The Effects of Lung Recruitment on the Phase III Slope

of Volumetric Capnography in Morbidly Obese Patients

Stephan H. Böhm, MD* BACKGROUND: In this study, we analyzed the effect of the alveolar recruitment
strategy (ARS) and positive end-expiratory pressure (PEEP) titration on Phase III
Stefan Maisch, MD* slope (SIII) of volumetric capnography (VC) in morbidly obese patients.
METHODS: Eleven anesthetized morbidly obese patients were studied. Lungs
were ventilated with tidal volumes of 10 mL 䡠 kg⫺1, respiratory rates of 12–14
Alexandra von Sandersleben, MD* bpm, inspiration:expiration ratio of 1:2, and Fio2 of 0.4. ARS was performed by
increasing PEEP in steps of five from 0 end-expiratory pressure to 15 cm H2O.
Oliver Thamm, MD*† During lung recruitment, plateau pressure was limited to 50 cm H2O, whereas tidal
volume was increased to the ventilator’s maximum value of 1400 mL, and PEEP
Isabel Passoni, PhD‡ was increased to 20 cm H2O for 2 min. Thereafter, PEEP was reduced in steps of 5
cm H2O, from 15 to 0. VC, arterial blood gases, and lung mechanics data were
Jorge Martinez Arca, MSc‡ determined for each PEEP step.
RESULTS: SIII decreased from 0.014 ⫾ 0.006 to 0.005 ⫾ 0.005 mm Hg/mL when 0
Gerardo Tusman, MD§ end-expiratory pressure was compared against 15 cm H2O of PEEP after ARS
(15ARS, P ⬍ 0.05). This decrement in SIII was accompanied by increases in Pao2
(27%, P ⬍ 0.002) and compliance (32%, P ⬍ 0.001), whereas Paco2 decreased by 8%
(P ⬍ 0.038) when comparing values before and after ARS. A good prediction of the
lung recruitment effect by SIII was derived from the receiver operating character-
istic curve analysis (area under the curve of 0.81, sensitivity of 0.75, and specificity
of 0.74; P ⬍ 0.001).
CONCLUSION: The SIII in VC was useful to detect the optimal level of PEEP after lung
recruitment in anesthetized morbidly obese patients.
(Anesth Analg 2009;109:151–9)

D uring general anesthesia, morbidly obese pa-

tients develop considerable amounts of atelectasis
Noninvasive means of monitoring the effects of
ARS and positive end-expiratory pressure (PEEP) on
in dependent lung areas, much more than normal lung function are needed at the bedside. In theory,
weight patients.1–5 Such “compression atelectasis” has real-time and breath-by-breath respiratory variables
a known negative effect on gas exchange and lung related to the collapse-recruitment physiology of the
mechanics.5–7 Lung recruitment maneuvers (RMs), lungs could help in performing RMs more safely and in
one of which is the alveolar recruitment strategy determining the optimal level of PEEP for each patient.
(ARS), are ventilatory strategies which were devel- Our group described the effect of lung recruitment
oped to reverse anesthesia-induced lung collapse.7,8 on volumetric capnography (VC) curves (the curve
The positive effects of lung recruitment on pulmonary formed by the expired CO2 concentration plotted
physiology have been demonstrated in anesthetized against expired tidal volume [VT]) not only in anes-
patients of varying body masses.7,9,10 thetized patients11,12 but also in an experimental
model of acute lung injury.13 Dead space and several
VC-derived variables changed favorably after lung
From the *Clinic of Anesthesiology, University Hospital, recruitment and at adequate levels of PEEP, as could
Hamburg-Eppendorf, Hamburg, Germany; †currently at Clinic of be documented by improved arterial oxygenation,
Plastic and Reconstructive Surgery, Burn Care Center, Hospital improved respiratory mechanics, and reduced areas of
Cologne-Merheim, University of Witten/Herdecke, Germany;
‡Department of Bioengineering, University of Mar del Plata, Argen- collapse on computed tomography images.
tina; and §Department of Anesthesiology, Hospital Privado de Of all VC-derived variables, the slope of Phase III
Comunidad, Mar del Plata, Argentina. (SIII) is of particular physiological interest because it is
Accepted for publication November 13, 2008. an indicator of both CO2 transport within the airways
Supported by the Clinic of Anesthesiology, University Hospital (ventilation) and the CO2 delivered to the pulmonary
Hamburg-Eppendorf, Germany.
capillaries (perfusion).14 –18 Thus, this variable has also
Address correspondence to Stephan H. Böhm, MD, CSEM
Centre Suisse d’Electronique et de Microtechnique SA, Research been associated with the ventilation/perfusion (V/Q)
Centre for Nanomedicine, Medical Sensors, Schulstr. 1, CH-7302 relationship in the lungs.19 –21 There is clear evidence
Landquart, Switzerland. Address e-mail to shb@csem.ch. suggesting that any increment in SIII is a reflection of
Copyright © 2009 International Anesthesia Research Society impaired V/Q matching, whereas decrements indicate
DOI: 10.1213/ane.0b013e31819bcbb5
its improvement.22–26 Considering the above concepts,

