Index
Carbon dioxide partial blood pressure is essential in the intensive care unit (ICU).
It is used to diagnose, treat and monitor the patient’s progress.
Given the limitations of an arterial puncture, alternative methods for estimating the partial pressure of carbon dioxide (PCO2) are highly desired.
When the gas exchange is stable and the patient is in a steady-state, the central venous partial pressure of carbon dioxide is an excellent substitute for the partial pressure of carbon dioxide (PCO2), as long as the cardiac output is not impaired.
Peripheral venous carbon dioxide partial pressure (Pco2) values obtained from venipuncture or indwelling catheters differ substantially from carbon dioxide partial pressure (PCO2).
These values should not be used to estimate the partial pressure of carbon dioxide (PCO2), especially for sicker patients or when conditions are changing.
The post-pneum pressure of carbon dioxide at the end of the tide (PetCO2) has the advantage of continuous monitoring, breath by breath, but because its correlation with the partial pressure of carbon dioxide (PCO2) depends on V / Q.
The pairing and expiration time should not be used to estimate the partial pressure of carbon dioxide (PCO2) during critical illness.
However, capnography is useful during endotracheal intubation and patient transport to confirm the correct endotracheal tube placement.
To assess the adequacy of resuscitation during cardiac arrest and estimate the partial pressure of carbon dioxide (PCO2) in healthy patients under general anesthesia.
The transcutaneous carbon dioxide pressure (PtcCO2) is a continuous measure. Still, it has also been shown to reflect the partial pressure of carbon dioxide (PCO2) within proper limits, even during severe illness.
Transcutaneous carbon dioxide (PtcCO2) pressure measurements remain accurate during hemodynamic instability and vasopressors as long as skin perfusion is not severely impaired.
It may be of particular value in non-hospital settings, for example:
In the evaluation and follow-up of outpatients undergoing long-term assisted ventilation for neuromuscular respiratory failure or when capnography is especially difficult, such as during non-invasive or high-frequency ventilation.
Transcutaneous carbon dioxide (PtcCO2) pressure measurements can help detect hypoventilation or hyperventilation in outpatient clinics, emergency departments, or out-of-hospital settings.
Since Haldane and Priestley’s (1905) work on the pressure of C02 in alveolar air, and Krogh’s (1910) on the mechanism of gas exchange in the lung, it has been generally believed that the partial pressure of C02 is the Equal (to a close approximation) in arterial blood and the alveolar air.
However, the relationship of the partial pressure of oxygen in alveolar air to that in arterial blood has not been determined with the same precision.
This uncertainty is partly due to the dispute over the role of the lungs in oxygen transfer, in part to the difference of opinion regarding the mechanisms of pulmonary ventilation and a lack of precise knowledge of the facts.
What is partial pressure?
It is the individual pressure exerted by a particular gas within a gas mixture.
The air we breathe is a mixture of gases: nitrogen, oxygen, and carbon dioxide.
So the air blowing into a balloon creates a pressure that causes the balloon to expand (and this pressure is generated like all the nitrogen, oxygen, and carbon dioxide molecules move and collide with the walls of the balloon).
However, the total pressure generated by the air is due in part to nitrogen, in part to oxygen, and in part to carbon dioxide.
That part of the total pressure generated by oxygen is the “partial pressure” of oxygen, while that caused by carbon dioxide is the “partial pressure” of carbon dioxide.
Therefore, the partial pressure of a gas is a measure of the amount of that gas present (for example, in the blood or alveoli).
The partial pressure exerted by each gas in a mixture equals the total pressure multiplied by the fractional composition of the gas in the mix.
So since the total atmospheric pressure (at sea level) is about 760mm Hg, and the air is about 21% oxygen, the partial pressure of oxygen in the air is 0.21 times 760mm Hg or 160 mm Hg.
Uses of Carbon Dioxide Partial Pressure
Carbon dioxide partial arterial pressure (PCO2) is an essential parameter in critically ill patients on mechanical ventilation.
To limit invasive procedures or more continuous monitoring of carbon dioxide partial blood pressure (PCO2), clinicians often rely on venous blood gases, capnography, or transcutaneous monitoring.
Each of these has advantages and limitations.
The central venous partial pressure of carbon dioxide (Pco2) allows an accurate estimation of the arterial partial pressure of carbon dioxide (PCO2), which differs from the amount described by the Fick principle.
As long as the cardiac output is relatively standard, the central venous partial pressure of carbon dioxide (Pco2) exceeds the arterial value by approximately 4 mm Hg.
In contrast, peripheral venous partial pressure of carbon dioxide (Pco2) is a poor predictor of arterial partial pressure of carbon dioxide (PCO2), and the use of peripheral venous partial pressure of carbon dioxide (Pco2) is not recommended for this way.
