What is Inspiratory Reserve Volume and How is it Measured?

Inspiratory reserve volume refers to the extra volume of air inspired by a person with the maximum effort once they have reached the end of regular inspiration.

A typical abbreviation used for inspiratory reserve volume is IRV, and it was introduced in 1975. It includes all the extra volume of air inhaled above the tidal range.

Here is all you need to know on Inspiratory reserve volume and other lung volumes that help measure the respiratory volumes.

Inspiratory Reserve Volume

What are lung volumes?

Respiratory volumes are also known as lung volumes and refer to the volume of gas inside the lungs during the respiratory cycle at a given time. If you wish to find out the total lung capacity of a person, you must know all the lung volumes. Summation of all the lung volumes helps derive the formula to calculate lung capacity.

The total lung capacity calculated on average in a human male is nearly 6 liters of air. Measuring lung volume is an essential aspect of pulmonary function tests. Lung volumes tend to differ based on different factors; these include:

Depth of respiration.
Body composition
Age
Gender
Ethnicity
Certain respiratory disorders.

There are four lung volumes-

Inspiratory lung volume.
Expiratory lung volume
Tidal volume
Residual volume

Three lung volumes can be measured using spirometry; these are Inspiratory reserve volume, tidal volume, and expiratory reserve volume. However, a residual volume measurement is different. Measurement of remaining residual volume, total lung capacity, and functional residual capacity is done through body plethysmography, helium dilution technique, and nitrogen washout.

Inspiratory reserve volume

It is the volume of air that is inhaled forcibly after the average volume, also called tidal volume. Inspiratory reserve volume is usually kept in reserve, and it is only used when a person requires more air in deep breathing activities. An average adult volume of IRV is 1900 to 3300 ml.

IRV is measured in terms of Inspiratory capacity. Inspiratory capacity or IC is the maximum volume or amount of air breathed in or inhaled after a resting state. It is calculated from the sum of tidal volume and inspiratory reserve volume.

IC= IRV+TV

Tidal Volume or Tv

This is the amount of air that can be breathed in and breathed out during the respiratory cycle of a person. This shows the function of respiratory muscles, respiratory centers, and mechanics of the chest wall and lungs.

The normal value in adults is 10% of the total vital capacity or VC. This is nearly 6 to 8 ml per kg or 300 to 500 ml, but it could increase as much as 50% of vital capacity after exercising or working out.

Expiratory Reserve Volume

It is the maximum volume or amount of air that can be exhaled forcibly after breathing out of average tidal volume. The standard ERV value of a person is 700 to 1200 ml. ERV is usually reduced with ascites or after an upper abdomen surgery and obesity.

Residual Volume

This is the amount of air remaining in your lungs after you have exhaled completely. The average adult value of this lung volume is regarded at 20 to 25 ml per kg or 1200 ml. It is measured indirectly from the summation of ERV and FRC, and you cannot measure it by spirometry.

In obstructive lung disorders with signs of incomplete emptying of your lungs or air trapping, residual volume might be high significantly. Therefore, the Residual Volume may also be expressed as the total lung capacity percentage.

Its value in abundance may raise the risk of pneumothorax, barotrauma, infection, and reduced venous return because of high intrathoracic pressure, as seen in patients having high residual volume who need mechanical ventilation and surgery. Thus patients require significant perioperative inflation pressures.

Inspiratory reserve volume is 3000ml in males, whereas it is only 2100 ml in females.

How to measure respiratory volumes?

If the doctor notices signs of chronic lung diseases, they might implement spirometry to determine how well your lungs are functioning. Spirometry is a vital tool for the diagnosis of different conditions in which lung volume is calculated. these include the following:

Asthma
Cystic fibrosis
Pulmonary fibrosis is a restrictive pulmonary disease
COPD- Chronic obstructive pulmonary disease
Emphysema
Chronic bronchitis

Once the reports come and you are diagnosed with any of the above chronic lung disorders, spirometry is implemented to monitor the progress and to find out if the breathing issues as a result of diseases are appropriately treated or not.

Once all the four lung volumes or respiratory volumes are known, one can calculate lung capacities. You have already seen how to calculate Inspiratory lung capacity or IC; let’s see how to calculate other capacities and how Inspiratory reserve volume is associated with other lung capacities.

Calculation of lung capacities

TLC (Total Lung Capacity)-

It is the highest volume of air your lungs can hold or the sum of all volume sections or volume of air in your lungs following the maximum inspiration. The standard value is approximately 6,000mL, or 4‐6 L. Total Lung Capacity is estimated by summation of all the four principal lung volumes (Tidal Volume, Inspiratory Reserve Volume, Expiratory Reserve Volume, Residual Volume).

Total Lung Capacity might be raised in patients suffering from obstructive diseases such as emphysema and reduced in patients suffering from restrictive deformities, including specific chest wall malformations and kyphoscoliosis.

Vital Capacity or VC

It is the inclusive amount of air breathed out after maximal breath in. The value is approximately 4800mL, and it differs as per age and body size. It is estimated by summation of tidal volume, Expiratory reserve volume, and Inspiratory reserve volume.

VC = TV+IRV+ERV.

Vital capacity refers to our ability to inhale deeply and cough, showing Expiratory and Inspiratory muscle strength. Vital capacity must be 3 times more than Tidal Volume for efficient cough. Vital capacity is often decreased in obstructive disorders, and it is continuously decreased in restrictive disorders.

Functional Residual capacity or FRC

It is the total amount of air left in the lungs after the end of a regular exhalation. It is estimated by summing together Expiratory and residual reserve volumes. The standard value is approximately 1800 – 2200 mL.

FRC = RV+ERV.

Functional Reserve Capacity does not depend on effort and focuses on the resting position when outer, and inner elastic recoils are equivalent or balanced. FRC is decreased in restrictive diseases. The ratio of Functional Reserve Capacity to Total Lung Capacity is an index of hyperinflation. In Chronic obstructive pulmonary disease, FRC is nearly 80% of Total Lung Capacity.

Is lung capacity the same for everyone?

The volume of lung capacity differs from one person to another depending upon physical makeup and the environment they reside in. For example, a person is likely to have larger lung volume if you are physically fit, lives at a high altitude, or is taller.

A person is more likely to have a small lung capacity or volume if they are short, obese, or live in lower altitude settings.

The Bottom Line

The inspiratory reserve volume is the maximum extra air that a person-above a tidal volume- can inhale during a forcibly breathed in. This mainly occurs when a person is exercising or working out.

Inspiratory reserve volume similar to other lung volumes like tidal volume and expiratory reserve volume can be measured during spirometry. IRV is measured as part of data collected in pulmonary function tests to find out issues related to obstructive and restrictive pulmonary disease.

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