Symptoms and signs of early hypercapnia include flushed skin, full pulse, tachypnea, dyspnea, extrasystoles, muscle twitches, hand flaps, reduced neural activity, and possibly a raised blood pressure. According to other sources, symptoms of mild hypercapnia might include headache, confusion and lethargy. Hypercapnia can induce increased cardiac output, an elevation in arterial blood pressure, and a propensity toward arrhythmias. Hypercapnia may increase pulmonary capillary resistance. In severe hypercapnia (generally PaCO greater than 10 kPa or 75 mmHg), symptomatology progresses to disorientation, panic, hyperventilation, convulsions, unconsciousness, and eventually death.

Hypercapnia, also known as hypercarbia and CO retention, is a condition of abnormally elevated carbon dioxide (CO) levels in the blood. Carbon dioxide is a gaseous product of the body's metabolism and is normally expelled through the lungs.

Hypercapnia normally triggers a reflex which increases breathing and access to oxygen (O), such as arousal and turning the head during sleep. A failure of this reflex can be fatal, for example as a contributory factor in sudden infant death syndrome.

Hypercapnia is the opposite of hypocapnia, the state of having abnormally reduced levels of carbon dioxide in the blood. Hypercapnia is from the Greek "hyper" = "above" or "too much" and "kapnos" = "smoke".

Respiratory failure results from inadequate gas exchange by the respiratory system, meaning that the arterial oxygen, carbon dioxide or both cannot be kept at normal levels. A drop in the oxygen carried in blood is known as hypoxemia; a rise in arterial carbon dioxide levels is called hypercapnia. Respiratory failure is classified as either Type I or Type II, based on whether there is a high carbon dioxide level. The definition of respiratory failure in clinical trials usually includes increased respiratory rate, abnormal blood gases (hypoxemia, hypercapnia, or both), and evidence of increased work of breathing.

The normal partial pressure reference values are: oxygen PaO more than , and carbon dioxide PaCO lesser than .

Disorders like congenital central hypoventilation syndrome (CCHS) and ROHHAD (rapid-onset obesity, hypothalamic dysfunction, hypoventilation, with autonomic dysregulation) are recognized as conditions that are associated with hypoventilation. CCHS may be a significant factor in some cases of sudden infant death syndrome (SIDS), often termed "cot death" or "crib death".

The opposite condition is hyperventilation (too much ventilation), resulting in low carbon dioxide levels (hypocapnia), rather than hypercapnia.

Acute respiratory acidosis occurs when an abrupt failure of ventilation occurs. This failure in ventilation may be caused by depression of the central respiratory center by cerebral disease or drugs, inability to ventilate adequately due to neuromuscular disease (e.g., myasthenia gravis, amyotrophic lateral sclerosis, Guillain–Barré syndrome, muscular dystrophy), or airway obstruction related to asthma or chronic obstructive pulmonary disease (COPD) exacerbation.

Respiratory acidosis can be acute or chronic.

- In "acute respiratory acidosis", the "Pa"CO is elevated above the upper limit of the reference range (over 6.3 kPa or 45 mm Hg) with an accompanying acidemia (pH <7.36).

- In "chronic respiratory acidosis", the "Pa"CO is elevated above the upper limit of the reference range, with a normal blood pH (7.35 to 7.45) or near-normal pH secondary to renal compensation and an elevated serum bicarbonate (HCO >30 mm Hg).

Type 1 respiratory failure is defined as a low level of oxygen in the blood (hypoxemia) without an increased level of carbon dioxide in the blood (hypercapnia), and indeed the PCO may be normal or low. It is typically caused by a ventilation/perfusion (V/Q) mismatch; the volume of air flowing in and out of the lungs is not matched with the flow of blood to the lungs. The basic defect in type 1 respiratory failure is failure of oxygenation characterized by:

This type of respiratory failure is caused by conditions that affect oxygenation such as:

- Low ambient oxygen (e.g. at high altitude)

- Ventilation-perfusion mismatch (parts of the lung receive oxygen but not enough blood to absorb it, e.g. pulmonary embolism)

- Alveolar hypoventilation (decreased minute volume due to reduced respiratory muscle activity, e.g. in acute neuromuscular disease); this form can also cause type 2 respiratory failure if severe

