The prognosis of pulmonary arterial hypertension (WHO Group I) has an "untreated" median survival of 2–3 years from time of diagnosis, with the cause of death usually being right ventricular failure (cor pulmonale). A recent outcome study of those patients who had started treatment with bosentan (Tracleer) showed that 89% patients were alive at 2 years. With new therapies, survival rates are increasing. For 2,635 patients enrolled in The Registry to Evaluate Early and Long-term Pulmonary Arterial Hypertension Disease Management (REVEAL Registry) from March 2006 to December 2009, 1-, 3-, 5-, and 7-year survival rates were 85%, 68%, 57%, and 49%, respectively. For patients with idiopathic/familial PAH, survival rates were 91%, 74%, 65%, and 59%. Levels of mortality are very high in pregnant women with severe pulmonary arterial hypertension (WHO Group I). Pregnancy is sometimes described as contraindicated in these women.

SIPE is estimated to occur in 1-2% of competitive open-water swimmers, with 1.4% of triathletes, 1.8% of combat swimmers and 1.1% of divers and swimmers reported in the literature.

The mechanisms by which SIPE occurs are controversial, and likely multiple factors are required for the phenomenon to manifest.

- Hydrostatic pressure from water immersion squeezes the extremities, and forces blood from the peripheral circulation (arms, legs) to the central circulation (heart, lungs, great vessels of the chest)

- Cold water may cause peripheral vasoconstriction and other neuro-humoral changes that contribute to central shift of the blood volume

- Wetsuits may add additional extrinsic compression to the extremities.

- Increased pressure somewhere in the pulmonary circulation (pulmonary artery hypertension, left heart diastolic dysfunction) leads to increased pressure gradient across the pulmonary capillaries

- Capillary stress from oxidative or physical injury leads to breach

SIPE is believed to arise from a "perfect storm" of some combination of these factors, which overwhelms the ability of the body to compensate, and leads to alveolar flooding.

The epidemiology of IPAH is about 125–150 deaths per year in the U.S., and worldwide the incidence is similar to the U.S. at 4 cases per million. However, in parts of Europe (France) indications are 6 cases per million of IPAH. Females have a higher incidence rate than males (2–9:1).

Other forms of PH are far more common. In systemic scleroderma, the incidence has been estimated to be 8 to 12% of all patients; in rheumatoid arthritis it is rare. However, in systemic lupus erythematosus it is 4 to 14%, and in sickle cell disease, it ranges from 20 to 40%. Up to 4% of people who suffer a pulmonary embolism go on to develop chronic thromboembolic disease including pulmonary hypertension. A small percentage of patients with COPD develop pulmonary hypertension with no other disease to explain the high pressure. On the other hand, obesity-hypoventilation syndrome is very commonly associated with right heart failure due to pulmonary hypertension.

The rate of BPD varies among institutions, which may reflect neonatal risk factors, care practices (e.g., target levels for acceptable oxygen saturation), and differences in the clinical definitions of BPD.

Let us consider some scenarios where there is a defect in ventilation and/ or perfusion of the lungs.

In condition such as pulmonary embolism, the pulmonary blood flow is affected, thus the ventilation of the lung is adequate, however there is a perfusion defect with defect in blood flow. Gas exchange thus becomes highly inefficient leading to hypoxemia as measured by arterial oxygenation. A ventilation perfusion scan or lung scintigraphy shows some areas of lungs being ventilated but not adequately perfused. This also leads to a high A-a gradient which is not responsive to oxygen

In conditions with right to left shunts, there is again a ventilation perfusion defect with high A-a gradient. However, the A-a gradient is responsive to oxygen therapy. In cases of right to left shunts more of deoxygenated blood mixes with oxygenated blood from the lungs and thus to a small extent the condition might neutralize the high A-a gradient with pure oxygen therapy.

Patient with parenchymal lung diseases will have an increased A-a gradient with moderate response to oxygen therapy.

A patient with hypoventilation will have complete response to 100% oxygen therapy

In terms of the epidemiology of air embolisms one finds that the "intra-operative" period to have the highest incidence. For example, VAE in neurological cases ranges up to 80%, and OBGYN surgeries incidence can climb to 97% for VAE (vascular air embolism). In divers the incidence rate is 7/100,000 per dive.

Peripheral cyanosis is the blue tint in fingers or extremities, due to an inadequate or obstructed circulation. The blood reaching the extremities is not oxygen-rich and when viewed through the skin a combination of factors can lead to the appearance of a blue color. All factors contributing to central cyanosis can also cause peripheral symptoms to appear but peripheral cyanosis can be observed in the absence of heart or lung failures. Small blood vessels may be restricted and can be treated by increasing the normal oxygenation level of the blood.

