FRC Full Form

The quantity of breath at the extreme of passive expiration is referred to as functional residual capacity (FRC). At FRC, the contrasting elastic recoil powers of the lungs, as well as chest wall, as well as chest wall are in balance and no diaphragm or any other respiratory muscles are engaged.

FRC is the total amount of expiratory reserve volume (ERV) as well as residual volume (RV), and it measures about 2500 mL in a 70 kg, average adult (or about 30ml/kg). Because it contains the residual volume, it cannot be predicted using spirometry. To precisely measure RV, a test including nitrogen washout, helium dilution, or body plethysmography would be required.  

About functional residual capacity (FRC)

Positioning is important in influencing FRC. It is greatest when upright and declines as one shift from vertical to supine/prone or Trendelenburg. FRC decreases the most when moving from 60° to completely supine at 0°. FRC does not change significantly as shifts from 0 degrees to Trendelenburg of close to 30°. However, beyond 30°, the fall in FRC is significant. 

The helium solubility technique and the pulmonary plethysmography are mainly two methods for determining lung functional residual capacity.

For large populations, the calculated value of FRC has been evaluated and posted in several references. FRC was discovered to differ depending on a patient’s age, tallness, and gender. The functional residual ability is directly proportionate to height and indirectly to obesity. It is limited by the presence of obesity due to a decrease in cardiac compliance.

Oxygen-enriched gas enters the alveoli during inspiration. The pulmonary capillary flow continues to remove oxygen during exhalation. FRC is the amount of gas remaining inside the alveoli at the end of expiration. This is the only cause of gas accessible for exhalation gas exchange. Pao2 falls sharply throughout expiration in illnesses with reduced FRC (for example ARDS, pulmonary edema), resulting in hypoxemia. PEEP as well as increasing the inspiratory time (Ti) are 2 ventilator methods used to enhance oxygenation in this kind of situation (Fig. 89.5). PEEP raises FRC, so although a longer Ti allows capillary permeability blood to be exposed to a greater concentration of O2 throughout inspiration for a longer period.

Importance of FRC

Conditions associated with low compliance reduce FRC. FRC is also diminished in the supine position as appeared differently in the erect position and during anesthesia. FRC is clinically important for two reasons. First, FRC serves as an important oxygen storage facility. For example, an apneic client who is inhaling 100% oxygen and has an oxygen uptake of 300 ml/min, as well as an FRC of 3000 ml, as well as an FRC of 3000 ml does have 10 minutes of saved oxygen. The same patient who is inhaling air has much less than 120 seconds of oxygen stored. In practice, oxidation warehouses never last this hard because FRC doesn’t ever contain 100% oxygen and progressive oxygen consumption outcomes in clinically important hypoxemia before 10 minutes. However, the example emphasizes the significance of deeply oxygenating patients before provoking apnea for intubation. The major reason FRC is important is its link to closing capacity.

HOW FRC IS MEASURED

The FRC tests rely on implicit calculations to ascertain the amount of air in your respiratory system after you exhale. These tests will involve your cooperation- operation, and you’ll be asked to inhale and exhale while following specific instructions.

Lung Plethysmography

This test, also known as pulmonary plethysmography or whole-body plethysmography, requires you to enter a small chamber and breathe through a mouthpiece. The air pressure within the chamber is evaluated as you take a breath in and out. The computation of one FRC is based on the mathematical formula that considers the amount of air you breathe in and out and the amount of air in your lungs. 

Helium Dilution Procedure

This test makes use of a spirometer, which would be a simple device into which you can breathe in and out. A helium-containing spirometer is used in a helium dilution test. The accumulation of helium surviving in the spirometer as you exhale and inhale could be used to determine your FRC. Perhaps if you have developed lung disease, the tests used to calculate your FRC are safe.

Conclusion 

FRC stands for functional residual capacity. It can be understood and determined by the compliance of chest and lung wall. The vital capacity and the residual lung capacity are correlated in a positive way to the functional residual capacity.