Vol. 109, No. 1, July 2009 151

it seems that SIII is an on-line breath-by-breath vari- mg 䡠 kg⫺1, sufentanyl 0.1– 0.5 ␮g 䡠 kg⫺1, and succinyl-
able that could provide valuable information about choline 1 mg 䡠 kg⫺1. Balanced anesthesia was main-
the effects of RMs and PEEP on lung function in tained with sevoflurane plus IV boluses of 0.1– 0.5
anesthetized patients. ␮g 䡠 kg⫺1 sufentanyl and 0.075– 0.15 mg 䡠 kg⫺1 rocuro-
The goal of this observational study was twofold: 1) nium as needed. After tracheal intubation, the lungs
to test the role of SIII as a noninvasive real-time were ventilated using a Cicero EM (Dräger, Lübeck,
VC-derived variable for monitoring the effects of ARS Germany) in a volume-controlled mode with a VT of
and PEEP during a titration process and 2) to identify 10 mL 䡠 kg⫺1 lean body weight, a respiratory rate of
other variables derived from VC that could describe the 10 –14 bpm, an inspiratory-to-expiratory ratio of 1:2,
lung collapse-recruitment phenomenon in a noninvasive Fio2 of 0.4, and initially without PEEP (ZEEP). Minute
way. This protocol was performed in morbidly obese ventilation was adjusted by changing the respiratory
patients, a good clinical model of anesthesia-induced rate to maintain Paco2 within the normal range.
atelectasis,4 –5 which magnifies the typical changes in
pulmonary physiology because of lung collapse rou- VC and Lung Mechanics Variables
tinely seen in normal weight patients. For VC and respiratory mechanics measurements, we
used the respiratory profile monitor COSMOplus (Re-
spironics, Wallingford, CT). Its Capnostat威 sensor was
METHODS inserted between the endotracheal tube and Y-piece of
The study was approved by the local ethics commit- the ventilator. Data were continuously recorded using
tee of the University Hospital Hamburg-Eppendorf, Ger- the software Aplus (Novametrix, Wallinford, CT).
many. We examined 11 patients with body mass indexes The VC is traditionally divided into three phases.21
(BMI, weight/height2) ⬎40 kg 䡠 m⫺2 undergoing bariat- Phase I represents expired gases free of CO2. Phase II
ric laparoscopic surgery. After obtaining written in- is formed by the rapid increase in expired CO2 coming
formed consent, we enrolled only patients without from lung acini, whereas Phase III consists of gas
known cardiovascular and pulmonary diseases. resident in the alveolar space. The maximum slope of
In the induction room, patients lay in the supine Phase II (SII) was calculated by using linear regression
position. We inserted a 20 G radial artery catheter from the data points between the start of Phase II and
under local anesthesia. After induction of anesthesia, a 40% of the expired volume. SIII was calculated by linear
central venous catheter (Certofix-Trio-Set 5730, B. regression, considering all data points between 40% and
Braun, Melsungen, Germany) and a pulmonary artery 80% of VC. Both slopes are expressed as mm Hg/mL.
catheter (Swan-Ganz, Baxter, Irvine, CA) were intro- VDaw is the airway dead space as determined by
duced into the same internal jugular vein. Cardiac Fowler method.28 Physiologic dead space (VDphys)
output was measured in triplicate using the thermodi- was calculated using Enghoff modification of Bohr
lution method. Pulmonary vascular pressures and all formula29 as:
other measurements were performed at the end of
expiration. The catheters for rather extensive hemody- VDphys ⴝ (Paco2 ⴚ Peco2)/Paco2 ⴛ VT
namic monitoring were inserted immediately after
where PeCO2 is the mean expired partial pressure of
induction of anesthesia but served primarily the needs
CO2.
of a subsequent intraoperative study in the same
Next, the ratio of physiological VD/VT was calcu-
patients, investigating the effects of capnoperitoneum
lated as:
and PEEP on heart and lung function. In the current
study, we analyzed the data of only a subset of 11 of VD/VT ⴝ VDphys/VT
19 morbidly obese patients enrolled in the original
study (in preparation for publication) who had com- VDalv was computed by subtracting VDaw from VDphys.
plete sets of CO2 and reference data. The ratio of alveolar dead space to alveolar VT
The current study protocol was completed before (VDalv/VTalv) was then obtained by dividing VDalv by
moving the patient into the operating room where the the alveolar part of the tidal volume (VTalv ⫽ VT ⫺ VDaw).
surgery took place. The arterial to end-tidal partial pressure of CO2 (Pa-ETco2)
One liter of colloidal solution (RheoHAES, B. was also calculated.
Braun, Melsungen, Germany) was administered for Dynamic compliance (CDYN) was calculated as VT
intravascular volume expansion before induction of divided by ␦ pressure (plateau pressure minus total
anesthesia. IV saline solution infusion was kept at 5 PEEP). Expiratory airway resistance (RAW) was com-
mL 䡠 kg⫺1 (lean body weight) during the study. This puted as ␦ pressure divided by expiratory flow. Peak
preload optimization was necessary to minimize po- expiratory flow (PEF) was defined as the maximum
tential hemodynamic impairment because of high value of expiratory flow of a breath. The expiratory
airway pressures if patients presented with a relative time constant (ETC) was calculated as the product of
or inadvertent hypovolemia.27 CDYN and RAW.
Oxygen (100%) was administered for 3 min. There- Two new variables derived from the VC and lung
after, anesthesia was induced with etomidate 0.15– 0.3 mechanics were developed and tested. These variables

