Resuscitation 83 (2012) 813 818
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Resuscitation
jo u rn al hom epage : www.elsevier.com/locate/resuscitation
Clinical paper
Factors complicating interpretation of capnography during advanced life support
in cardiac arrest A clinical retrospective study in 575 patients
Bård E. Heradstveita,", Kjetil Sundeb, Geir-Arne Sundea, Tore Wentzel-Larsenc,d,e, Jon-Kenneth Heltnea,f
a
Department of Anaesthesia and Intensive Care, Haukeland University Hospital, Bergen, Norway
b
Surgical Intensive Care Unit Ullevål, Department of Anaesthesiology, Division of Critical Care, Oslo University Hospital, Oslo, Norway
c
Centre for Clinical Research, Haukeland University Hospital, Bergen, Norway
d
Centre for Child and Adolescent Mental Health, Eastern and Southern Norway, Oslo, Norway
e
Norwegian Centre for Violence and Traumatic Stress Studies, Oslo, Norway
f
Department of Medical Sciences, University of Bergen, Bergen, Norway
a r t i c l e i n f o a b s t r a c t
Article history:
Background: End tidal carbon dioxide (ETCO2) monitoring during advanced life support (ALS) using
Received 21 November 2011
capnography, is recommended in the latest international guidelines. However, several factors might com-
Received in revised form 7 February 2012
plicate capnography interpretation during ALS. How the cause of cardiac arrest, initial rhythm, bystander
Accepted 15 February 2012
cardiopulmonary resuscitation (CPR) and time impact on the ETCO2 values are not completely clear. Thus,
we wanted to explore this in out-of-hospital cardiac arrested (OHCA) patients.
Methods: The study was carried out by the Emergency Medical Service of Haukeland University Hospital,
Keywords:
Bergen, Norway. All non-traumatic OHCAs treated by our service between January 2004 and December
Cardiac arrest
2009 were included. Capnography was routinely used in the study, and these data were retrospectively
Outcome
reviewed together with Utstein data and other clinical information.
Capnography
Results: Our service treated 918 OHCA patients, and capnography data were present in 575 patients.
Capnometry
Advanced life support Capnography distinguished well between patients with or without return of spontaneous circulation
Pulmonary embolism
(ROSC) for any initial rhythm and cause of the arrest (p < 0.001). Cardiac arrests with a respiratory cause
Prognostics
had significantly higher levels of ETCO2 compared to primary cardiac causes (p < 0.001). Bystander CPR
affected ETCO2-recordings, and the ETCO2 levels declined with time.
Conclusions: Capnography is a useful tool to optimise and individualise ALS in cardiac arrested patients.
Confounding factors including cause of cardiac arrest, initial rhythm, bystander CPR and time from cardiac
arrest until quantitative capnography had an impact on the ETCO2 values, thereby complicating and
limiting prognostic interpretation of capnography during ALS.
© 2012 Elsevier Ireland Ltd. All rights reserved.
1. Introduction placement. Monitoring of ETCO2 during cardiopulmonary resusci-
tation (CPR) was first described by Kalenda, who used ETCO2 as
The partial pressure of end tidal carbon dioxide (ETCO2) esti- a guide to the efficacy of CPR. A drop in ETCO2 was an indicator
mates alveolar carbon dioxide (CO2) tension, and reflects its for when to change the person providing chest compressions, due
production, transport to, and elimination from the lungs; hence it to inadequate compression efficacy.4 This was later followed by
generally reflects cardiac output.1,2 Alteration in one of these fac- studies reporting its use during CPR in experimental models.1,5 The
tors will affect the measurement. The technique was first described positive correlation between ETCO2 and outcome of cardiac arrest
during anaesthesia in the 1950s,3 in order to verify correct tube in patients has been well described in several studies,5 11 and a
significant increase in ETCO2 during CPR has been associated with
return of spontaneous circulation (ROSC).12,13 The 2010 guidelines
from European Resuscitation Council (ERC) now encourage the use

of capnography to guide CPR during Advanced Life Support (ALS).14
A Spanish translated version of the summary of this article appears as Appendix
in the final online version at doi:10.1016/j.resuscitation.2012.02.021.
