upright position?

Answer 15

The apices are well expanded/ventilated (towards the right hand side of the compliance curve)

The middle zone and bases are less expanded/ventilated (in the middle zone of the compliance curve) therefore more compliant

Any collapsed alveoli aka in atelectasis would be on the left hand side of the compliance curve.

Question 16

Describe how perfusion varies in the lung in a patient in an upright position?

Answer 16

The effect of gravity reduces the perfusion pressure by 1 cm H2O for every cm in height above the level of the heart, with a similar increase below.

Apices poorly perfused.

Bases well perfused.

Question 17

In the upright position which area of the lung has the best V/Q matching?

Answer 17

The lung bases as they are well perfused, and they are on the middle zone of the compliance curve (partially expanded but not collapsed)

Question 18

Define shunt?

Answer 18

Shunt is where there is an area of lung which is not ventilated but is being perfused.

This results in non-oxygenated blood being mixed with oxygenated blood resulting in a drop in arterial O2.

This would be caused by: atelectasis/lobar collapse

Question 19

What will happen in a patient with a pulmonary embolism?

Answer 19

The patients’ lungs will still be ventilated but will not be perfused.

Note the patients end tidal CO2 will drop as the gas not involved in respiratory exchange will dilute the CO2 concentration coming from areas of lung that have been.

Question 20

Describe the functional residual capacity in relation to the forces which influence it?

Answer 20

It represents the point at which the elastic recoil force of the lung (inward force) is in equilibrium with the elastic recoil of the chest wall (outward force).

Question 21

Why does the FRC drop during general anaesthesia?

Answer 21

Due to loss of inspiratory muscle tone of the muscles acting on the rib cage, aka they have less elastic recoil.

As FRC is the point at which there is equilibrium of the elastic recoil of the lung (inward force) vs the chest wall (outward force).

Due to the increase of the inward force there is a reduction in the FRC.

Note the effect of this will be more pronounced in the obese

Question 22

What is the significance of the drop of FRC under anaesthesia?

Answer 22

It moves the lung down the compliance curve, therefore the bases are more likely to be collapsed, creating a greater V/Q mismatch.