What Is Tidal Volume?

Tidal volume (Vt or TV) is a physiological term used to describe the amount of air typically moved during inspiration and expiration while you are at rest. It is measured by spirometry.

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

Your Vt is an important determinant in many different breathing functions and measurements that are used in analyzing your respiratory system, such as minute and alveolar ventilation.

Minute Ventilation

Minute ventilation (VE) is an important measurement that's related to Vt. 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

Alveolar ventilation (VA) is another important measurement related to Vt. VA measures VE without the inclusion of airway dead space.

Dead space is the volume of air you breathe without active gas exchange in the lungs. It 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 Is Tidal Volume Measured?

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 your pulmonologist determine if you have either obstructive or restrictive lung disease.

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

Abnormal Tidal Volume Symptoms

Abnormally low and abnormally high Vt 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 (hypersomnolence)
• 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, resulting in respiratory acidosis.

Abnormally High Tidal Volume

Hyperventilation (over-breathing) can cause a high Vt. Symptoms of hyperventilation are often more distressing than the symptoms related to hyperventilation.

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. You may notice frequent and deep sighing, as well as anxiety and emotional distress with chronic hyperventilation.

Diagnoses Related To Low Tidal Volume

There are many different causes of hypoventilation that can typically be identified in one of the following categories:

• 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

Using physical exam, medical history, spirometry and a variety of blood tests or radiologic imaging will be helpful in determining the cause of hyperventilation.

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. And you also breathe faster, which increases your VE.

Emotions and stress leading to anxiety or a panic attack can lead to an acute episode 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 is to use a tidal volume setting on the ventilator referred to as low tidal volume ventilation (LTVV) which approximates your natural Vt.