152 Volumetric Capnography in Obese Patients ANESTHESIA & ANALGESIA

are related to the CO2 elimination kinetic during
expiration:
1. The time-constant for eliminating CO2 (Tau-CO2),
represented as the amount of CO2 eliminated dur-
ing 1 expiratory time-constant.30 It is calculated as:

Tau-CO2 ⴝ C DYN ⴛ R AW ⴛ VTCO2,br

where VTCO2,br is the amount of CO2 eliminated

in 1 breath as derived from an integration of the
area under the CO2 versus volume curve.
2. Because of the fact that ARS, in conjunction with
adequate levels of PEEP, increased CDYN, while
at the same time decreasing RAW and having
some transient effects on VTCO2,br (at least if
observation intervals are shorter than the time
for complete equilibration, as in this study), we
modified the initial Tau-CO2 formula by placing
RAW in the denominator of the above equation.
This new variable derived from the “modified”
Tau-CO2 formula was referred to as CO2flow:
Figure 1. Symmetrical study protocol showing phases of
CO2flow ⴝ C DYN ⴛ VTCO2,br/R AW increasing and decreasing levels of positive end-expiratory
pressure (PEEP) separated by an alveolar recruitment strat-
egy (ARS). Numbers on x axis show the level of PEEP
Protocol applied during each 3-min period, as depicted by a rect-
PEEP titration and alveolar recruitment were initi- angle. IndexARS ⫽ level of PEEP after an alveolar recruit-
ated before the start of surgery. ARS was slightly ment strategy. * ⫽ time points at which data were collected.
modified from our previous publications7,9,11 to meet
H2O. Each level of PEEP before and after ARS was
the higher pressure requirements of this study popu-
maintained for exactly 3 min.
lation. Eichenberger et al.4 showed that morbidly obese
Data for analyzing VC and respiratory mechanics
patients developed more atelectatic areas in the periop-
were taken starting at the third minute, and a mean
erative period when compared with patients of nor-
value for each variable was calculated from approxi-
mal weight. Additionally, Rothen et al.8 observed that
mately 12 breaths. Hemodynamic values and arterial
1 of 16 patients still had atelectasis after lung recruit-
blood for blood gas analysis were taken at the end of
ment with 40 cm H2O of Paw. These authors found a
the third minute, just before changing PEEP. Samples
close correlation between the Paw needed to reexpand
were processed without delay by the blood gas ana-
the atelectatic lungs and the BMI. Thus, we reasoned
lyzer ABL System 615 (Radiometer, Copenhagen,
that the pressures needed to open all collapsed lung
Denmark). From previous protocols, it was known
units in morbidly obese patients with otherwise
that during the actual RM no steady-state condition
healthy lungs would be higher than those in normal
would be reached. Therefore, we elected not to take
weight patients because of the decreased transpulmo-
blood samples for gas analysis at this step.
nary pressures caused by additional abdominal and
thoracic amounts of fatty tissue.3,4,9 The lung’s open- Analysis of the Recruitment Effect
ing pressures were assumed to be around 50 cm H2O Pao2, shunt, CDYN, and the study of CO2 kinetics
of plateau pressure. Therefore, airway pressures were (Paco2 or dead space) are known markers of the
limited to 50 cm H2O, whereas both PEEP and VTs effects of lung recruitment.13,31,32 Therefore, we used
were increased to the machine’s maximum values of them to determine the physiological efficacy of PEEP