- Diffusion problem (oxygen cannot enter the capillaries due to parenchymal disease, e.g. in pneumonia or ARDS)

- Shunt (oxygenated blood mixes with non-oxygenated blood from the venous system, e.g. right to left shunt)

Hypoventilation may be caused by:

- A medical condition such as stroke affecting the brainstem

- Voluntary breath-holding or underbreathing, for example, hypoventilation training or Buteyko

- Medication or drugs, typically when taken in accidental or intentional overdose. Opioids in particular are known to cause respiratory depression. Examples of opioids include pharmaceuticals such as oxycodone and hydromorphone.

- Hypocapnia, which stimulates hypoventilation

- Chronic mountain sickness, a mechanism to conserve energy.

Obesity hypoventilation syndrome is a form of sleep disordered breathing. Two subtypes are recognized, depending on the nature of disordered breathing detected on further investigations. The first is OHS in the context of obstructive sleep apnea; this is confirmed by the occurrence of 5 or more episodes of apnea, hypopnea or respiratory-related arousals per hour (high apnea-hypopnea index) during sleep. The second is OHS primarily due to "sleep hypoventilation syndrome"; this requires a rise of CO levels by 10 mmHg (1.3 kPa) after sleep compared to awake measurements and overnight drops in oxygen levels without simultaneous apnea or hypopnea. Overall, 90% of all people with OHS fall into the first category, and 10% in the second.

The signs and symptoms of ARDS often begin within two hours of an inciting event, but can occur after 1–3 days. Signs and symptoms may include shortness of breath, fast breathing, and a low oxygen level in the blood due to abnormal ventilation.

Obesity hypoventilation syndrome (also known as Pickwickian syndrome) is a condition in which severely overweight people fail to breathe rapidly enough or deeply enough, resulting in low blood oxygen levels and high blood carbon dioxide (CO) levels. Many people with this condition also frequently stop breathing altogether for short periods of time during sleep (obstructive sleep apnea), resulting in many partial awakenings during the night, which leads to continual sleepiness during the day. The disease puts strain on the heart, which eventually may lead to the symptoms such as heart failure, leg swelling and various other related symptoms. The most effective treatment is weight loss, but it is often possible to relieve the symptoms by nocturnal ventilation with positive airway pressure (CPAP) or related methods.

Obesity hypoventilation syndrome is defined as the combination of obesity (body mass index above 30 kg/m), hypoxemia (falling oxygen levels in blood) during sleep, and hypercapnia (increased blood carbon dioxide levels) during the day, resulting from hypoventilation (excessively slow or shallow breathing). The disease has been known since the 1950s, initially as "Pickwickian syndrome" in reference to a Dickensian character but currently under a more descriptive name.

Diffuse compromise of the pulmonary system resulting in ARDS generally occurs in the setting of critical illness. ARDS may be seen in the setting of severe pulmonary (pneumonia) or systemic infection (sepsis), following trauma, multiple blood transfusions (TRALI), severe burns, severe inflammation of the pancreas (pancreatitis), near-drowning or other aspiration events, drug reactions, or inhalation injuries. Some cases of ARDS are linked to large volumes of fluid used during post-trauma resuscitation.

Prolonged high oxygen delivery in premature infants causes necrotizing bronchiolitis and alveolar septal injury, with inflammation and scarring. This results in hypoxemia. Today, with the advent of surfactant therapy and high frequency ventilation and oxygen supplementation, infants with BPD experience much milder injury without necrotizing bronchiolitis or alveolar septal fibrosis. Instead, there are usually uniformly dilated acini with thin alveolar septa and little or no interstitial fibrosis. It develops most commonly in the first 4 weeks after birth.

The newer National Institute of Health (US) criteria for BPD (for neonates treated with more than 21% oxygen for at least 28 days) is as follows:,

- Mild

- Breathing room air at 36 weeks post-menstrual age or discharge (whichever comes first) for babies born before 32 weeks, or

- breathing room air by 56 days postnatal age, or discharge (whichever comes first) for babies born after 32 weeks gestation.

