What Is Tidal Volume?

Tidal volume (Vt or TV) is the amount of air you move through your lungs each time you inhale and exhale while you're at rest. Tidal volume typically measures around 500 milliliters (mL) for those assigned male at birth and 400 mL for those assigned female.

Vt is measured by spirometry, which is a non-invasive test. Your tidal volume is an important determinant in many different breathing functions and measurements that are used in analyzing your respiratory system.

This article discusses what tidal volume is, how it is calculated, and conditions associated with low or high tidal volume.

Minute Ventilation and Tidal Volume

Minute ventilation (VE) is an important measurement that's related to tidal volume. It's a measurement of the volume of inhaled and exhaled air over 60 seconds. A typical adult VE ranges around 4 to 6 liters in 60 seconds.

You can increase your VE by either taking deeper breaths (increasing Vt) or by breathing faster (increasing your respiratory rate).

Alveolar Ventilation and Tidal Volume

Alveolar ventilation (VA) is another important measurement related to tidal volume. VA measures VE without the inclusion of airway dead space, or the volume of air you breathe without active gas exchange in the lungs.

Dead space is the air that remains above the vocal cords in the upper respiratory tract (nasal passages, sinuses, pharynx, and larynx) as well as below the vocal cords in the lower respiratory tract (trachea, bronchi, and bronchioles). Dead space represents approximately a third of the air volume that's moved during casual breathing.

Breathing harder can increase your alveolar ventilation.

How Tidal Volume Is Calculated

Your Vt can be measured with spirometry, which involves breathing into a machine to measure how much air is moved during different patterns of breathing. Your Vt can help a pulmonologist, or doctor who specializes in respiratory conditions, determine if you have either obstructive or restrictive lung disease.

To calculate your tidal volume during a spirometry test, you will place your mouth over a mouthpiece attached to the machine and breathe in and out as you breathe normally.

Abnormal Tidal Volume Symptoms

Abnormally low and abnormally high tidal volume can cause a number of symptoms.

Abnormally Low Tidal Volume

A low Vt can be caused by hypoventilation (respiratory depression). In the early stages of hypoventilation, you may not experience any symptoms.

As hypoventilation progresses, symptoms may include:

• Difficulty breathing (dyspnea) at rest
• Excessive daytime/nighttime sleepiness
• Anxiety
• Delirium
• Lethargy

With moderate to severe hypoventilation, you can experience decreased oxygen levels in your blood (hypoxemia) as well as increased carbon dioxide levels in your blood (hypercapnia).

Severe hypercapnia increases the level of hydrogen ions in your blood causing an increase in its acidity. This can cause respiratory acidosis, which is when the lungs can't get rid of all the carbon dioxide produced in the body.

Abnormally High Tidal Volume

Hyperventilation (over-breathing) can cause a high Vt. Symptoms associated with hyperventilation are often more distressing than over-breathing.

Symptoms may include:

• Agitation
• Sense of terror
• Chest pain
• Burning or prickly sensation around the mouth, hands, arms, or legs
• Stiffness of arms and/or fingers
• Lightheaded (presyncope)
• Passing out (syncope)

The noticeable symptoms are most common with acute hyperventilation. The effects of chronic hyperventilation are not usually as obvious. Among the signs to watch for are frequent and deep sighing, anxiety, and emotional distress.

Diagnoses Related to Low Tidal Volume

Hypoventilation, which can cause low Vt, can be related to a number of conditions;

• Carotid body resection: Removal of the chemical receptors located where the common carotid artery splits (bifurcates)
• Chronic obstructive pulmonary disease (COPD)
• Deformities of the wall of the chest (such as kyphoscoliosis)
• Drugs: Barbiturates, benzodiazepines, or narcotics
• Severe hypothyroidism with myxedema
• Neurologic disease: Brainstem disease, encephalitis, multiple sclerosis (MS), poliomyelitis
• Neuromuscular disease: Amyotrophic lateral sclerosis (ALS), muscular dystrophy, Guillain-Barré, myasthenia gravis
• Obesity hypoventilation syndrome

Diagnoses Related to High Tidal Volume

There are not many pathologic (caused by disease) reasons for high Vt. It is commonly increased with moderate exercise. When you exercise, you breathe deeper, which increases your Vt. You also breathe faster, which increases your VE.

Emotions and stress leading to anxiety or a panic attack can also cause an acute episode of hyperventilation. Using physical exam, medical history, spirometry and a variety of blood tests or radiologic imaging can help determine the cause of hyperventilation.

Other disorders can cause high tidal volumes due to changes within the body, particularly if it affects blood acidity such as in diabetic ketoacidosis.

Tidal Volume in the Intensive Care Unit

If you are in an intensive care unit, you may require a breathing tube (endotracheal tube) with a ventilator. A pulmonologist or anesthesiologist will determine your ventilator settings. Typically a respiratory therapist will manage the ventilator settings, which guides your breathing pattern.

Your Vt plays an important role in your progression to getting off the ventilator. High tidal volumes (greater than 10 mL/kg) can be harmful and may delay advancement to independent breathing.

Using low tidal volumes on a ventilator has been shown to improve the survival rate in acute respiratory distress syndrome (ARDS).

It is common practice to use a tidal volume setting on the ventilator referred to as low tidal volume ventilation (LTVV) which approximates your natural Vt.