Interpretation of ETCO2 during resuscitation from cardiac arrest
"
Corresponding author. Tel.: [phone]; fax: [phone].
is still challenging and has several pitfalls. Especially the cause of
E-mail addresses: [e-mail] (B.E. Heradstveit),
the arrest seems to have impact on the ETCO2, and recent studies
[e-mail] (K. Sunde), [e-mail]
have described higher ETCO2 in asphyxial arrests compared with
(G.-A. Sunde), [e-mail] (T. Wentzel-Larsen),
[e-mail] (J.-K. Heltne). arrests of cardiac aetiology.15,16 Further, the influence of bystander
0300-9572/$  see front matter © 2012 Elsevier Ireland Ltd. All rights reserved.
doi:10.1016/j.resuscitation.2012.02.021
814 B.E. Heradstveit et al. / Resuscitation 83 (2012) 813 818
CPR may impact on ETCO2 as well as variations over time, but this Netherlands). Recording of ETCO2-values were initiated upon the
has not been documented in clinical studies. arrival of the HEMS, and after placement of a secured airway, and
Thus, the aims of the study were to document levels of ETCO2 in were continuously observed by the treating anaesthesiologist.
patients with out-of-hospital cardiac arrest (OHCA). We hypothe-
sised that although capnography will give valuable feedback to the 2.5. Study design and data collection
ALS providers, initial heart rhythm, cause of the arrest, presence of
bystander CPR and time dependency will limit and complicate its All patients with ALS initiated non-traumatic cardiac arrest
interpretation. treated at our HEMS between January 2004 and December 2009
were included in the study. Pre-hospital data were recorded accord-
2. Material and methods ing to the Utstein model.21 Time records from the dispatch centre
supplemented ambulance records regarding response times. In
2.1. Ethics cases where the exact time of cardiac arrest was unknown, the
time was estimated based on the current information available.
This retrospective study was carried out at the Emergency Med- In patients with unknown arrest time of over 60 min, all response
ical Service (EMS), Haukeland University Hospital, Norway. The times were increased by 60 min.
Privacy Protection Supervisor approved the study and the Regional In patients with ROSC admitted to hospital, the cause of
Committees for Medical Research Ethics had no objections. The arrest was determined based on hospital records and all available
need of an informed consent from the patients or the families was information. Patients were classified in four categories; cardiac,
waived. respiratory, pulmonary embolism (PE) and unknown. Based on
the initial heart rhythm, patients were classified in three groups;
2.2. Organisation ventricular fibrillation/pulseless ventricular tachycardia (VF/VT),
asystole (AS) or pulseless electric activity (PEA). For those pro-
Our region has a population of approximately 470,000 peo- nounced dead at the scene, the anaesthesiologist stated the
ple (15,000 km2). Since 1988, the Helicopter Emergency Medical assumed cause of the arrest according to the Utstein-criteria. This
Service (HEMS) at Haukeland University Hospital has assisted the assumption was based on previous medical history, comparative
decentralised ambulances treating cardiac arrests. The paramedics information from family, witnesses and bystanders and all avail-
are trained in ALS and are yearly certified. The HEMS is served by able clinical or environmental data or signs.21 For example, a PE
an anaesthesiologist by helicopter or rapid response car. Regarding was decided as the cause of the arrest if a clinical suspicion of a deep
cardiac arrest, the emergency dispatch centre provides telephone vein thrombosis (presumably with initial AS or PEA) was present.
guided CPR to lay people if the patient is unconscious with abnor- Those patients with an unclear cause of the arrest were grouped as
mal breathing. In parallel, both the nearest ambulance and the  unknown in order to have as clean groups as possible.
HEMS are immediately despatched for initiation of ALS. Local Patients who gained ROSC before arrival of HEMS, patients
first-responders providing basic life support with defibrillation transported to the hospital with ongoing resuscitation (hypother-
(fire-fighters) are also despatched, who may arrive at the patient mic patients or other special circumstances), and patients with
before the ambulance/HEMS. unknown initial heart rhythm were excluded from the study.
ETCO2 were recorded after the HEMS crew arrived at the patient
2.3. ALS treatment as previously described. After one minute of normal ventilation,
the average, minimal and maximal values in the following 15 min
All patients in the present study were treated accord- of ALS (or until ROSC if it occurred before 15 min) were recorded
ing to the current international guidelines with our national manually by the anaesthesiologist. The Tidal Wave® capnograph
adjustments.17 19 Both the ambulances and the HEMS were has no automatic recording of data, and the average value dur-
equipped with Lifepak® 12 Defibrillator (Physio-Control Inc., Red- ing these 15 min was not an average in a strict sense, but was
mond, WA, USA), while the first responders used the fully automatic based on the anaesthesiologist s judgement. ETCO2 measurements
Lifepak CR® (Physio-Control). To permit continuous chest com- were then analysed based upon the initial heart rhythm, cause of
pressions, the patients had airways secured with a supraglottic the arrest and presence of bystander CPR, and further classified
laryngeal tube (LTS-D, VBM Medizintechnik GmbH, Germany) by depending on ROSC or no ROSC. Association of ETCO2 related to
the paramedics, or endotracheal tube by the anaesthesiologists. The bystander CPR, time of measurement, initial rhythms and cause
first responders used mouth-to-mouth ventilation with a pocket of the arrest were also classified. Other factors that may influence
mask. All patients were manually ventilated according to the cur- ETCO2 like epinephrine, quality of CPR and ventilation data were
rent guidelines.17 19 ALS drugs were used according to national not available.
