# Volumes and Capacities

Explain the measurement of lung volumes and capacities, and factors that influence them

State the normal values of lung volumes and capacities

Define closing capacity and its clinical significance and measurement

The lung has four volumes and four (main) capacities:

- A
**volume**is measured directly - A
**capacity**is a**sum of volumes**

## Volumes

**Tidal volume**(V_{T})

Volume of air during normal, quiet breathing.- Normal is
**7ml.kg**, or 500ml^{-1}

- Normal is

**Inspiratory reserve volume**(IRV)

Volume of air that can be inspired**above tidal volume**.- Normal is 45ml.kg
^{-1}, or**2500ml**

- Normal is 45ml.kg

**Expiratory reserve volume**(ERV)

Volume of air that can be expired following tidal expiration.- Normal is 15ml.kg
^{-1}, or**1500ml**

- Normal is 15ml.kg

**Residual volume**(RV)

Volume of air in the lungs following a maximal expiration.- Normal is 15-20ml.kg
^{-1}, or**1500ml**

- Normal is 15-20ml.kg

## Capacities

**Functional Residual Capacity**(FRC)

FRC = RV + ERV.- Normal is
**30ml.kg**or 3000ml^{-1} - FRC decreases 20% when supine, and a further 20% under general anaesthesia

- Normal is

**Vital Capacity**(VC)

VC = ERV + V_{T}+ IRV.- Normal is 4500ml

**Inspiratory Capacity**(IC)

IC = V_{T}+ IRV.- Normal is 3000ml

### Functional Residual Capacity

The FRC has many important physiological functions:

**Gas exchange**

The FRC allows blood in the pulmonary circulation to become oxygenated throughout the respiratory cycle (if there was no FRC, then at expiration the lungs would be empty and no oxygenation would occur).**Oxygen Reserve**

FRC is the only clinically modifiable oxygen store in the body, and allows continual oxygenation of blood during apnoeic periods.

**Minimise Work of Breathing**

Work of breathing is a function of lung resistance and compliance.- The lung sits on the
**steepest**part of the**compliance occurs**at FRC**Compliance is optimised**as:- Alveoli are open and minimally distended

**Airway resistance decreases**as airway radius increases as lung volume increases

- The lung sits on the

**Minimise RV Afterload**

PVR is minimal at FRC.

**Maintain lung volume above closing capacity**

If closing capacity (see below) exceeds FRC, then shunt will occur.

Factors affecting FRC:

- FRC is reduced by:
- Supine positioning

Falls by ~20%. - Anaesthesia

Falls by ~20%. - Raised intra-abdominal pressure
- Impaired lung and chest wall compliance

- Supine positioning
- FRC is increased by:

## Measurement of Lung Volumes and Capacities

- ERV, V
_{T}, and IRV can all be measured directly using spirometry- A
**spirometer**is a flow meter- The patient exhales as fast as possible through the flow meter
- A flow-time curve is produced
- This curve can be integrated to find volume

- A
- Any capacity which is a sum of these (IC, VC) can therefore be calculated

- RV
**cannot**be measured by spirometry, as it can't be exhaled - RV can be measured using:
- Gas dilution
- Body plethysmography

### Gas Dilution

- Gas dilution relies on two principles:
- Conservation of Mass
- Helium has poor solubility and will not diffuse into circulation

**Limitations**of gas dilution:- Only gas communicating gas can be measured - will underestimate FRC in gas-trapping

- Method:
- Patient takes several breaths from a gas mixture containing a known concentration of helium (giving time for equilibration)
- The concentration of expired helium is then measured

From the law of conservation of mass:

$$C_1V_1 = C_2V_2$$

### Body Plethysmography

- Body plethysmography relies on:
- Boyles law

Pressure and volume are inversely proportional at a constant temperature, i.e. ($$P \times V = k$$).

- Boyles law

- Method:
- Patient is placed in a closed box, with a mouthpiece that exits the box
- The patient inhales against a closed mouthpiece:
- When the patient inhales, the volume of gas in the box decreases (the patient takes up more space) and therefore the pressure increases
- The change in volume of the box is given by:

$$P_1V_1 = P_2V_2$$, where:- $$V_2$$ is the change in box volume, or $$V_1 - \Delta V$$
- Therefore:

$$P_1V_1 = P_2(V_1 - \Delta V)$$

As $$\Delta V$$ is the only unknown value, it can be calculated.

- The change in volume of the lung must be the same as the volume of the box ($$\Delta V$$)

## Closing Capacity

- Closing capacity is
**volume at which small airways begin to close**

Closing capacity is the sum of residual volume and closing volume.- Because dependent lung is compressed by gravity, dependent (typically basal) airways are of smaller calibre than non-dependent (typically apical) airways
- During expiration, these airways are compressed first

Alveoli connected to these airways are isolated, and**V/Q scatter or shunt**occurs. - If
**closing capacity exceeds FRC**, then airway closure occurs during normal tidal breathing

This occurs when: - This is clinically relevant during
**preoxygenation**, as it will limit the denitrogenation that can occur

### Measurement of Closing Capacity

Closing capacity is measured using Fowlers Method, and is covered under Dead Space.

## References

- Chambers D, Huang C, Matthews G. Basic Physiology for Anaesthetists. Cambridge University Press. 2015.