20 cm H2O and 1400 mL, respectively. However, applied before and after the ARS. Shunt was calculated
because of the pressure limitation the latter value according to the following formula and assumptions:
would never be reached. The high-flow rate together Because the CvO2 needed for the formula CO ⫽
with the pressure limitation resulted in a decelerating VO2/CaO2 ⫺ CvO2 could not be obtained during our
flow pattern even with the ventilator’s volume- short study periods, the assumption that VO2 ⫽ VCO2 ⫻
controlled mode of ventilation. As shown in Figure 1, RQ (0.85) was used instead. Thus, CvO2 ⫽ VO2/CO ⫺
PEEP was initially increased from 0 to 15 cm H2O in CaO2, now allowing the classic shunt formula to be
steps of 5 cm H2O. applied as: Shunt ⫽ CcO2 ⫺ CaO2/CcO2 ⫺ CvO2.
After 2 min at maximal airway pressures, previous
ventilator settings were applied, starting at a PEEP of Statistical Analysis
15 cm H2O (15ARS), which was then decreased in steps Statistical analysis was performed using the pro-
of 5 cm H2O to 10 (10ARS), 5 (5ARS), and 0 (0ARS) cm gram SPSS version 15.0 (SPSS, Chicago, IL). Variables
Vol. 109, No. 1, July 2009 © 2009 International Anesthesia Research Society 153
were analyzed by one-way analysis of variance and was prolonged and reached significant differences
Tukey honestly significant difference criterion. Results after ARS when compared with ventilation without
are presented as mean and standard deviation. Sensi- recruitment. All values of PEF after lung recruitment
tivity and specificity of a 20% decrease in SIII to detect were lower than those before ARS.
a lung recruitment effect were evaluated by construct- Despite the fact that VTCO2,br did not change
ing the receiver operating characteristics curve significantly throughout the entire protocol, Tau-CO2
(ROC)33 (Area Under the Curve, AUC ⫽ 0.7, P ⬍ 0.05). showed consistent increments after lung recruitment,
An AUC of 0.5 would mean no predictive value at all describing a combined positive effect of ARS on lung
(pure chance), whereas an AUC of 1 signifies the best mechanics and CO2 elimination (Fig. 3). CO2flow, how-
possible prediction. A P ⬍ 0.05 was considered signifi- ever, described the impact of lung recruitment in a
cant. We considered changes in Pao2 ⱖ 20%, in CDYN ⱖ more pronounced way, showing a pattern similar to
40%, in VDalv/VTalv ⱕ20%, and in Paco2 ⱕ 5% as cutoffs Pao2 and CDYN in Figure 2.
for defining a recruitment effect. Hemodynamic data are provided in Table 4. Mean
systemic and pulmonary arterial pressures, cardiac
filling pressures, and cardiac output remained stable
RESULTS and within normal ranges throughout the entire pro-
Nine men and two women morbidly obese patients tocol while shunt presented an almost exact mirror
undergoing anesthesia for laparoscopic gastric banding image of Pao2. No complications occurred.
were studied. The mean patient age was 38 ⫾ 6 yr. Mean
height was 1.68 ⫾ 0.1 m and mean weight was 144 ⫾ 29 DISCUSSION
kg, corresponding to a mean BMI (weight/height2) of This study in morbidly obese anesthetized patients
51 ⫾ 10 kg/m2. showed that an ARS in conjunction with adequate
The effect of lung recruitment on SIII is presented in levels of PEEP decreased the SIII of the VC curve,
Figure 2. SIII showed significantly higher values before reflecting improved global lung function. This sus-