guidelines,17 19 and no bicarbonate buffer was administered dur-
ing the study period. If ROSC did not occur and the resuscitation 2.6. Statistics
attempt was deemed to be futile by the attending doctor, ALS was
terminated on the scene. In the presence of profound hypothermia, All numbers are presented as mean Ä… SD. Continuous data were
or in other special circumstances, patients were transported with compared using independent samples t-tests. Linear regression
ongoing CPR to Haukeland University Hospital. analysis was used to determine the relationship of average mea-
surement on ETCO2 with bystander CPR, time of measurement,
2.4. Capnography use rhythm and cause of the arrest. Regression analysis used all obser-
vations where average ETCO2 was known. Since some covariates
The HEMS has routinely used waveform capnography in all intu- from different patients were missing, the regression analysis was
bated patients since 1999. Initially, the purpose of capnography was run using multiple imputation, a well described general procedure
to verify correct tracheal tube placement. However, in cardiac arrest to use as much information as possible.22 In this procedure, several
patients, we also used it as a surrogate marker of circulation.20 completed data sets (200 in our case) are constructed and analyses
ETCO2-monitoring was performed using a mainstream sensor, from these completed data sets are combined. Some continuous
using single beam, non-dispersive infrared absorption, ratio- covariates were entered nonlinearly, when deviations from a
metric measurements (Tidal Wave®, Philips Respironics, The linear relationship was suspected. A p-value <0.05 was considered
B.E. Heradstveit et al. / Resuscitation 83 (2012) 813 818 815
Table 2
significant. The R (The R Foundation for Statistical Computing,
Average ETCO2 (kPa) during CPR in patients with or without ROSC, regarding the
Vienna, Austria) packages rms and Hmisc were used for regression
cause of the arrest.
analysis, imputation and assessment of which covariates that
Cause Overall ETCO2, ROSC, No-ROSC, p-Valuea
should be entered nonlinearly. SPSS version 17-18 (IBM SPSS,
mean Ä… SD mean Ä… SD mean Ä… SD
Somers, NY, USA) was used for presentation of the data and for
Cardiac 2.8 Ä… 1.3 3.4 Ä… 1.2 2.4 Ä… 1.2 <0.001
other statistical analyses.
Respiratory 3.5 Ä… 2.2 4.5 Ä… 2.2 2.3 Ä… 1.5 <0.001
Pulmonary embolism 1.7 Ä… 1.1 2.2 Ä… 1.0 0.9 Ä… 0.5 0.023
Unknown/Other 2.0 Ä… 1.2 2.7 Ä… 1.0 1.3 Ä… 1.1 <0.001
3. Results
a
Contrast between ROSC and no-ROSC using independent samples t-test.
A total of 918 patients received ALS after OHCA during the study
period. Patient flow chart with included and excluded patients is
Table 3
shown in Fig. 1. Of 724 eligible patients, ETCO2 recordings were
ETCO2 (kPa) in patients presenting asystole with respiratory and cardiac causes to
the arrest.
present in 575 (82%) patients who were included in the final study.
Patients with ETCO2 measurements did not differ from the miss-
ETCO2 Cardiac cause, Respiratory cause, p-Valuea
ing/excluded group regarding gender, age, initial heart rhythm,
mean Ä… SD mean Ä… SD
response times or outcome. All baseline characteristics are pre-
Average 2.3 Ä… 1.4 3.5 Ä… 2.3 <0.001
sented in Table 1. Data only relates to patients in whom a clear
Min. 1.5 Ä… 1.0 2.4 Ä… 2.0 <0.001
airway and controlled ventilation were established and confirmed Max. 3.4 Ä… 2.3 5.1 Ä… 3.5 <0.001
by capnography before data collection started. Additionally all a
Contrast between cardiac and respiratory causes using independent samples
tube placements were confirmed by signs of effective ventilation.
t-test.
