Hypocapnia - Symptoms, Causes, Diagnosis, and Treatment

Introduction:

Hypocapnia refers to the decrease in the alveolar and blood carbon dioxide (CO2) levels below the normal reference range of 35 mm of Hg. Carbon dioxide is a metabolic product of the many cellular processes within the body involved in the processing of carbohydrates, lipids, and proteins. It is also known as hypocarbia. The primary organ systems involved in regulating carbon dioxide homeostasis are the renal and pulmonary systems. Aberrations that result in hypocarbia typically result in respiratory alkalosis.

What Is the Pathophysiology of Hypocapnia?

Hypocapnia mainly occurs due to hyperventilation. When the ventilation to the alveolar space increases, gaseous carbon dioxide gets removed easily. This results in the increased level of diffusion gradient of carbon dioxide from the blood to the alveoli. Therefore, carbon dioxide is more readily removed from the body. The partial pressure of carbon dioxide (PaCO2) ranges from 35 to 45 mm of Hg. The hydrogen concentration is detected by the central chemoreceptors present in the brain and the peripheral chemoreceptors present in the carotid bodies. These receptors also regulate the pH and partial pressure of carbon dioxide. If hyperventilation is persistent, it leads to hypocapnia because alveolar ventilation exceeds the amount of carbon dioxide being produced.

The estimation of change in the pH with hyperventilation can be done with the below-mentioned values:

• Acute Respiratory Alkalosis: Change in pH =0.008 x (40 - partial pressure of carbon dioxide).

• Chronic Respiratory Alkalosis: Change in pH = 0.017 X (40 - partial pressure of carbon dioxide).

What Are the Clinical Symptoms of Hypocapnia?

The clinical presentation of the hypercapnia depends on the duration, underlying cause, and severity of the illness. The medical history and physical exam findings of the patient are highly variable as there are several pathologies that induce respiratory alterations. These include:

• Acute onset dyspnea (shortness of breath).

• Chills.

• Fever.

• Peripheral edema.

• Weakness.

• Orthopnea.

• Chest pain.

• Wheezing.

• Hemoptysis (coughing up blood)

• Trauma.

• Asthma.

• Chronic obstructive pulmonary disease.

• Abdominal pain.

• Acute focal neurological signs.

• Nausea.

• Vomiting.

• Tinnitus (ringing sound in ears).

• Weight loss.

• Paresthesias of extremities and mouth, cramps, carpopedal spasm (involuntary muscle contractions in the hands and feet).

• Hyperventilation might sometimes cause a painful and tingling sensation in the hands and feet.

• Numbness.

• Sweating.

• Cerebral vasoconstriction might occur after hypocarbia. Sometimes, the patient might experience dizziness, confusion, nausea, and syncope.

• Hypocarbia also affects vitamin D metabolism leading to hypovitaminosis D, which may present as tetany and fibromyalgia.

• Secondary to respiratory alkalosis, there may be positive Trousseau and Chvostek signs, causing decreased serum calcium due to the shift of calcium from the blood to albumin- which has become more negative in an alkalotic state.

• Tachypnea is a frequent finding, as many patients with hyperventilation syndrome are anxious and tachycardic.

• Pulmonary diseases are often accompanied by respiratory alkalosis. In these conditions, the physical findings are based on the underlying pathology. For example, fine crackles are present in left ventricular failure, coarse crackles in pneumonia, and wheezing.

What Are the Causes for Hypocapnia?

• Hypocarbia occurs as a consequence of reduced carbon dioxide levels or an increase in carbon dioxide loss.

• The pulmonary system effectively removes carbon dioxide from the body through the diffusion procedure.

• The carbon dioxide gradients are developed and maintained where the partial pressure of carbon dioxide in the blood is directly proportional to the rate of metabolic carbon dioxide production and inversely proportional to the rate of carbon dioxide eliminated by the lung through increased alveolar ventilation.

• Alveolar volume is the removal of alveolar air into the environment, refers to the expired minute volume that reaches the alveoli, and is determined by minute ventilation and the ratio of dead space to tidal volume.

