Hyperventilation

# Overview
Hyperventilation is the state of rapid breathing which results in the reduction in carbon dioxide levels (below normal) thereby leading to hypocapnia. During rapid breathing, the body loses more carbon dioxide (CO2) than it can produce resulting in net reduction of CO2 levels. This state of rapid/faster breathing is most commonly seen in stress and anxiety and termed as hyperventilation syndrome. Kussmaul breathing is also a type of hyperventilation and done to reduce the acidity of body as seen in metabolic acidosis. Hyperventilation may also be voluntarily manifested following episodes of rapid deep breathing. The symptoms of hyperventilation are variable. Some patients are completely asymptomatic while others may present with minimal symptoms such as headache and numbness or tingling in the hands, feet, and lips. More severe symptoms include dizziness, lightheadedness, and fainting. Some patient also report having chest pain and slurred speech particularly when accompanied by the Valsalva maneuver. Voluntary deep breathing and induction of hyperventilation is a common practice among young individuals to attain focus and adrenaline rush. Other similar terms that are completely different from hyperventilation include hyperpnea and tachypnea.
Hyperpnea is commonly seen with exercise or any major physical activity as well as in response to hypoxic states. In these conditions the energy demand of the body either goes up or is not adequately met. To meet this energy deficit, the body increases the depth and rate of breathing which is known as hyperpnea. Other common examples of hyperpneic state include sepsis, anemia, and individuals living at high altitudes. Tachypnea is derived from a Greek word which means "rapid breathing". Tachypnea means rapid and shallow breathing and is also seen with exercise as a compensatory mechanism to increase the oxygen content of the body.

# Pathophysiology

## Physiology of breathing
The basic physiology of normal breathing is as follows:
- Under normal conditions, breathing is regulated by the central nervous system (CNS).
- The CNS regulates the depth and frequency of each breath to maintain normal levels of carbon dioxide (CO2) and oxygen (O2) in the blood and tissues.
- The CNS measures the amount of CO2 in the body to regulate the breathing process.
- As the CO2 and O2 gas exchange mechanism is simultaneous and continuous, any condition resulting in increased high carbon dioxide concentration primarily signals a low oxygen concentration.
- In addition, metabolism in the body uses O2 and results in the production of CO2 as a byproduct.

## Hyperventilation
Under physiologic conditions, the volume of alveolar gas is in equilibrium with the arterial gas. 
- With each breath approximately 10% of the alveolar gas is replaced with atmospheric air.
- The rate and depth of breathing determines the level of CO2 in the body.
- A rapid and deep breath will lead to a better alveolar - atmospheric gas exchange leading to low CO2 levels.
- It is to be noted that atmospheric air has 21% O2 content as compared to just 0.03% of CO2 content.
- This results in low CO2 content (hypocapnia) with each rapid and deep breath.

## Mechanism of Alkalosis
The mechanism of the development of alkalosis following hyperventilation is as follows:
- Majority of the CO2 in human body is stored as carbonic acid and is a major factor in determining the pH of the body.
- Loss of CO2 results in blood becoming more alkaline and increase in blood pH.
- In the normal individual, the resultant alkalosis would automatically be countered by reduced breathing except when the neural control is altered or disturbed.

## Response of brain vasculature to hyperventilation and hypoventilation
The response of vessels in the brain to hyperventilation and hypoventilation is as follows:
- Conditions causing high CO2 levels (hypercapnia) results in the body assuming that the O2 levels are low.
- As a result, to increase the O2 supply the blood vessels in the brain dilate.
- Alternatively, hypocapnia results in constriction of brain's blood vessels causing diminished blood flow leading to lightheadedness.
- Thus, though it seems counterintuitive, breathing too much can result in a decrease in the oxygen supply to the brain.
- Physicians often artificially induce hyperventilation after head injury to reduce the intracranial pressure, although the treatment has potential risks.