than after lung recruitment at 15ARS. SIII increased pected improvement was confirmed by simulta-
again when PEEP decreased to values below 15 cm neous and positive changes in arterial oxygenation
H2O. Regression analysis showed that SIII together and CO2, shunt, dead space, and respiratory mechan-
with SII and VTCO2,br predict the real Paco2 value ics, which were most pronounced after the lung
according to the following equation: Paco2 ⫽ 719.2 ⫻ recruitment.7,9,13,31,32
SIII ⫹ 23.74 ⫻ SII ⫹ 1.039 ⫻ VTCO2,br ⫹ 4.107 (R2 ⫽ Two main clinical implications can be deduced
0.84, P ⬍ 0.05). No good correlations were found from this study. First, the association between VC-
between SIII and Pao2, VDalv/VTalv, or CDYN at a high derived variables and the collapse-recruitment physi-
variability among these 11 patients (Fig. 2). However, ology suggests that monitoring these variables on a
the receiver operating characteristic analysis showed breath-by-breath basis could guide RMs and identify
good sensitivity and specificity of SIII to detect lung the appropriate level of PEEP in mechanically venti-
recruitment effects (Table 1). lated patients. Second, lung recruitment together with
Arterial oxygenation improved with lung recruit- a PEEP of 15 cm H2O showed the best effect on
ment (Fig. 2). Pao2 values after lung recruitment were respiratory physiology in these morbidly obese pa-
higher than those at the same level of PEEP before tients undergoing general anesthesia. Thus, it can be
ARS. The highest Pao2 value was observed at 15ARS reasoned that it was the active lung recruitment that
and was statistically different from baseline (ZEEP). induced these improvements in lung function while
Dead space variables during the protocol are PEEP maintained them. This same level of PEEP
shown in Table 2. In general, the inefficiency of without lung recruitment did not show the same
ventilation decreased with lung recruitment and ven- physiological effect in morbidly obese patients, as also
tilation at 15 cm H2O of PEEP. VDalv, VDalv/VTalv, observed by Santesson34 and Erlandsson et al.35 Their
and Pa-ETco2 showed significant changes after ARS results showed some similarities with our data, and 15
compared with ZEEP ventilation before recruitment. cm H2O of PEEP without prior lung recruitment had
Both Pao2 and dead space variables declined when an inconsistent and limited impact on cardiopulmo-
PEEP was decreased stepwise from 15ARS to 0ARS, nary physiology.
suggesting a derecruitment of previously recruited
lung areas. Effects of Lung Recruitment on SIII
Data on ventilation and lung mechanics are shown The origin of SIII of the expired CO2-volume curve
in Table 3. Lung mechanics improved after ARS. CDYN has been investigated over the last decades.12–17 From
increased with PEEP and lung recruitment, reaching the ventilatory point of view, SIII is caused by two kinds
the highest values at 15ARS. RAW decreased with of inhomogeneities in the transport of gas within the
increasing PEEP, reaching the lowest values at 15 cm airways: “convection-dependent inhomogeneity,” which
H2O both before and after recruitment. Similar to gas is a large-scale inhomogeneity within and between dif-
exchange, CDYN and RAW showed a progressive dete- ferent ventilated lung zones mediated by a convective
rioration when PEEP went from 15ARS to 0ARS. ETC transport of CO2, and “diffusion-convection-dependent

