Bookcase at National Library of Medicine.

DataPearls.StatPearls Publishing; Treasure Island, FL; 2022 Jan.


Continuing Education Activity

The definition of hypercapnia is a partial pressure of carbon dioxide (PaCO2) above 45 millimeters of mercury (mmHg).In the body, carbon dioxide is produced as a metabolic byproduct of various cellular processes, and there are several physiological mechanisms that ensure the body keeps its carbon dioxide levels under control.The acid-base buffering system involves the balance of bicarbonate and carbon dioxide.Hypercapnia, therefore, leads to an acid-base imbalance.In this activity, participants will learn about the assessment, diagnosis, and management of hypercapnia and will also consider the role that team-based interprofessional care can have on the treatment of affected patients.

The objectives are: Provide a list of the causes of hypercapnia. Analyze the presentation of a patient in hypercapnia. Give an overview of the treatment options for hypercapnia. Describe the importance of interprofessional care coordination to improve outcomes for patients with hypercapnia. You can access free multiple choice questions on this topic.


As the partial pressure of carbon dioxide (PaCO2) rises above 45 mm Hg, hypercapnia is present.The body's metabolic processes include metabolism of lipids, carbohydrates, and proteins, resulting in carbon dioxide (CO2).There are a number of physiological mechanisms that control CO2 levels.The pH buffering system between hydrogen carbonate (HCO3) and carbon dioxide is one of them.Hypercapnia can cause acid-base imbalances because of this relationship.


In the simplest terms, hypercapnia is caused by either metabolically increased CO2 production or respiratory failure.Fever, thyrotoxicosis, and steroid use can increase CO2 production, as can overeating, metabolic acidosis, and exercise.In this patient population, respiratory failure is a failure to eliminate carbon dioxide from the pulmonary system, which is synonymous with hypoventilation caused by decreased respiratory rate or decreased tidal volume*4>. This may be caused by decreased respiratory drive in the central nervous system, anatomical defects, a decreased neuromuscular response, or increased dead space in the lungs.Additionally, exposure and inhalation of CO2-rich air can cause hypercapnia.Hypercapnia can be an acute or chronic condition, depending on its etiology.This can be distinguished from acute hypercapnia by evaluating the pH blood values. *5>Acute hypercapnia can be identified by elevated PaCO2 above 45 mm Hg and high HCO3 levels at approximately 30 mm Hg while pH readings decrease proportionally below 7.35.This condition allows the kidney to compensate for the elevated levels of CO2.Thus, PaCO2 will be elevated above the normal range of 45 mm Hg, and HCO3 level will also be elevated proportionally, resulting in an imbalance of pH in the low-normal range.


A single cause of hypercapnia does not exist. Instead, it refers to a syndrome of illnesses and not one single disease.Thus, the precise epidemiology depends on the specific etiology.


Among the functions of the lungs is the elimination of CO2 from the body through gas diffusion.A diffusion gradient must be established between the relatively high concentration arteriolar blood and the low concentration ambient air.The CO2 gradients, therefore, are maintained where PaCO2 in arterial blood is directly related to the rate of CO2 metabolic production and indirectly related to the rate of CO2 ejection by the lungs via increased alveolar ventilation.Air from the alveoli is expelled into the environment by alveolar ventilation, defined as the minute volume that reaches the alveoli and is related to minute ventilation and dead space ratio.Our bodies are extremely adapted and capable of eliminating excess CO2.A significant loss in pulmonary ventilation will not cause hypercapnia in metabolic processes.Using mathematics, this relationship is determined as follows:

The PaCO2 value is equal to 0.863 times VCO2/VA

As well as

The VA equation is VE - VD


The product of RR and TV is VE


The TV is equal to the RR times the dead-space volume

CO2 is the metabolic energy produced by the body, VA is alveolar ventilation, VE is minute ventilation, VD is dead space ventilation, RR is respiration rate, and TV is the tidal volume.

The relationship between respiratory rate and tidal volume indicates that they are the two components of ventilation that can be controlled, either physiologically or artificially, to moderate CO2 elimination.Consequently, a failure in one or both of these fields will cause hypercapnia.In addition, oxygenation decreases as PACO2 increases.To explain this, we can use the alveolar gas equation:

History and Physical

The physical findings and history are highly variable according to the source of hypercapnia.An individual may present with flushed skin, lethargy, inability to focus, headaches, disorientation, dizziness, shortness of breath, dyspnea on exertion, nausea, vomiting, and/or fatigue.Some of the more severe complaints include confusion, paranoia, depression, unusual muscle twitches, palpitations, hyperventilation, hypoventilation, seizures, anxiety, and/or syncope.Often, patients with a history of asthma or chronic obstructive pulmonary disease (COPD) present with symptoms of exacerbation.

Findings on physical examination are typically vague, but may indicate underlying health conditions.


Hypocapnia must be evaluated based on clinical suspicion, as there are many differential diagnoses.Standard tests in most patients include:

Treatment / Management

In the treatment of hypercapnia, the underlying cause should be addressed.The lungs can be ventilated with external aids to assist the removal of CO2.BiPAP (BiLevel Positive Airway Pressure) ventilation assist, CPAP (Continuous Positive Airway Pressure) ventilation, and intubation with mechanical ventilation for severely ill patients are among the most common.BiPAP is typically preferred by an alert, awake patient who is able to protect his or her airway, as it provides alternate levels of positive pressure support for better air exchange between the alveolar space and ambient air.Patients who need to splint their airways use CPAP.Although, it has a lower CO2 exchange rate compared to BiPAP. .Despite being the most invasive option, mechanical ventilation provides the physician with added control over both respiratory rate and tidal volume while also implementing FiO2 and pressure support.If a patient is not awake, alert, or able to protect their airway, mechanical ventilation should be strongly considered.It is not a curative treatment on its own, but it is a supportive measure as a stabilizing measure while the underlying etiology is corrected.Whatever the oxygenation support used, maintaining O2 saturation of 90% or higher is essential.*18>*19>*20>

Differential Diagnosis

A variety of diseases can result in hypercapnia.Among them are: