Non-invasive blood gas interpretation
Lawrence Martin, M.D.

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5. PetCO₂ in lieu of PaCO₂ in intubated patients

Summary: Measurement of end-tidal PCO₂, called capnography, has been shown to track PaCO₂ in stable patients. Because the measurement requires a closed system, PetCO₂ monitoring works best in intubated patients. Once a correlation is made between PetCO₂ and PaCO₂, the latter need no longer be measured (or measured as frequently) in intubated patients, including those being weaned from the ventilator.

Discussion
Measurement of the "end-tidal" PCO₂, symbolized PetCO₂, is achieved non-invasively by analyzing the fraction of CO₂ in expired air. PetCO₂ can often substitute for PaCO₂ and thus obviate drawing an arterial blood sample. This is so because, in most clinical situations, PetCO₂ either closely approximates PaCO₂ or differs from PaCO₂ by some fairly constant amount.
The measurement of CO₂ in expired gas is called capnography. Capnography can be done on a continuous basis, using either an infrared analyzer or a mass spectrometer (Clark 1992; Wright 1992; Stock 1995. Most intensive care units employ infrared capnograpy, since it is much cheaper to monitor a group of patients than with a mass spectrometer.

In ventilated patients the sensor for exhaled carbon dioxide is placed in the ventilator tubing so that it can measure both inspired and expired air. PetCO₂ is continuously displayed on the bedside monitor. The curve registering PetCO₂ goes up and peaks or plateaus with expiration and returns to baseline (zero) with inspiration (Figure 3-3). A bedside monitor can graphically display the breath-to-breath changes in CO₂, as well as the digital value of the end-tidal CO₂ as a percentage or as a pressure.

The figure shows a normal tracing of expired PCO₂, measured during a tidal volume breath and printed out for placing in a patient's chart. Note the following about this figure:

Inhaled PCO₂ is zero (the amount in inhaled air is negligible).
The first part of exhaled air reflects air recently inhaled, and also has a PCO₂ of zero. This is followed by air that is a mixture of alveolar and dead space gas that contains increasing amounts of CO₂, until finally a peak or plateau is reached that, ideally, reflects air only from the alveoli.
This peak or plateau is the 'end-tidal' point and its PCO₂ is the end-tidal PCO₂ (PetCO₂). The tracing shows that the end-tidal PCO₂ (PetCO₂) is approximately 38 mm Hg, which was very close to the PaCO₂ at that point, which was 40 mm Hg.

PetCO₂ in health and disease

In health, because carbon dioxide is never diffusion limited, alveolar PCO₂ (PACO₂) is assumed equal to the end-capillary PCO₂ which in turn is equal to arterial PCO₂ (PaCO₂). Thus if you could sample alveolar PCO₂ you would have a proxy for PaCO₂. This in fact is possible because the last bit of air exhaled in each breath is the end-of-tidal-volume ('end-tidal') air and comes from the alveoli. By sampling exhaled gas breath-to-breath, we can obtain a continuous display of the end-tidal PCO₂. This continuous reading should equal, or be very close (within 1-2 mm Hg) to the alveolar and arterial PCO₂. To summarize:

in health: PetCO₂<= PACO₂ <= PaCO₂

In patients with lung disease and ventilation-perfusion (V-Q) imbalance, PetCO₂ will likely not be equal or close to alveolar and arterial PCO₂. This is because of increased physiologic dead space common in states of V-Q imbalance. Increases in dead space come about when a group of alveoli are un-perfused or under perfused, so that air reaching them becomes wholly or partly dead space air. Air in unperfused or under perfused alveoli keeps a low PCO₂, since little or no CO₂ is added to it (remember, inhaled air has almost zero CO₂).

The above figure is a breath-by-breath tracing of exhaled CO₂ from a patient with severe COPD. The patient was tachypneic so that the breaths appear compressed. Note the slight variation in PetCO₂. In this patient the PetCO₂ averaged approximately 50 mm Hg but his PaCO₂ was 74 mm Hg, for a PaCO₂-PetCO₂ difference of 24 mm Hg. In this situation, the diseased alveoli do not empty evenly, and the end-tidal sample includes considerable dead space air.
There is no way to know how large the (PaCO₂-PetCO₂) will be in a given patient or a given type of lung disease. And while 24 mm Hg is a relatively high difference between PaCO₂ and PetCO₂, a large difference does not obviate usefulness of PetCO₂. Like any blood gas measurement, the result must be validated and interpreted in light of the full clinical picture. The average PetCO₂ in this sequence of breaths is about 50 mm Hg. The patient's PaCO₂ measured at the same time was 74 mm Hg, for a PaCO₂-PetCO₂ of 24 mm Hg.
Proper use of end-tidal PCO₂

