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Careful
management is required to avoid the extremes of PaCO2 associated
with mechanical ventilatory support. Noninvasive TcPCO2 and
PetCO2 monitors
have been proposed as means to reduce blood sampling while providing near
continuous assessment. Currently, TcPCO2 is
commonly used as an adjunct PaCO2 or
PcapCO2,
while PetCO2 monitoring
is used less frequently.
Performing a TcPCO2 measurement
consists of creating a skin-electrode unit of increased local perfusion
by controlled hyperemia. The neonate’s thin epidermal layer has a small
metabolic CO2 production,
and diffusing CO2 molecules
change the pH of an electrolyte solution. However, poor perfusion can result
in overestimation due to reduced removal of the CO2 produced
locally. Many TcPCO2 devices
include a metabolism correction factor, and TcPCO2 is
often post-calibrated to PaCO2 or
PcapCO2.
Further, in preterm infants, heating can produce injury, which limits application
time at a single site.
Among the studies reviewed herein, Aliwalas
et al reported acceptable TcPCO2 bias
in infants of ≤ 28 weeks (w) of gestational age (GA), but warned of
between-patient variability; Tingay et al reported a small overestimation
bias in TcPCO2 that
did not change at higher PaCO2 during
transport; and Bernet-Buettiker et al reported that bias in estimation
of PaCO2 with
an ear TcPCO2 electrode
was small but was accompanied by variability between patients, and that
TcPCO2 was
tightly related to PcapCO2.
Although PcapCO2 may
not always reflect PaCO2,
this correlation is important because in preterm infants, invasive arterial
lines are not commonly in place beyond the critical period of respiratory
failure.
PetCO2 monitoring
is performed by placing infrared sensors mainstream, or by sidestream gas
sampling. Both the Aliwalas and Wu studies showed acceptable bias in PetCO2 with
respect to PaCO2 in
preterm infants after birth, and in older preterm and term infants using
low-flow sidestream and mainstream measurements, respectively. Both studies
showed important between-patient variability. However, they differed in
their recommendations. Further, Tingay found PetCO2 underestimated
PaCO2 during
ground transport.
PetCO2 is
more responsive to changes in PaCO2 than
TcPCO2.
However, preterm infants have a relatively large anatomical dead space
and PetCO2 depends
on tidal volume (VT)
size.1 PetCO2 is
also influenced by the arterial-alveolar gradient in infants with lung
disease. Use of this gradient as a longitudinal correlate to advancing
lung disease should be further examined.
Capnography may be informative in the preterm
infant but can present some shortcomings. Claure et al described capnographic
patterns associated with rebreathing, while others suggested patterns associated
with lung disease.2 New
mainstream PetCO2 sensors
have small dead space volumes (<1 ml), and — although this has
not been documented in small preterm infants — have been shown to
induce rebreathing in preterm infants in a manner similar to flow sensors
of comparable size. In larger preterm infants, a mainstream PetCO2 sensor
of 2 ml dead space increased TcPCO2 due
to rebreathing.3 High
sidestream sampling flows can provide inaccurate data by diluting the end
expiratory gas. Low-flow sidestream PetCO2 monitors
have only recently become available.
In comparison to the older literature, these
newer reports show improved bias with these monitoring techniques. However,
there is poor precision, as indicated by wide between-patient variability.
Under routine clinical conditions, the lack of certainty when a CO2 reading
is outside the desirable range is an important limitation. In most cases,
this finding requires repeat preparation and application procedures, and
often requires additional blood sampling.
The availability of monitoring data over
time is important, but the ability to trend such data has not been fully
exploited. Preterm infants undergo different phases of respiratory disease,
and an association has been found between extremes and fluctuations in
PaCO2 with
IVH.4 Detection
of ensuing changes that lead to extreme hypo- or hypercarbia may be important
triggers for intervention. In many centers, TcPCO2 monitoring
is started ahead of a blood sample to assure stability, and monitoring
often continues if the ventilatory support is changed. Although TcPCO2 or
PetCO2 readings
alone should not be used to adjust the support, they may give an early
warning of impaired ventilation.
