How to Calculate the Inspiratory & Expiratory Ratio

Breathing is one of the few things you do constantly, and in fact can't get away with not doing for very long without giving the process a lot of thought, at least when you are at rest.

A portion of your brain stem called the medulla oblongata is responsible for the maintenance of your breathing as an autonomic (basically, automatic) function. Of course, you can consciously manipulate your breathing rate as well, unlike your heartbeat and blood pressure, other autonomically regulated functions.

The number of breaths you take per minute is usually dependent on your body's oxygen needs. Correspondingly, during periods when you cannot breathe on your own, such as when under anesthesia for surgery, medical professionals have to know how to set up ventilators (breathing machines) based on your body's health, specific needs and other personal factors.

Lung Volumes Defined

Ventilation​ is the process by which oxygen (O2) and carbon dioxide (CO2) are transported to and from the lungs. The ​alveoli​ are the tiny sacs deep in the lungs where gas exchange occurs between lungs and the blood stream.

Tidal volume (VT)​ is the amount of gas expired in each breath, usually about half a liter.

Dead space volume (VD)​ is the sum of the "anatomic" dead space, which is wasted airway space, and the "physiologic" dead space, resulting from alveoli that are getting air but not being supplied with enough blood for useful gas exchange. Minute volume (VE) is the total amount of gas expired per minute.

Alveolar ventilation (VA)​ is the amount of gas which reaches functional respiratory units (i.e., the alveoli) per minute.

V_A=(V_T-V_D)\times \text{ respiratory rate in breaths per minute}

Other lung volumes:

  • FRC (Functional residual capacity)​ is the amount of air you can ​exhale​ after exhaling ​normally​ – about 2 L.
  • TLC (total lung capacity)​, about 6 L. 
  • MIV (Maximum inspiratory volume )​ is the amount of air you can ​inhale​ after a ​normal​ exhalation, about 4 L.

Pulmonary Function Tests

All of these metrics can be collected in a standard series of pulmonary function tests (PFTs) in which you breathe into tube in a machine under the instructions of lab technicians. The machine includes flow rate sensors and gas analyzers and provides the results of tests in easy-to-read graphical form.

You may be asked to consider taking a PFT if you should signs of obstructive lung disease, such as asthma, or restrictive lung diseases, such as pulmonary fibrosis.

What Is the I/E Ratio?

The I/E ratio (I:E ratio), or inspiratory expiratory ratio, is just the ratio of inhalations to exhalations during steady breathing. At rest, it is usually about 1:2, meaning that you exhale more slowly than you inhale. This ratio drops toward 1:1, however, with exertion. Most people take about 15 breaths a minute at rest.

Of interest to people operating ventilators is the ​cycle time​, which is just the reciprocal of the number of breaths a minute and represents the total time of a single inhalation-exhalation cycle.

The Alveolar Ventilation Equation

The ​alveolar ventilation equation​ relates the amount of CO2 in a patient's arterial blood to the overall metabolic rate of the person being analyzed (​V​CO2). ​VA (ml/min) × PACO2 (mm Hg) = ​V​CO2 (ml/min) × K

Here, ​VA is alveolar ventilation, PACO2 is the partial pressure of carbon dioxide in the alveoli (which had to have come from within the body, as normal air actually has very little CO2) and K is a constant. Higher exercise rates imply more carbon dioxide produced as waste and a higher ventilatory excretion of the gas.

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