A less commonly understood, but equally informative, measurement in critical settings is the level of carbon dioxide (CO2) in the breath. This simple, non-invasive reading offers a real-time window into two fundamental processes of the body: how effectively the lungs are moving air and how efficiently the heart is circulating blood. By tracking the concentration of CO2 expelled, healthcare providers gain unique insights into a patient’s ventilation and overall circulatory status. Monitoring this specific gas exchange is now considered a standard of care, providing an early warning system for respiratory and cardiac compromise.
Defining End-Tidal CO2 and How It Is Measured
End-Tidal CO2 (EtCO2) is defined as the maximum concentration or partial pressure of carbon dioxide reached at the very end of an exhaled breath. This measurement is expressed in millimeters of mercury (mmHg) and serves as a reliable, non-invasive estimate of the CO2 level present in the blood returning to the lungs. The gas is produced by metabolic activity in the tissues, transported by the blood to the lungs, and then eliminated through respiration.
The method used to capture this reading is called capnography, which translates the concentration of the gas into both a numeric value and a continuous graphical waveform. The device, known as a capnograph, uses infrared light to measure the CO2 concentration in the sampled gas. This technique provides an instantaneous, breath-by-breath analysis of ventilation status. The resulting waveform, or capnogram, offers detailed information about the entire respiratory cycle, allowing clinicians to assess gas exchange efficiency.
The Standard Range and Why It Is Physiologically Important
For a healthy individual, the standard range for End-Tidal CO2 falls between 35 and 45 mmHg. This narrow range is physiologically significant because CO2 plays a direct role in maintaining the body’s acid-base balance. Carbon dioxide dissolves in the blood to form carbonic acid, meaning that its concentration directly influences the acidity (pH) of the blood.
Maintaining CO2 within this range is achieved by balancing two processes: metabolic production and ventilatory removal. The body’s cells produce CO2 constantly as a waste product of metabolism, and the lungs must eliminate it at an equal rate to keep the blood pH stable. If the EtCO2 is too high, the blood becomes more acidic, a state known as respiratory acidosis; if it is too low, the blood becomes too alkaline, leading to respiratory alkalosis.
EtCO2 levels are also closely tied to the coupling of ventilation and perfusion, often referred to as V/Q matching. Carbon dioxide must be delivered to the lungs by circulating blood (perfusion) and then removed by the breath (ventilation). A change in the EtCO2 reading can therefore signal an alteration in either the breathing function or the blood circulation.
Causes and Clinical Significance of Low End-Tidal CO2
A low EtCO2 reading, below 35 mmHg, is termed hypocapnia and indicates a net loss of carbon dioxide from the body. This drop can stem from one of two distinct physiological problems: excessive CO2 removal or inadequate CO2 delivery to the lungs. The most straightforward cause is hyperventilation, where a person is breathing faster or deeper than metabolically necessary. This can be a conscious action, such as due to anxiety or pain, or a compensatory mechanism, like the deep, rapid breaths seen in metabolic acidosis.
A low EtCO2 can also signal a severe circulatory issue, indicating that not enough blood is reaching the lungs to participate in gas exchange. Conditions like cardiac arrest, severe shock, or a massive pulmonary embolism cause a drastic reduction in the blood flow (perfusion) to the lungs. In these scenarios, the CO2 being measured is low because the blood is failing to deliver the waste gas from the tissues, even if the lungs are ventilating normally.
During cardiopulmonary resuscitation (CPR), an EtCO2 reading below 10 mmHg suggests that chest compressions are not effectively circulating blood. Conversely, a sudden increase in EtCO2 often indicates the return of spontaneous circulation.
Causes and Clinical Significance of High End-Tidal CO2
Conversely, an EtCO2 reading consistently above 45 mmHg is known as hypercapnia and signifies that the body is retaining too much carbon dioxide. This condition is primarily linked to hypoventilation, which is inadequate breathing that fails to remove CO2 as quickly as it is produced. The respiratory center in the brain may be suppressed, such as from the effects of opioid medications or general anesthesia, causing a dangerously slow breathing rate.
High EtCO2 can also result from mechanical issues that impede airflow and gas exchange. Examples include a severe airway obstruction or an exacerbation of chronic obstructive pulmonary disease (COPD), which traps air and CO2 in the lungs. Less commonly, an elevated reading can be caused by a state of increased metabolic production, such as the rapid CO2 generation that occurs during malignant hyperthermia or intense shivering. Uncorrected hypercapnia leads to respiratory acidosis, which can disrupt cellular function and necessitate immediate intervention.