Respiratory Volumes

Introduction

The volume of air in the lungs at different stages of the respiratory cycle is called Respiratory volumes definition and lung capacities. An adult human male’s total lung capacity is around 6 litres of air.

Normal resting breathing is tidal breathing, and the tidal volume is the amount of air inhaled or expelled in a single breath. At birth, the average human respiratory rate is 30–60 breaths per minute, dropping to 12–20 breaths per minute in adults. Respiratory volumes are an important part of pulmonary function tests since they may tell you a lot about how healthy your lungs are.

Respiratory Volumes

Respiratory volumes are another name for lung volumes. During the respiratory cycle, it refers to the amount of gas in the lungs at any one time. To calculate lung capacities, the sum of several lung volumes is used. An adult human male’s total lung capacity is around 6 litres of air. The measurement of lung volumes is an important element for the pulmonary function test. They vary based on the depth of breathing, ethnicity, gender, age, body composition, and the presence of specific respiratory illnesses. Spirometry can measure a multitude of lung volumes, including tidal volume, inspiratory reserve volume, and expiratory reserve volume. However, body plethysmography, nitrogen washout, and helium dilution techniques determine residual volume, functional residual capacity, and total lung capacity.

Classification of Respiratory Volumes

1. Tidal volume: The amount of air inhaled or expelled during one respiratory cycle is known as tidal volume. The usual adult value is 10% of vital capacity (VC), or about 300-5tidal volume 100ml (6.8 ml/kg), although it can rise to 50% of VC during exercise.
2. Inspiratory Reserve Volume : The Inspiratory Reserve Volume (IRV) is the amount of air that can be forcibly breathed after a normal tidal volume. IRV is generally maintained on standby, but it is activated when deep breathing is required.1900-3300ml is the average adult value.
3. Expiratory Reserve Volume (ERV): The amount of air that can be forcibly expelled after the usual tidal volume has been exhaled. 700-1200ml is the average adult value. Obesity, ascites, and upper abdominal surgery all diminish ERV. 
4. The volume of Remainder (RV): It’s the amount of air left in your lungs after you’ve exhaled completely. It can be calculated indirectly from the sum of FRC and ERV and cannot be assessed with the technique of spirometry.

RV may be greatly elevated in obstructive lung disorders with inadequate lung emptying and air trapping symptoms. The RV can also be expressed as a percentage of total lung capacity, and values over 140% significantly increase the risks of barotrauma pneumothorax, infection, and high intrathoracic pressures cause venous return, as seen in patients with a high RV who require surgery and mechanical breathing, necessitating high perioperative inflation pressures.

Classification of Lung Capacities

• Inspiratory capacity: The largest volume of air that can be inhaled after a resting condition is called inspiratory capacity (IC). It is calculated using the sum of inspiratory reserve volume and tidal volume
• Total lung capacity: It is the sum of all volume compartments or the volume of air in the lungs following maximum inspiration, or the maximum volume of air the lungs can accommodate. 6,000mL (46 L) is the typical value.The total lung capacity (TLC) is computed (TV, IRV, ERV, RV)
• TLC levels are higher in patients with obstructive defects like emphysema and lower in patients with restrictive abnormalities such as chest wall abnormalities and kyphoscoliosis

• Vital Capacity (VC): The entire volume of air expelled after maximal inhalation is the critical capacity (VC). The value is around 4800mL and varies depending on age and body size. It is calculated using the sum of tidal volume, inspiratory reserve volume, and expiratory reserve volume
• Function residual capacity: The amount of air left in the lungs after a normal exhale is known as functional residual capacity (FRC). It can be calculated by adding the quantities of residual and expiratory reserves. The usual range is between 1800 and 2200 millilitres. RV+ERV = FRC

FRC does not need effort and emphasises the resting position, which is achieved when the inner and outer elastic recoils are balanced. In restricted disorders, FRC is diminished. Hyperinflation is measured by the FRC/TLC ratio. 

Volumes of the Lungs Measured

Respiratory volume measurements are necessary for a precise physiological diagnosis, but their role in determining disease severity, functional handicap, disease progression, and treatment response is debatable. Spirometry, body plethysmography, nitrogen washout, and helium dilution are among the techniques that can be used to measure static lung volumes. 

Factors that affect Respiratory Volume

Several factors influence respiratory volume, some of which can be adjusted and others that cannot.

• When someone who lives at or near sea level travels to high altitudes (e.g., the Andes; Denver, Colorado; Tibet; the Himalayas), they can suffer altitude sickness because their lungs are able to remove enough carbon dioxide but are unable to take in enough oxygen. (The fundamental predictor of respiratory drive in healthy people is carbon dioxide.)

• The respiratory volumes are either dynamic or static from a physiological standpoint. Both subtypes are assessed at various degrees of inspiration or expiration, but dynamic lung volumes typically rely on airflow rate. The static lung volumes/capacity are further separated into four standard volumes (tidal, inspiratory reserve, expiratory reserve, and residual volumes) and four standard capacities (tidal, inspiratory reserve, expiratory reserve, and residual volumes) (inspiratory, functional residual, vital and total lung capacities). The essential capacity is used to calculate the dynamic lung volumes. While dynamic lung volumes are critical for obstructive lung disease diagnosis and follow-up, static lung volumes are as significant for evaluating obstructive and restrictive ventilatory abnormalities. The purpose of this paper is to provide the reader with an overview of the physiological basis, clinical implications, and interpretive methodologies for typical static lung volumes and capacities.

Conclusion

Spirometry can be used to detect, characterize, and quantify the severity of lung disease by measuring the volume of air inhaled and exhaled. Absolute lung volumes, residual volume (RV), functional residual capacity (FRC), and total lung capacity (TLC) are more difficult to measure technically, limiting their usage in clinical practice. In infants, children, and adults, the role of respiratory volume measures in assessing illness severity, functional disability, disease course, and therapy response has yet to be defined. However, lung volume measurements are important for a precise physiological diagnosis in some cases.