154 Volumetric Capnography in Obese Patients ANESTHESIA & ANALGESIA

Figure 2. Individual responses to the lung recruitment maneuver for Pao2, Paco2, dead space (VD)alv/tidal volume (VT)alv,
the slope of Phase III (SIII), dynamic compliance (CDYN) and CO2flow. ⌬ ⫽ differences in the mean values summarizing all
conditions before and after the alveolar recruitment strategy (ARS) are presented as percentage values above each graphic.
A P value ⬍0.05 was considered significant. * ⫽ significant difference between 0 positive end-expiratory pressure (PEEP) and
15ARS.

inhomogeneity,” which is caused by the interaction of From the perfusion point of view, SIII is caused by the
both convective and diffusive CO2 transport within continuous elimination of CO2 molecules through the
asymmetric small airways.36 As an example, asthmatic alveolar-capillary membrane, as this gas is delivered
and emphysematous patients show high SIII because of to the lungs by the pulmonary blood flow. It was
the known increases in RAW, which also affect the CO2 postulated that the effect of lung perfusion on SIII is
transport within the lungs.22,23 responsible for approximately 10% of the sloping.18

Vol. 109, No. 1, July 2009 © 2009 International Anesthesia Research Society 155
Table 1. Receiver Operating Characteristic (ROC) Analysis
Variable Cutoff (%) AUC Sensitivity Specificity P
Pao2 ⬎20 0.81 0.75 0.74 ⬍0.001
CDYN ⬎40 0.84 0.78 0.76 ⬍0.001
VDalv/VTalv ⬍20 0.67 0.65 0.62 ⬍0.001
Paco2 ⬍5 0.81 0.67 0.66 ⬍0.001
Prediction of the recruitment effect by a decrease in the slope of Phase III (SIII) was tested by calculating the area under the curve (AUC) of the ROC curve comparing zero positive end-expiratory
pressure (PEEP) (ZEEP) before lung recruitment against the value at 15 cm H2O after it. We used a % change in SIII ⱕ20% and a cutoff in the partial pressure of oxygen (PaO2) ⱖ20%, in dynamic
compliance (CDYN) ⬎40%, in the ratio of alveolar dead space to alveolar tidal volume (VDalv/VTalv) ⱕ20% and in the partial pressure of carbon dioxide (PaCO2) ⱕ5%. A P value ⬍0.05 was
considered statistically significant.

Table 2. Data on Dead Space and Other Volumetric Capnography Variables During the Protocol
PEEP (cm H2O) ARS

0 5 10 15 15ARS 10ARS 5ARS 0ARS

VDaw (mL) 113 ⫾ 13* 116 ⫾ 13 124 ⫾ 12 135 ⫾ 15 117 ⫾ 16 107 ⫾ 15* 105 ⫾ 15* 107 ⫾ 13*
VDalv (mL) 131 ⫾ 46† 122 ⫾ 43 110 ⫾ 38 101 ⫾ 44 84 ⫾ 46 99 ⫾ 45 106 ⫾ 42 110 ⫾ 48
VDphys (mL) 244 ⫾ 48 239 ⫾ 43 234 ⫾ 42 236 ⫾ 48 201 ⫾ 48 206 ⫾ 50 211 ⫾ 46 216 ⫾ 51
VD/VT 0.39 ⫾ 0.08 0.39 ⫾ 0.07 0.38 ⫾ 0.06 0.38 ⫾ 0.07 0.32 ⫾ 0.06 0.33 ⫾ 0.07 0.34 ⫾ 0.07 0.35 ⫾ 0.08
Pa-ETco2 (mm Hg) 6.3 ⫾ 0.3† 5.7 ⫾ 0.3 5.6 ⫾ 0.5 5.6 ⫾ 0.6 2.9 ⫾ 0.4 4.4 ⫾ 0.4 5.5 ⫾ 0.3 6.5 ⫾ 0.3
VTCO2,br (mL) 16.6 ⫾ 2.3 16.6 ⫾ 2.5 16.2 ⫾ 2.5 16.6 ⫾ 3.4 16.8 ⫾ 3.3 17.3 ⫾ 2.8 17.3 ⫾ 3.1 17.2 ⫾ 2.9
SII (mm Hg/mL) 0.39 ⫾ 0.08 0.41 ⫾ 0.09 0.39 ⫾ 0.13 0.36 ⫾ 0.07 0.34 ⫾ 0.07 0.35 ⫾ 0.07 0.36 ⫾ 0.07 0.38 ⫾ 0.08
Data are presented as mean ⫾ SD.
IndexARS ⫽ level of positive end-expiratory pressure (PEEP) applied after an alveolar recruitment strategy (ARS); VDaw ⫽ airway dead space; VDalv ⫽ alveolar dead space; VDphys ⫽ physiological
dead space; VD/VT ⫽ ratio of physiological dead space to tidal volume; Pa-ETCO2 ⫽ arterial to end-tidal partial pressure differences of CO2; VTCO2,br ⫽ carbon dioxide elimination per breath;
and SII ⫽ slope of Phase II.
* Vs 15, P ⬍ 0.05.
† Vs 15ARS, P ⬍ 0.05.