• The respiratory rate and tidal volume are the two components of ventilation that are physiologically controlled to moderate carbon dioxide elimination. Subsequently, the causes that induce hypocarbia are any disease that increases ventilation rate or tidal volume. The increased respiratory rate is the primary inciting factor involved.

A wide variety of illnesses may induce this condition. These include:

1) Neurologic Causes:

• Pain.
• Panic disorder.
• Central causes are stroke, head injury, hyperthyroidism, anxiety-hyperventilation, pain, stress, fear, medications such as salicylates, and various toxins.

2) Endocrine Causes:

• Thyrotoxicosis (a condition that refers to an increased level of thyroid hormone).
• Pregnancy.

3) Respiratory Disease:

• Hypocapnia can be caused by any pulmonary disease (e.g., pneumonia, asthma, pneumothorax, pulmonary edema, pulmonary embolism).
• Hypoxemia can cause hypocapnia.
• Pulmonary irritation can also drive dyspnea and increase ventilation, leading to hypocapnia. Abnormal chest X-rays suggest pulmonary disease as the cause. Normal chest X-rays still occur with some respiratory etiologies: Pericardial tamponade (compression of the heart caused by fluid collection), asthma, mild cardiogenic pulmonary edema, and pulmonary embolism.

4) Cirrhosis of the Liver:

• Cirrhosis is one of the most common causes of respiratory alkalosis that persists among intubated patients.

5) Sepsis:

• The early sign of sepsis is respiratory alkalosis, which is accompanied by hypoxemia and hypotension.

6) Iatrogenic Causes:

• Iatrogenic causes are commonly due to hyperventilation in intubated patients on mechanical ventilation.

How to Diagnose?

In most situations, arterial blood gas is an important diagnostic factor in pH imbalances. The electrolytes present in the blood, such as sodium, magnesium, potassium, phosphate, and calcium levels, should be measured as aberrations. These may lead to further complications.

• Evaluation of Bicarbonate Levels:

In acute cases, the bicarbonate concentration level decreases by 2 mEq per liter for each decrease of 10 mm of Hg in the partial pressure of carbon dioxide level. In chronic cases, the bicarbonate level declines by 5 mEq per liter for each decrease of 10 mm of Hg in the partial pressure of carbon dioxide level. But in cases of primary respiratory alkalosis, the bicarbonate levels usually never decrease below 12 mm of Hg.

• Evaluation of A- a (alveolar-arterial) Gradient:

If the A-a gradient is wide, it is suspicious of pulmonary embolism, and the investigation is carried out appropriately. It is important to calculate the A-a gradient to determine the etiology and further diagnosis.

• Chest X-ray:

It helps to differentiate an anatomical or infectious cause and may rule out pulmonary edema. In necessary cases, a chest computed tomography (CT) scan is taken to achieve a proper diagnosis.

• Magnetic Resonance Imaging (MRI):

In the case of neurologic insult, magnetic resonance imaging of the head may be appropriate along with the lumbar puncture for white blood cell (WBC), glucose, and protein analysis.

What Are the Treatment Approaches for Hypocapnia?

• The primary aim is to treat the underlying pathological condition to reduce the respiratory rate.

• In anxious patients, anxiolytics (used to relieve anxiety or stress) are recommended.

• In the case of infectious disease, antibiotics targeting sputum or blood cultures are recommended.

• In the case of embolic disease, anticoagulants (drugs that help to prevent blood clots- Eg, Aspirin, Warfarin) are necessary.

• Ventilator support is indicated in cases of acute, chronic pulmonary obstructive disease, acute asthma, and acute respiratory failure.

• In ventilator-controlled patients, it is necessary to reevaluate the ventilator settings to reduce respiratory rate.

• If hyperventilation is intentional, monitoring arterial or venous blood gas values is necessary.

Conclusion:

Patients suffering from hypocarbia related to anxiety and panic disorder must be educated and made aware of the importance of controlling the breathing rate when experiencing severe anxiety symptoms. The doctors do not recommend rebreathing into a paper bag to increase alveolar carbon dioxide. As hypocapnia is generally well tolerated by patients, there are no major complications that are due only to low partial pressure carbon dioxide.