Tidal volume: what it is, measurements and related conditions

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Tidal volume (Vt or TV) is a physiological term used to describe the amount of air that normally moves during inhalation and exhalation when at rest. Measured by spirometry.

On average, adults breathe 7 milliliters (ml) per kilogram (kg) of ideal body weight. The average adult woman has a Vt of approximately 400 ml and the average adult male has a Vt of approximately 500 ml.

Your Vt is an important determining factor in many different respiratory functions and measurements that are used to analyze your respiratory system, such as minute and alveolar ventilation .

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Minute ventilation

Minute ventilation (VE) is an important measure related to Vt. This is a measure of the volume of air inhaled and exhaled in 60 seconds. A typical adult EV is 4 to 6 liters in 60 seconds.

You can increase VE by breathing more deeply (increasing Vt) or breathing more rapidly (increasing the respiratory rate) .

Alveolar ventilation

Alveolar ventilation (VA) is another important measure related to Vt. VA measures VE without considering airway dead space.

Dead space is the volume of air you breathe without actively exchanging gas in your lungs. This is the air that remains above the vocal cords in the upper airways (nasal passages, sinuses, pharynx, and larynx) and below the vocal cords in the lower airways (trachea, bronchi, and bronchioles). Dead space is about a third of the volume of air that moves during normal breathing.

More difficult breathing can improve alveolar ventilation .

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How is tidal volume measured?

Your Vt can be measured using spirometry, which involves breathing into a device to measure how much air is moving during different breathing patterns. Your Vt can help your pulmonologist determine if you have obstructive or restrictive lung disease .

To measure Vt during a spirometry test, place your mouth on a mouthpiece attached to the machine and breathe in and out informally as usual.

Symptoms of abnormal tidal volume

An abnormally low and abnormally high Vt can cause a variety of symptoms.

Abnormally low tidal volume

Low Vt may be due to hypoventilation (respiratory depression). In the early stages of hypoventilation, you may not experience any symptoms.

As hypoventilation progresses, symptoms can include :

With moderate to severe hypoventilation, there may be a decrease in the level of oxygen in the blood (hypoxemia) , as well as an increase in the level of carbon dioxide in the blood (hypercapnia) . Severe hypercapnia increases the level of hydrogen ions in the blood, causing it to become acidic and lead to respiratory acidosis .

Abnormally high tidal volume

Hyperventilation (excessive breathing) can cause a high Vt. The symptoms of hyperventilation are often more concerning than those associated with hyperventilation.

Symptoms can include :

  • Agitation
  • Feeling of terror
  • Chest pain
  • Burning or tingling sensation around the mouth, hands, or feet.
  • Stiffness in the hands and / or fingers.
  • Dizziness (lightheadedness)
  • Fainting (fainting)

Notable symptoms occur most often with acute hyperventilation. The effects of chronic hyperventilation are often less obvious. You may notice frequent, deep breaths, as well as anxiety and emotional distress with chronic hyperventilation .

Tidal volume during pregnancy

During the first trimester of pregnancy, Vt increases, followed by increased respiration. An increase in Vt during pregnancy causes an increase in VE. Displacement of the chest during pregnancy-related body changes is the main factor influencing the increase in Vt .

Low tidal volume diagnostics

There are many different causes of hypoventilation, which generally fall into one of the following categories:

High tidal volume diagnostics

The use of a physical exam, medical history, spirometry, and various blood tests or radiological images can help determine the cause of hyperventilation.

High Vt does not have many pathological (disease-related) causes. It usually increases with moderate exercise. As you exercise, you breathe more deeply, increasing your Vt. And you also breathe faster, which increases your VE.

It is important to understand that hyperventilating does not necessarily mean a high Vt, as you can hyperventilate because you breathe very fast and shallow.

Emotions and stress that trigger anxiety or a panic attack can lead to an acute hyperventilation attack .

Other disorders can cause high tidal volumes due to changes in the body, especially if they affect the acidity of the blood, as in diabetic ketoacidosis .

Tidal volume in the intensive care unit

If you are in the ICU, you may need a breathing tube (endotracheal tube) with a ventilator. Your pulmonologist or anesthesiologist will determine your ventilator settings. Typically, a respiratory therapist controls the ventilator settings that determine how you breathe.

Your Vt plays an important role in a fan's transition. High tidal volumes (greater than 10 ml / kg) can be harmful and delay the transition to spontaneous respiration.

The use of low tidal volumes of the ventilator has been shown to improve survival in acute respiratory distress syndrome (ARDS) .

It is common practice to use a tidal volume setting on the ventilator, called low tidal volume ventilation (LTVV) , that approximates its natural Vt.

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