- Moderate

- Need for <30% oxygen at 36 weeks postmenstrual age, or discharge (whichever comes first) for babies born before 32 weeks, or

- need for <30% oxygen to 56 days postnatal age, or discharge (whichever comes first) for babies born after 32 weeks gestation.

- Severe

- Need for >30% oxygen, with or without positive pressure ventilation or continuous positive pressure at 36 weeks postmenstrual age, or discharge (whichever comes first) for babies born before 32 weeks, or

- need for >30% oxygen with or without positive pressure ventilation or continuous positive pressure at 56 days postnatal age, or discharge (whichever comes first) for babies born after 32 weeks' gestation.

Other important or common causes of shortness of breath include cardiac tamponade, anaphylaxis, interstitial lung disease, panic attacks, and pulmonary hypertension. Cardiac tamponade presents with dyspnea, tachycardia, elevated jugular venous pressure, and pulsus paradoxus. The gold standard for diagnosis is ultrasound. Anaphylaxis typically begins over a few minutes in a person with a previous history of the same. Other symptoms include urticaria, throat swelling, and gastrointestinal upset. The primary treatment is epinephrine. Interstitial lung disease presents with gradual onset of shortness of breath typically with a history of a predisposing environmental exposure. Shortness of breath is often the only symptom in those with tachydysrhythmias. Panic attacks typically present with hyperventilation, sweating, and numbness. They are however a diagnosis of exclusion. Around 2/3 of women experience shortness of breath as a part of a normal pregnancy. Neurological conditions such as spinal cord injury, phrenic nerve injuries, Guillain–Barré syndrome, amyotrophic lateral sclerosis, multiple sclerosis and muscular dystrophy can all cause an individual to experience shortness of breath. Shortness of breath can also occur as a result of vocal cord dysfunction (VCD).

Anaemia that develops gradually usually presents with exertional dyspnea, fatigue, weakness, and tachycardia. It may lead to heart failure. Anaemia caused by low haemoglobin levels is often a cause of dyspnea. Menstruation, particularly if excessive, can contribute to anaemia and to consequential dyspnea in women. Headaches are also a symptom of dyspnea in patients suffering from anaemia. Some patients report a numb sensation in their head, and others have reported blurred vision caused by hypotension behind the eye due to a lack of oxygen and pressure; these patients have also reported severe head pains, many of which lead to permanent brain damage. Symptoms can include loss of concentration, focus, fatigue, language faculty impairment and memory loss.

CHS is associated with respiratory arrests during sleep and, in some cases, to neuroblastoma (tumors of the sympathetic ganglia), Hirschsprung disease (partial agenesis of the enteric nervous system), dysphagia (difficulty swallowing) and anomalies of the pupilla. Other symptoms include darkening of skin color from inadequate amounts of oxygen, drowsiness, fatigue, headaches, and an inability to sleep at night. Those suffering from Ondine's curse also have a sensitivity to sedatives and narcotics, which makes respiration even more difficult. A low concentration of oxygen in the red blood cells also may cause hypoxia-induced pulmonary vasoconstriction and pulmonary hypertension, culminating in cor pulmonale or a failure of the right side of the heart. Associated complications may also include gastro-esophageal reflux, ophthalmologic issues, seizures, recurrent pneumonia, developmental delays, learning disabilities and episodes of fainting and temperature disregulation.

Central hypoventilation syndrome (CHS) is a respiratory disorder that results in respiratory arrest during sleep. CHS can either be congenital (CCHS) or acquired (ACHS) later in life. It is fatal if untreated. It is also known as Ondine's curse.

ACHS can develop as a result of severe injury or trauma to the brain or brainstem. Congenital cases are very rare and involve a failure of autonomic control of breathing. In 2006, there were only about 200 known cases worldwide. As of 2008, only 1000 total cases were known. The diagnosis may be delayed because of variations in the severity of the manifestations or lack of awareness in the medical community, particularly in milder cases. However, as there have been cases where asymptomatic family members also were found to have CCHS, it may be that these figures only reflect those found to require mechanical ventilation. In all cases, episodes of apnea occur in sleep, but in a few patients, at the most severe end of the spectrum, apnea also occurs while awake.