Capnography measures the final partial pressure of carbon dioxide (PetCO2), a value close to the partial pressure of carbon dioxide (PCO2) when the lung is healthy.
It has the advantage of being non-invasive and continuously available.
However, in mechanically ventilated patients with lung disease, the absolute partial pressure of carbon dioxide (PetCO2) often differs from the partial arterial pressure of carbon dioxide (PCO2), sometimes significantly, constantly seriously underestimating the value. Arterial.
The dependence of absolute partial pressure of carbon dioxide (PetCO2) on alveolar dead space and ventilator expiration time limits their value for predicting arterial partial pressure of carbon dioxide (PCO2).
When lung function or ventilator settings change, the final partial carbon dioxide (PetCO2) and the partial arterial pressure of carbon dioxide (PCO2) can vary in different directions, leading to more significant uncertainty.
Measurement of the transcutaneous partial pressure of carbon dioxide (Pco2) has become practical and reliable.
It shows promise for judging steady-state values for partial arterial pressure of carbon dioxide (PCO2) unless there is overt vasoconstriction of the skin.
Furthermore, it may be helpful in conditions where capnography fails (high-frequency ventilation) or where arterial blood gas analysis is burdensome (clinical or home administration of mechanical ventilation).
Partial blood pressure for carbon dioxide (PCO2) is one of the most important physiological parameters measured in patients in the intensive care unit (ICU).
Carbon dioxide partial blood pressure (PCO2) is essential for analyzing acid-base disorders, monitoring ventilation adequacy, guiding machine settings during mechanical ventilation, and discerning a wide range of intoxications, metabolic disorders, and conditions. of shock.
Obtaining arterial blood is invasive, requiring an arterial puncture or arterial catheter.
Although many critically ill patients with shock or ventilatory failure are routinely managed with an arterial catheter, there is substantial variation in practice.
This practice has been criticized because the results do not seem better when catheters are used.
In addition, blood gas values are sampled periodically rather than continuously, and clinically significant changes may be missing, especially in populations where close control of partial arterial pressure of carbon dioxide (PCO2) is desired.
At the same time, new technologies have supplanted the information obtained through arterial catheters by non-invasively estimating blood gas tensions, blood pressure, and stroke volume.
Carbon dioxide partial pressure is important.
Carbon dioxide partial pressure (PCO2) is just one of the factors measured in the arterial blood gas (ABG) test.
It also evaluates the partial pressure of oxygen (PaO2), bicarbonate (HCO3), and the pH level of the blood.
Changes in that pressure can cause the blood to lose too much oxygen or build up too much carbon dioxide. Neither is considered good.
Too little carbon dioxide can lead to alkalosis, a condition in which you have too many bases in your blood (carbon dioxide is an acid).
What causes changes in the partial pressure of carbon dioxide (PCO2)?
Diseases can work simultaneously, altering the partial pressure that ensures the balanced transfer of carbon dioxide molecules. Several conditions can change these levels:
- Obstructive lung diseases such as chronic obstructive pulmonary disease and asthma .
- Central nervous system impairment (including head injury and drug use).
- Neuromuscular diseases such as amyotrophic lateral sclerosis (ALS).
- A low hemoglobin concentration is used to transport oxygen and carbon dioxide through the blood.
The normal and abnormal partial pressure of carbon dioxide (PCO2)
An arterial blood gas test is usually performed on the wrist’s radial artery or the groin’s femoral artery.
It is an uncomplicated procedure, but it can be painful since the arteries are deeper in the body than the veins. Swelling and bruising can sometimes occur.
The usual range of partial pressure of carbon dioxide is between 40 and 45 mm Hg. If it is more than 45 mm Hg, you have too much carbon dioxide in your blood. Less than 40mm Hg, and you have very little.
Elevated levels of carbon dioxide are commonly seen in cases of:
- Obstructive pulmonary disease.
- Severe vomiting
- Excessive use of mercury-based diuretics.
- Aldosteronism (a type of hormonal disorder that causes high blood pressure ).
In contrast, carbon dioxide decline is frequently seen with:
- Kidney dysfunction or failure.
- Severe diarrhea
- Anorexia / starvation.
- Overuse of chlorothiazide diuretics (used to reduce the risk of stroke and heart attack).
- Diabetic acidosis
Importance of carbon dioxide partial pressure in chronic obstructive pulmonary disease
Carbon dioxide is in equilibrium with bicarbonate (HCO3) in the blood. When carbon dioxide rises, it creates an acidic environment.
In people with chronic obstructive pulmonary disease who have severe breathing problems, increased carbon dioxide levels can lead to what we call respiratory acidosis.