Among the 575 included, 232 patients (40%) gained ROSC and were
transported to the hospital. For all initial heart rhythms and dif-
ferent causes, ETCO2 were significantly higher in those achieving
ROSC compared to those not achieving ROSC (Tables 2 and 3).
3.1. ETCO2 and different causes
98 ROSC before
918
HEMS arrival
HEMS arrival
ALS after OCHA
There were significant differences in ETCO2 depending on the
38 Transport
cause of the arrest (p < 0.001) (Table 2), with respiratory arrests hav-
with ALS
ing increased levels compared to primary cardiac caused arrests.
194
194
Excluded Furthermore, a significantly lower level of ETCO2 was present in
19 Traumatic
arrests
patients with PE compared to patients with respiratory and car-
diac causes, regardless of ROSC or not (Table 2). Patients with ROSC
724
11 Special
and PE, had similar values as patients without ROSC and all other
Eligible patients
circumstances
causes (and actually tended to be even lower) (Table 2). In patients
with initial asystole, the minimum, maximum and average ETCO2
149
were characteristically higher among those patients with respira-
ETCO2 not recorded
tory compared to cardiac causes (p < 0.001) (Table 3). More patients
gained ROSC in the respiratory compared to the cardiac group, 49%
575
vs. 15%.
Included patients
3.2. ETCO2 and different initial rhythms
Fig. 1. Included and excluded patients  flow chart.
Initial VF/VT was present in 198 patients (34%), AS in 266
patients (46%), and PEA in 111 (19%) patients (Table 4). Regression
Table 1
Baseline characteristics in study population (n = 575).
analysis did reveal differences in the ETCO2 with respect to initial
rhythms (p = 0.004). Within each rhythm, there were significant
Variable Mean Ä… SD
contrasts between patients with and without ROSC (Table 4). In
Age (year) 60.7 Ä… 17.8
the presence of ROSC, patients with initial asystole had the highest
Female/male 145/430
ETCO2, and PEA the lowest, whereas in absence of ROSC, patients
Witnessed 414 (72%)
Bystander CPR 438 (76%) with initial VF/VT had the highest levels (Table 4).
Arrest-CPR (min) 8.6 Ä… 15.4
Arrest-ACLS (min) 14.7 Ä… 16.9
Table 4
Arrest-CO2 recording (min) 22.5 Ä… 17.5
Average ETCO2 (kPa) during CPR in patients with or without ROSC, regarding the
Admitted hospital with ROSC 232 (40%)
initial heart rhythm.
Any ROSC (%) 286 (50%)
Termination of resuscitation (min)a 43.3 Ä… 22.3
Initial heart rhythm ETCO2 ROSC, No-ROSC, p-Valuea
Cause of the arrest
mean Ä… SD mean Ä… SD
Cardiac 336 (58%)
Respiratory 117 (20%) VF/VT (n = 198) Average 3.4 Ä… 1.1 2.8 Ä… 1.2 <0.001
Min. 2.6 Ä… 1.0 1.8 Ä… 0.9 <0.001
Pulmonary embolism 12 (2%)
Max. 5.1 Ä… 2.2 4.3 Ä… 1.9 0.009
Unknown/other 110 (19%)
Initial rhythm AS (n = 266) Average 4.1 Ä… 2.1 2.0 Ä… 1.3 <0.001
Min. 2.9 Ä… 1.8 1.4 Ä… 1.0 <0.001
Ventricular fibrillation 195 (34%)
Max. 5.9 Ä… 3.3 3.0 Ä… 2.1 <0.001
Ventricular tachycardia 3 (1%)
Asystole 266 (46%) PEA (n = 111) Average 3.1 Ä… 1.5 2.2 Ä… 1.3 0.001
Min. 2.2 Ä… 1.4 1.3 Ä… 1.0 <0.001
Pulseless electrical activity 111 (19%)
Max. 4.4 Ä… 2.5 3.1 Ä… 1.9 0.003
CPR, cardio pulmonary resuscitation; ACLS, advanced cardiac life support.
a
a
Contrast between ROSC and No-ROSC using independent samples t-test.
Time between arrest and termination of resuscitation.
816 B.E. Heradstveit et al. / Resuscitation 83 (2012) 813 818
(a) (b)
4.5
4
4.0
3
3.5
2
3.0
1
2.5
0
2.0
0 10 20 30 40 50 60
0 20 40 60 80
Time of onset CPR (min)
Time of measurement (min)
Fig. 2. (a) End tidal CO2 and time of onset bystander CPR after the arrest, adjusted for time of measurement, initial rhythms and cause of the arrest (estimated values with
95% CI). (b) Measurement of end tidal CO2 at different times after the arrest, adjusted for bystander CPR, initial rhythms and cause of the arrest (estimated values with 95%
CI).