# Causes
Following are the various causes of hyperventilation.

## Life-Threatening Causes
- Diabetic ketoacidosis
- Heat stroke
- Pulmonary embolism
- Respiratory alkalosis
- Sepsis

## Common Causes
- Acute altitude sickness
- Anxiety
- Ascites
- Asthma
- Chronic obstructive pulmonary disease
- Congestive heart failure
- Drugs such as:
Amphetamine
Aspirin
- Amphetamine
- Aspirin
- Encephalitis
- Exercise
- Fever
- Graves' disease
- Head injury
- Hyperthyroidism
- Meningitis
- Panic disorder
- Pneumonia
- Pneumothorax
- Pregnancy
- Pulmonary edema
- Pulmonary embolus
- Pulmonary fibrosis
- Stress
- Stroke

## Causes by Organ System

## Causes in Alphabetical Order
- Abdominal surgery
- Acid-base imbalance
- Acute liver failure
- Acute porphyria
- Acute stress disorder
- Adulation
- Adult panic-anxiety syndrome
- Agoraphobia
- Altitude sickness, acute
- Aminophylline
- Ammonium chloride
- Anxiety
- Apneustic respirations
- Arginosuccinic aciduria
- Aspirin
- Asthma
- Ataxic respiration
- Biguanide intolerance
- Biot's respiration
- Briquet's syndrome
- Brain trauma
- Bronchial neoplasm
- Camphor
- Carbamoylphosphate synthetase 1 deficiency disease
- Carbonic anhydrase va deficiency
- Central neurogenic hyperventilation
- Cerebrovascular accident
- Cheyne-stokes respirations
- Childbirth
- CNS infection
- Coffee abuse
- Combat stress reaction
- Congestive cardiac failure
- Cree leukoencephalopathy
- Crying or severe distress
- Da costa syndrome
- Damaged respiratory pathways
- Depression
- Diabetic ketoacidosis
- Diffuse pulmonary fibrosis
- Distress
- Emphysema
- End stage liver failure
- Epilepsy
- Epileptic encephalopathy, early infantile, 2
- Excitement
- Fear
- Fever
- hereditary fructose-1,6-bisphosphatase deficiency
- Functional disorders
- Glycol ether
- Grief
- Head injury
- Heat stroke
- Hepatic cirrhosis
- Hyperventilation syndrome
- Hypochondriasis
- Hysteria
- Inhalation of irritants
- Intestinal fistula
- Intracranial space-occupying lesion
- Lactic acidosis
- Lung damage
- Malingering
- Meningoencephalitis
- Metabolic acidosis
- Methanol
- Pain
- Panic attack
- Persistent hypoxemia
- Phaeochromocytoma
- Phobia
- Pitt-hopkins syndrome
- Pleural effusion
- Pneumonia
- Pneumothorax
- Pralidoxime
- Primary habit disorder
- Pseudoallergic reactions
- Pulmonary embolism
- Pulmonary oedema
- Pyloric stenosis
- Raised intracranial pressure
- Renal disease
- Rett's syndrome
- Rib tumor
- Salicylate poisoning
- Schizophrenia
- Sponatneous pneumothorax
- Stress
- Stroke
- Strong emotions
- Surgical relocation of ureters in ileum or colon
- Tiagabine
- Vasovagal attacks

# Differentiating Hyperventilation from other Diseases
Various diseases that can cause hyperventilation may include diseases of respiratory, cardiovascular, endocrine, central nervous system, and musculoskeletal system. Pregnancy, sepsis, and hepatic failure are other conditions that may also result in hyperventilation. Given below is a differential diagnosis of diseases presenting with hyperventilation.
Abbreviations: ABG (arterial blood gas); ACE (angiotensin converting enzyme); hCG (beta human chorionic gonadotropin);  BMP (basic metabolic panel); BNP (brain natriuretic peptide); CBC (complete blood count); COPD (chronic obstructive pulmonary disease); CSF (cerebrospinal fluid); CXR (chest X-ray); CT (computed tomography); DLCO (diffusing capacity of the lung for carbon monoxide); DOE (dyspnea on exercise); ECG (electrocardiogram); FEF (forced expiratory flow rate); FEV1 (forced expiratory volume); FT4 (free T4); FVC (forced vital capacity); HRCT (high resolution computed tomography); JVD (jugular vein distention); LFTs (liver function tests); MCV (mean corpuscular volume);  MEN (multiple endocrine neoplasia); MRI (magnetic resonance imaging); P2 (pulmonic heart sound); Plt (platelet); PT (prothrombin time); RBC (red blood cell); RV (residual volume); SIADH (syndrome of inappropriate antidiuretic hormone); S3 ( third heart sound); S4 (fourth heart sound); T3 ((Triiodothyronine); TLC (total lung capacity); TSH (thyroid stimulating hormone); VC (vital capacity); VMA(vanillylmandelic acid); Vt (tidal volume); WBC (white blood cell);