The usefulness of CO2 monitoring
may be enhanced when combined with ventilation data. As Claure et al reported
(reviewed herein), in preterm infants, increases in PetCO2 and
TcPCO2 correlated
with rebreathing and occurred in parallel to a rise in total ventilation
due to increased spontaneous breathing. In prior studies, during volume-targeted
ventilation, a lower target VT led to an increase in TcPCO2 and
spontaneous ventilation while total ventilation remained unchanged.5 Further,
when preterm infants were switched from nasal CPAP to noninvasive pressure
support, there was a greater increase in ventilation than among preterm
infants that started with a higher baseline TcPCO2.6
Detection of exhaled CO2 has
been used as an adjunct to standard methods to assess proper intubation.
The report by Repetto et al suggests a means to more quickly detect improper
intubation. However, PetCO2 alone
may not be recommended, since conditions of reduced pulmonary circulation
could lead to poor CO2 detection
in spite of a correct intubation. A recent (2007) report recommends caution
due to the risk of false-positive color change in colorimetric CO2 detectors
in the presence of epinephrine, atropine, calfactant, and naloxone administered
through the endotracheal tube.7
The recent peer-reviewed reports summarized
herein, evaluating TcPCO2 and
PetCO2 usefulness
in estimating PaCO2,
indicate that TcPCO2,
in spite of its limitations, can be used as an acceptable estimate of CO2 measured
with arterial and capillary blood gases. These newer data also suggest
improved accuracy of PetCO2.
However, most of these reports, similar to prior ones, do not justify substitution
of PaCO2 analysis
with these non-invasive methods. Nonetheless, the use of continuous CO2 monitoring
as an adjunct to standard care has been shown beneficial in newborns in
respiratory failure and, perhaps more particularly, in labile preterm infants
by providing CO2 trending
and variability data.
References
| 1. |
Greer
KJ, Bowen WA, Krauss AN. End-tidal
CO2 as
a function of tidal volume in mechanically ventilated infants. Am
J Perinatol. 2003;20(8):447-451. |
 |
| 2. |
Hagerty
JJ, Kleinman ME, Zurakowski D, Lyons AC, Krauss B. Accuracy
of a new low-flow sidestream capnography technology in newborns: a
pilot study. J Perinatol. 2002;22(3):219-225. |
|
| 3. |
McEvedy
BA, McLeod ME, Kirpalani H, Volgyesi GA, Lerman J. End-tidal
carbon dioxide measurements in critically ill neonates: a comparison
of side-stream and mainstream capnometers. Can J Anesth. 1990(3);37:322-326. |
|
| 4. |
Fabres
J, Carlo WA, Phillips V, Howard G, Ambalavanan N. Both
extremes of arterial carbon dioxide pressure and the magnitude of fluctuations
in arterial carbon dioxide pressure are associated with severe intraventricular
hemorrhage in preterm infants. Pediatrics. 2007; 119(2):299-305. |
|
| 5. |
Herrera
CM, Gerhardt T, Everett R, Musante G, Thomas C, Bancalari E. Effects
of volume-guaranteed synchronized intermittent mandatory ventilation
in preterm infants recovering from respiratory failure. Pediatrics. 2002:110(3):529-533. |
|
| 6. |
Ali
N, Claure N, Alegria X, D’Ugard C, Organero R, Bancalari E. Effects
of non-invasive pressure support ventilation (NI-PSV) on ventilation
and respiratory effort in very low birth weight infants. Pediatr
Pulmonol. 2007; 42(8):704-710. |
|
| 7. |
Hughes
SM, Blake BL, Woods SL, Lehmann CU. False-positive
results on colorimetric carbon dioxide analysis in neonatal resuscitation:
potential for serious patient harm. J Perinatol. 2007;27(12):800-1. |
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