The principal advantages of capnography over intermittent measurement of arterial blood gases are: 1) to obviate the need for blood sampling; and 2) to allow for continuous reading of PetCO₂. Thus, to the extent that PetCO₂ can be used as a proxy for PaCO₂, it has obvious utility in the intensive care setting. When properly used, PetCO₂ (along with pulse oximetry) can often reduce arterial blood sampling and allow for safe patient monitoring.

There are two keys to properly using PetCO₂: first, make a correlation with PaCO₂ and second, know the pitfalls that can affect interpretation of PetCO₂.

1) Correlate PetCO₂ with PaCO₂.
A sizable PaCO₂-PetCO₂ difference does not obviate the value of the PetCO₂ for physiologic monitoring. A rise in PetCO₂ still indicates a rise in PaCO₂, but one cannot equate the numerical value of PetCO₂ with PaCO₂. Therefore, for physiologic monitoring at least one or two comparisons should be made between PetCO₂ and PaCO₂ before following PetCO₂. Once the difference between the two values is established, and providing the patient remains clinically stable, the PetCO₂ can be followed in lieu of the PaCO₂.

Below are some data from a stable 38-year-old man receiving mechanical ventilation. Assuming there is no major change in his clinical condition, how could you use PetCO₂ in weaning him from the ventilator?

Arterial blood gas and PetCO₂ data
FIO₂ .40
pH 7.41
PaCO₂ 56 mm Hg
PetCO₂ 35 - 40 mm Hg
PaO₂ 70 mm Hg
SaO₂ 93%
HCO₃^- 36 mEq/L

Note that his PetCO₂ is 16 to 21 mm Hg lower than arterial PCO₂. If he is stable, this difference can be added to the PetCO₂ during weaning to determine PaCO₂ at any time (帶 few mm Hg). If the patient remains clinically stable, one should not have to obtain an arterial sample to measure PaCO₂. Of course other parameters should also be followed, such as pulse oximetry, respiratory rate, etc.
2) Know the pitfalls that can affect interpretation of PetCO₂.
There are several potential pitfalls, but in the aggregate they are not common and should not dissuade one from using PetCO₂ as a proxy for PaCO₂ in intubated patients. While PetCO₂ is not as widely used as is pulse oximetry, and is therefore a less familiar measurement, properly used it can be a valuable monitoring aid. Discussed below are three potential pitfalls.

With cardiac arrest, shock, or any other state of low pulmonary blood flow, PetCO₂ may be significantly and variably lower than PaCO₂, too much to use as a proxy for PaCO₂. In such conditions it is best to directly measure PaCO₂. Interestingly, however, changes in PetCO₂ can be used as a guide to changes in cardiac output (see below).

PetCO₂ may be higher than PaCO₂. Although this has been reported, it is not common (Moorthy 1984; Stock 1995). It may occur transiently, as for example if a high inspired O₂ concentration displaces CO₂ from hemoglobin, or if there is a sudden surge in CO₂ production. Since ultimately exhaled air must include some air recently inhaled (anatomic dead space air with zero CO₂), the reversal of gradient should be transient. Probably the most common cause for 'reversal of gradient' is when PaCO₂ and PetCO₂ are not measured at exactly the same time. Another potential cause is the inherent variation of laboratory measurements, so that if each is accurate 帶 few mm Hg, on occasion PetCO₂ may on be higher. In any case, even when PetCO₂ is higher than PaCO₂, the difference is generally small, less than 5 mm Hg, and PetCO₂ can still be used to trend the PaCO₂ providing the patient is clinically stable. "Clinically stable" means no large changes in blood pressure, minute ventilation, temperature, or degree of physical activity (agitation, seizures, etc.) Within this framework, clinicians in the ICU know whether their patient is stable or not.

Another pitfall is not making a fresh correlation with PaCO₂ when there has been a significant clinical change. Sometimes PetCO₂ is followed for a prolonged period without blood gas correlation. Changes in the patient's metabolic condition, minute ventilation, and cardiopulmonary status can all affect the difference between end-tidal and arterial PCO₂. When there has been significant clinical or biochemical change, one should again make a correlation between PetCO₂ and PaCO₂.

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