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Ideally, the oxygen provided via ventilation would be just enough to saturate the blood fully. In the typical adult, 1 litre of blood can hold about 200 mL of oxygen; 1 litre of dry air has about 210 mL of oxygen. Therefore, under these conditions, the ideal ventilation perfusion ratio would be about 0.95.
Lung volumes and lung capacities are measures of the volume of air in the lungs at different phases of the respiratory cycle. The average total lung capacity of an adult human male is about 6 litres of air. [1] Tidal breathing is normal, resting breathing; the tidal volume is the volume of air that is inhaled or exhaled in only a single such ...
The ratio of partial pressure arterial oxygen and fraction of inspired oxygen, known as the Horowitz index or Carrico index, is a comparison between the oxygen level in the blood and the oxygen concentration that is breathed. This helps to determine the degree of any problems with how the lungs transfer oxygen to the blood. [5]
The respiratory exchange ratio (RER) is the ratio between the metabolic production of carbon dioxide (CO 2) and the uptake of oxygen (O 2). [3] [4] The ratio is determined by comparing exhaled gases to room air. Measuring this ratio is equal to RQ only at rest or during mild to moderate aerobic exercise without the accumulation of lactate.
The ideal V/Q ratio is 1, the most efficient state of pulmonary function when the amount of oxygen entering the lungs equals the amount of oxygen delivered to the body. Furthermore, adequate achievement of ventilation and perfusion matching is essential as it ensures the continuous supply of oxygen and withdrawal of waste products from the body.
The alveolar oxygen partial pressure is lower than the atmospheric O 2 partial pressure for two reasons.. Firstly, as the air enters the lungs, it is humidified by the upper airway and thus the partial pressure of water vapour (47 mmHg) reduces the oxygen partial pressure to about 150 mmHg.
Example: patient who is receiving an FiO2 of .5 (i.e., 50%) with a measured PaO2 of 60 mmHg has a PaO 2 /FiO 2 ratio of 120. In healthy lungs, the Horowitz index depends on age and usually falls between 350 and 450. A value below 300 is the threshold for mild lung injury, and 200 is indicative of a moderately severe lung injury.
Vital capacity (VC) is the maximum amount of air a person can expel from the lungs after a maximum inhalation. It is equal to the sum of inspiratory reserve volume, tidal volume, and expiratory reserve volume. It is approximately equal to Forced Vital Capacity (FVC). [1] [2] A person's vital capacity can be measured by a wet or regular spirometer.