Peripheral cyanosis may be due to the following causes:

- All common causes of central cyanosis

- Reduced cardiac output (e.g. heart failure or hypovolaemia)

- Cold exposure

- Chronic obstructive pulmonary disease (COPD)

- Arterial obstruction (e.g. peripheral vascular disease, Raynaud phenomenon)

- Venous obstruction (e.g. deep vein thrombosis)

Ventilation Perfusion mismatch or "V/Q defects" are defects in total lung ventilation perfusion ratio. It is a condition in which one or more areas of the lung receive oxygen but no blood flow, or they receive blood flow but no oxygen due to some diseases and disorders.

The V/Q ratio of a healthy lung is approximately equal to 0.8, as normal lungs are not perfectly matched., which means the rate of alveolar ventilation to the rate of pulmonary blood flow is roughly equal.

The ventilation perfusion ratio can be measured by measuring the A-a gradient i.e. the alveolar-arterial gradient.

Central cyanosis is often due to a circulatory or ventilatory problem that leads to poor blood oxygenation in the lungs. It develops when arterial oxygen saturation drops to ≤85% or ≤75%.

Acute cyanosis can be as a result of asphyxiation or choking, and is one of the definite signs that respiration is being blocked.

Central cyanosis may be due to the following causes:

1. Central nervous system (impairing normal ventilation):

- Intracranial hemorrhage

- Drug overdose (e.g. heroin)

- Tonic–clonic seizure (e.g. grand mal seizure)

2. Respiratory system:

- Pneumonia

- Bronchiolitis

- Bronchospasm (e.g. asthma)

- Pulmonary hypertension

- Pulmonary embolism

- Hypoventilation

- Chronic obstructive pulmonary disease, or COPD (emphysema)

3. Cardiovascular diseases:

- Congenital heart disease (e.g. Tetralogy of Fallot, right to left shunts in heart or great vessels)

- Heart failure

- Valvular heart disease

- Myocardial infarction

4. Blood:

- Methemoglobinemia * Note this causes "spurious" cyanosis, in that, since methemoglobin appears blue, the patient can appear cyanosed even in the presence of a normal arterial oxygen level.

- Polycythaemia

- Congenital cyanosis (HbM Boston) arises from a mutation in the α-codon which results in a change of primary sequence, H → Y. Tyrosine stabilises the Fe(III) form (oxyhaemoglobin) creating a permanent T-state of Hb.

5. Others:

- High altitude, cyanosis may develop in ascents to altitudes >2400 m.

- Hypothermia

- Obstructive sleep apnea

Pulmonary venoocclusive disease is rare, difficult to diagnose, and probably frequently misdiagnosed as idiopathic pulmonary arterial hypertension. Prevalence in parts of Europe is estimated to be 0.1-0.2 cases per million.

PVOD appears to occur as frequently in men as in women, and age at diagnosis ranges from 7–74 years with a median of 39 years. PVOD may occur in patients with associated diseases such as HIV, bone marrow transplantation, and connective tissue diseases. PVOD has also been associated with several chemotherapy regimens such as bleomycin, BCNU, and mitomycin.

There have been rare cases of air embolism being caused by air entering the bloodstream from the uterus or tears in female genitalia. The risk appears to be greater during pregnancy. Cases have been reported that resulted from attempts to perform an abortion by syringing. These appear to have been due to damage to the placenta allowing air to enter the bloodstream.

Baylor College of Medicine in Houston, Texas has conducted ACD research since 2001.

There is evidence to show that steroids given to babies less than 8 days old can prevent bronchopulmonary dysplasia. However, the risks of treatment may outweigh the benefits.

It is unclear if starting steroids more than 7 days after birth is harmful or beneficial. It is thus recommended that they only be used in those who cannot be taken off of a ventilator.

The prevalence of pulmonary interstitial emphysema widely varies with the population studied. In a 1987 study 3% of infants admitted to the neonatal intensive care unit (NICU) developed pulmonary interstitial emphysema.

It is sometimes treated with surgery, which involves rerouting blood from the right atrium into the left atrium with a patch or use of the Warden procedure. However, interest is increasing in catheter-based interventional approaches, as well as medical therapy for less severe cases.

Studies reflecting international frequency demonstrated that 2-3% of all infants in NICUs develop pulmonary interstitial emphysema. When limiting the population studied to premature infants, this frequency increases to 20-30%, with the highest frequencies occurring in infants weighing fewer than 1000 g.