3.3. ETCO2 and bystander CPR ETCO2. First, we found increased ETCO2 with onset of CPR within
the first four minutes after the cardiac arrest. Thereafter, the ETCO2
The impact of bystander CPR affected the ETCO2 significantly seemed to decrease with delayed onset of CPR beyond four minutes.
(p = 0.003). Initiation of bystander CPR within four minutes after the Survival after cardiac arrest depends on time from arrest until CPR
cardiac arrest resulted in higher values of ETCO2 while CPR started and successful defibrillation,17,18,23 and thereby reduces with later
later resulted in lower values (Fig. 2a). Over time, the trend was onset of bystander CPR.24,25 Besides the hypoxic component, this
decreasing values of ETCO2. can also be related to development of stone heart with thickening
of the myocardium and decrease in left ventricular volume. This
3.4. ETCO2 and time of measurement has been demonstrated in untreated cardiac arrest in pigs.26 Our
data confirm that delayed initiation of CPR leads to lower ETCO2.
The average ETCO2 was significantly affected by the time of This might be explained by less effective chest compressions due
recording after the arrest (p = 0.037), and the values declined with to development of stone heart. The reported delay between time
delayed measurement (Fig. 2b). of arrest and ETCO2-recording may seem long, but can partly be
explained by the fact that also unwitnessed arrests were included.
4. Discussion An interesting result in our study was the low levels of ETCO2
in patients with PE. ETCO2 in PE patients are characteristically
In the present study we have documented that several factors lower because of diminished pulmonary perfusion and increased
complicate the interpretation of ETCO2 during ALS. Although ETCO2 alveolar dead space, and consequently decreased CO2 elimination
differs well between patients with and without ROSC, there is no capability.27 28 Low ETCO2 and clinical suspicion of PE, might there-
clear generalised cut-off value determining whether ROSC will be fore be an indication for trombolysis during ongoing ALS, since
achieved or not. Several confounding factors such as cause of the individually adjusted treatment with fibrinolytics for these patients
arrest, initial rhythm, bystander CPR and changes over time from previously can be successful.29 Only 12/575 patients in the present
arrest until ETCO2 recordings seem to influence this. study had a PE confirmed as the cause of their arrest. This is far
Patients with respiratory causes and initial AS had in general less than previously reported,30 and emphasizes the fact that PE is
higher levels of ETCO2 than those with a primary cardiac cause. difficult to diagnose in cardiac arrest. Low ETCO2 combined with a
Similarly, Grmec et al. have previously reported higher ETCO2 non-shockable rhythm can be suspectible of PE.
immediately after intubation in patients with asphyxial compared In clinical studies, ETCO2 > 2.4 kPa after 20 min has been shown
to primary cardiac arrests.15 Lah et al. from the same group demon- to predict ROSC, and values <1.3 kPa have been associated with no
strated that this difference normalised within three to five minutes ROSC.15,16 Our data demonstrates that such cut-off values must
after initiation of ALS.16 They also reported that the initial ETCO2 be used with caution. Too many confounding factors impact on
could not be used as a prognostic factor due to these aetiology the actual ETCO2. Importantly, cut-off values from observational
differences.16 We speculate that capnography for CPR guidance studies are only based on the actual dataset, and cannot be gen-
during ALS is easier to interpret in patients with cardiac causes eralised to other systems. Strict use of cut-off values in patient
than in patients with asphyxial arrests. treatment will lead to treatment withdrawal based on self-fulfilling
The higher ETCO2 in patients with asphyxial arrests are presum- prophecy. Furthermore, the compression site on sternum might
ably not due to better cardiac output, but due to CO2 accumulation presumably affect haemodynamics and thereby cardiac output and
in the tissue and venous blood due to asphyxia and absence of ETCO2, as recently shown in a clinical study.31 This fits well with
ventilation.15 This assumption introduces the possibility for con- our impression that levels of ETCO2 in each patient varied depend-
founding in the presence of bystander CPR, which affected the ing on the rescuer performing chest compressions. Thus, since
Average endtidal CO2 (kPa)
Average endtidal CO2 (kPa)
B.E. Heradstveit et al. / Resuscitation 83 (2012) 813 818 817
both compression site and quality of chest compressions impact References
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