Although rare, cases of long-term untreated CCHS have been reported and are termed late onset CCHS (LO-CCHS). Cases that go undiagnosed until later life and middle age, although the symptoms are usually obvious in retrospect. There have, however, even been cases of LO-CCHS where family members found to have it have been asymptomatic. Again, lack of awareness in the medical community may cause such a delay. CCHS susceptibility is not known to be affected by gender.

Extrapulmonary restriction is a type of restrictive lung disease, indicated by decreased alveolar ventilation with accompanying hypercapnia. It is characterized as an inhibition to the drive to breathe, or an ineffective restoration of the drive to breathe.

Extrapulmonary restriction can be caused by central and peripheral nervous system dysfunctions, over-sedation, or trauma (such as a broken rib).

Although central and obstructive sleep apnea have some signs and symptoms in common, others are present in one but absent in another, enabling differential diagnosis as between the two types:

Signs and symptoms of sleep apnea generally

- Signs:

- Symptoms:

Signs and symptoms of central sleep apnea

- Signs:

- Symptoms:

Signs and symptoms of and conditions associated with obstructive sleep apnea

- Signs:

- Symptoms:

- Associated conditions:

Congenital central hypoventilation syndrome (CCHS), often referred to by its older name "Ondine's curse," is a rare and very severe inborn form of abnormal interruption and reduction in breathing during sleep. This condition involves a specific homeobox gene, PHOX2B, which guides maturation of the autonomic nervous system; certain loss-of-function mutations interfere with the brain's development of the ability to effectively control breathing. There may be a recognizable pattern of facial features among individuals affected with this syndrome.

Once almost uniformly fatal, CCHS is now treatable. Children who have it must have tracheotomies and access to mechanical ventilation on respirators while sleeping, but most do not need to use a respirator while awake. The use of a diaphragmatic pacemaker may offer an alternative for some patients. When pacemakers have enabled some children to sleep without the use of a mechanical respirator, reported cases still required the tracheotomy to remain in place because the vocal cords did not move apart with inhalation.

Persons with the syndrome who survive to adulthood are strongly instructed to avoid certain condition-aggravating factors, such as alcohol use, which can easily prove lethal.

There are several terms which were in general use, but are no longer recommended.

The World Health Organization in 2005 defined drowning as "the process of experiencing respiratory impairment from submersion/immersion in liquid". This definition does not imply death, or even the necessity for medical treatment after removal of the cause, nor that any fluid enters the lungs. The WHO further recommended that outcomes should be classified as: death, morbidity, and no morbidity. There was also consensus that the terms wet, dry, active, passive, silent, and secondary drowning should no longer be used.

Experts differentiate between distress and drowning.

- Distress – people "in trouble", but who still have the ability to keep afloat, signal for help and take actions.

- Drowning – people "suffocating" and in "imminent danger of death within seconds".

The most obvious sign that meconium has been passed during or before labor is the greenish or yellowish appearance of the amniotic fluid. The infant's skin, umbilical cord, or nailbeds may be stained green if the meconium was passed a considerable amount of time before birth. These symptoms alone do not necessarily indicate that the baby has inhaled in the fluid by gasping in utero or after birth. After birth, rapid or labored breathing, cyanosis, slow heartbeat, a barrel-shaped chest or low Apgar score are all signs of the syndrome. Inhalation can be confirmed by one or more tests such as using a stethoscope to listen for abnormal lung sounds (diffuse 'wet' crackles and rhonchi), performing blood gas tests to confirm a severe loss of lung function (respiratory acidosis as a consequence of hypercapnia), and using chest X-rays to look for patchy or streaked areas on the lungs. Infants who have inhaled meconium may develop respiratory distress syndrome often requiring ventilatory support. Complications of MAS include pneumothorax and persistent pulmonary hypertension of the newborn.

Ventilator-associated lung injury (VALI) is an acute lung injury that develops during mechanical ventilation and is termed ventilator-induced lung injury (VILI) if it can be proven that the mechanical ventilation caused the acute lung injury. In contrast, ventilator-associated lung injury (VALI) exists if the cause cannot be proven. VALI is the appropriate term in most situations because it is virtually impossible to prove what actually caused the lung injury in the hospital.