In TAPVC without obstruction, surgical redirection can be performed within the first month of life. The operation is performed under general anesthesia. The four pulmonary veins are reconnected to the left atrium, and any associated heart defects such as atrial septal defect, ventricular septal defect, patent foramen ovale, and/or patent ductus arteriosus are surgically closed. With obstruction, surgery should be undertaken emergently. PGE1 should be given because a patent ductus arteriosus allows oxygenated blood to go from the circulation of the right heart to the systemic circulation.

The annual incidence of ARDS is 13–23 people per 100,000 in the general population. Its incidence in the mechanically ventilated population in intensive care units is much higher. According to Brun-Buisson "et al" (2004), there is a prevalence of acute lung injury (ALI) of 16.1% percent in ventilated patients admitted for more than 4 hours.

Worldwide, severe sepsis is the most common trigger causing ARDS. Other triggers include mechanical ventilation, sepsis, pneumonia, Gilchrist's disease, drowning, circulatory shock, aspiration, traumaespecially pulmonary contusionmajor surgery, massive blood transfusions, smoke inhalation, drug reaction or overdose, fat emboli and reperfusion pulmonary edema after lung transplantation or pulmonary embolectomy. Pneumonia and sepsis are the most common triggers, and pneumonia is present in up to 60% of patients and may be either causes or complications of ARDS. Alcohol excess appears to increase the risk of ARDS. Diabetes was originally thought to decrease the risk of ARDS, but this has shown to be due to an increase in the risk of pulmonary edema. Elevated abdominal pressure of any cause is also probably a risk factor for the development of ARDS, particularly during mechanical ventilation.

The death rate varies from 25–40% in centers using up-to-date ventilatory strategies and up to 58% in all centers.

A number of congenital heart defects can cause Eisenmenger syndrome, including atrial septal defects, ventricular septal defects, patent ductus arteriosus, and more complex types of acyanotic heart disease.

If the inciting defect in the heart is identified "before" it causes significant pulmonary hypertension, it can normally be repaired through surgery, preventing the disease. After pulmonary hypertension is sufficient to reverse the blood flow through the defect, however, the maladaptation is considered irreversible, and a heart–lung transplant or a lung transplant with repair of the heart is the only curative option.

Transplantation is the final therapeutic option and only for patients with poor prognosis and quality of life. Timing and appropriateness of transplantation remain difficult decisions. 5-year and 10-year survival ranges between 70% and 80%, 50% and 70%, 30% and 50%, respectively. Since the average life expectancy of patients after lung transplantation is as low as 30% at 5 years, patients with "reasonable functional status" related to Eisenmenger syndrome have "improved survival with conservative medical care" compared with transplantation.

Various medicines and therapies for pulmonary hypertension are under investigation for treatment of the symptoms.

Shunting refers to blood that bypasses the pulmonary circulation, meaning that the blood does not receive oxygen from the alveoli. In general, a shunt may be within the heart or lungs, and cannot be corrected by administering oxygen alone. Shunting may occur in normal states:

- Anatomic shunting, occurring via the bronchial circulation, which provides blood to the tissues of the lung. Shunting also occurs by the smallest cardiac veins, which empty directly into the left ventricle.

- Physiological shunts, occur due to the effect of gravity. The highest concentration of blood in the pulmonary circulation occurs in the bases of the pulmonary tree compared to the highest pressure of gas in the apexes of the lungs. Alveoli may not be ventilated in shallow breathing.

Shunting may also occur in disease states:

- Acute lung injury and adult respiratory distress syndrome, which may cause alveolar collapse. This will increase the amount of physiological shunting, and unlike many forms of shunting, can be managed by administering 100% Oxygen.

- Pathological shunts such as patent ductus arteriosus, patent foramen ovale, and atrial septal defects or ventricular septal defects. These states are when blood from the right side of the heart moves straight to the left side, without first passing through the lungs. This is known as a right-to-left shunt, which is often congenital in origin.

Death may occur rapidly with acute, massive pulmonary bleeding or over longer periods as the result of continued pulmonary failure and right heart failure. Historically, patients had an average survival of 2.5 years after diagnosis, but today 86% may survive beyond five years.

Acute respiratory distress syndrome (ARDS) has some similarities to IRDS. Transient tachypnea of the newborn presents with respiratory distress syndrome in the preterm newborn.

The genetic cause of pulmonary veno-occlusive disease is mutations in EIF2AK4 gene. Though this does not mean other possible causes do not exist, such as viral infection and risk of toxic chemicals (chemotherapy drugs).