What Is Tidal Volume?

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

On average adults breathe 7 milliliters per kilogram of the ideal body weight. The average adult female has a tidal volume of around 400 mililiters (mL), while the average adult male has a tidal volume of around 500 mL.

Your tidal volume is an important determinant in many different breathing functions and measurements used in analyzing your respiratory system such as minute and alveolar ventilation.

Minute Ventilation

Minute ventilation (VE) is an important measurement related to tidal volume where the volume of inhaled and exhaled is measure over 60 seconds. A typical adult volume ranges around 4 to 6 liters in 60 seconds.

You can increase your minute ventilation by either taking deeper breaths (increasing tidal volume) or by breathing faster (increasing your respiratory rate).

Alveolar Ventilation

Alveolar ventilation (VA) is another important measurement related to tidal volume. Alveolar ventilation is minute ventilation without inclusion of airway dead space. Dead space represents approximately a third of the volume moved during casual breathing.

Dead space is the volume of air that is not actively exchanged 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, bronchioles, and alveoli).

Unlike minute ventilation, where you can increase minute ventilation by increasing tidal volume or increasing your respiratory rate, breathing harder (increasing tidal volume) is more effective at increasing your alveolar ventilation.

How Is Tidal Volume Measured?

Tidal volume is measured during spirometry, which involves breathing into a machine to measure how much air is moved during different patterns of breathing. Your tidal volume is an important measurement that is used during this test in helping a pulmonologist determine if you have either obstructive or restrictive lung disease.

To measure your tidal volume 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

There are symptoms associated with both abnormally low and abnormally high tidal volume.

Abnormally Low Tidal Volume

Tidal volume that is abnormally low leads to symptoms related to respiratory depression (hypoventilation). In 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

In progressive 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

Tidal volume that is abnormally high leads to symptoms related to overbreathing (hyperventilation). Symptoms of hyperventilation are often more distressing than the symptoms related to hypoventilation. 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 symptoms listed above are most common with sudden changes that cause hyperventilation. Chronic hyperventilation can be harder to detect. You may notice frequent and deep sighing, when hyperventilation has been long-standing as well as some anxiety and emotional distress.

Diagnoses Related To Low 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 hypoventilation. 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
• Hypothyroidism (severe) (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 tidal volumes. A common way tidal volume is increased physiologically is to exercise. When you exercise you breath deeper, which increases your tidal volume, and breath faster, which combined increases your minute ventilation.

Emotions and stress leading to anxiety or a panic attack can lead to an acute episode of excessively high tidal volumes leading to hyperventilation which causes too low of a decrease in the carbon dioxide in your blood leading to symptoms listed above included lightheadedness and dizziness.

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) and be connected to a ventilator. Typically a respiratory therapist will manage the ventilator, and your breathing pattern, under the orders of a pulmonologist or anesthesiologist.

Tidal volume plays an important role in your progression to getting off the ventilator. In the past, it was common for ventilators to provide you with a larger tidal volume than you would typically have.

High tidal volumes (greater than 10 mL/kg) while intubated should no longer be practiced in modern medicine. Increased tidal volumes have been shown to worsen disease states while on a ventilator.

Common practice is to use a tidal volume setting on the ventilator commonly referred to as low tidal volume ventilation (LTVV) which really is basically the same as physiologic tidal volume ventilation, meaning close to the same as you normally breathe.

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