Table 3. Data on Ventilation and Lung Mechanics

PEEP (cm H2O) ARS

0 5 10 15 20ARS 15ARS 10ARS 5ARS 0ARS

PIP (cm H2O) 27 ⫾ 4* 28 ⫾ 4* 30 ⫾ 5 34 ⫾ 4 50 ⫾ 1 30 ⫾ 3 27 ⫾ 5* 26 ⫾ 6* 25 ⫾ 5*
PLP(cm H2O) 21 ⫾ 6 23 ⫾ 5 24 ⫾ 6 27 ⫾ 7 50 ⫾ 1 24 ⫾ 4 20 ⫾ 6 20 ⫾ 4 20 ⫾ 5
Paw (cm H2O) 11 ⫾ 2*†‡§ 13 ⫾ 2†‡ 17 ⫾ 3† 21 ⫾ 3‡ 29 ⫾ 3 20 ⫾ 2 16 ⫾ 2 13 ⫾ 3 11 ⫾ 3
VT (mL) 613 ⫾ 72 614 ⫾ 69 609 ⫾ 80 618 ⫾ 62 1007 ⫾ 132 614 ⫾ 68 617 ⫾ 62 611 ⫾ 65 616 ⫾ 62
RR (bpm) 12 ⫾ 1.3 12 ⫾ 1.2 12 ⫾ 1.1 11 ⫾ 1.5 11 ⫾ 1.4 11 ⫾ 1.2 11 ⫾ 1.2 11 ⫾ 1.0 11 ⫾ 1.1
CDYN (mL/cm 30 ⫾ 6*†‡ 36 ⫾ 9†‡ 42 ⫾ 10†‡ 48 ⫾ 10† 39 ⫾ 15 68 ⫾ 13 57 ⫾ 16 44 ⫾ 12 33 ⫾ 8
H2O)
RAW (mL 䡠 cm 18 ⫾ 3.9 17 ⫾ 3.2 16 ⫾ 2.0 15 ⫾ 1.8 17 ⫾ 4.1 15 ⫾ 3.7 16 ⫾ 3.6 19 ⫾ 4.3 19 ⫾ 3.7
H2O⫺1 䡠 s⫺1)
PEF (L/s) 36 ⫾ 6 37 ⫾ 6㛳 39 ⫾ 5‡㛳 40 ⫾ 5†‡ 45 ⫾ 6 32 ⫾ 6 31 ⫾ 5 31 ⫾ 5 32 ⫾ 5*
ETC (s) 0.98 ⫾ 0.2†‡ 0.96 ⫾ 0.1†‡ 0.92 ⫾ 0.2†‡ 0.91 ⫾ 0.1†‡ 0.81 ⫾ 0.1 1.27 ⫾ 0.3 1.25 ⫾ 0.3 1.20 ⫾ 0.2 1.16 ⫾ 0.2
Data are presented as mean ⫾ SD.
IndexARS ⫽ level of positive end-expiratory pressure (PEEP) after an alveolar recruitment strategy; PIP ⫽ peak airway pressure; PLP ⫽ plateau pressure; Paw ⫽ mean airway pressure; VT ⫽
tidal volume; RR ⫽ respiratory rate; CDYN ⫽ respiratory dynamic compliance; RAW ⫽ expiratory airway resistance; PEF ⫽ expiratory peak flow; ETC ⫽ expiratory time constant.
* Vs 15, P ⬍ 0.05.
† Vs 15ARS, P ⬍ 0.05.
‡ Vs 10ARS, P ⬍ 0.05.
§ Vs 10, P ⬍ 0.05.
㛳 Vs 5ARS, P ⬍ 0.05.

Our group has confirmed the perfusion-dependent into the airway opening. First, increasing the area for
mechanism in the genesis of SIII in anesthetized pa- gas exchange by actively recruiting previously col-
tients.19 Therefore, SIII depends highly on the spatial lapsed lung acini will naturally decrease the resistance
and temporal distribution of ventilation and perfusion to CO2 diffusion through the alveolar-capillary mem-
and can therefore be considered a general index of brane. Second, increasing the cross-sectional area of
V/Q matching.18 –20 the airways will result in decreased resistance to
Considering the above concepts, changes in SIII can intrapulmonary CO2 transport by both diffusion and
be explained best by the effect that lung recruitment convection. The latter mechanism seems to be the
has on the transport of CO2 from the capillary blood most relevant for decreasing SIII. This hypothesis has

156 Volumetric Capnography in Obese Patients ANESTHESIA & ANALGESIA

 on the mechanical properties of the respiratory sys-
tem. Thus, any change in respiratory mechanics in-
duced by lung recruitment will play a major role in the
genesis of SIII because the expiratory flow patterns
change accordingly. As opposed to CO2 kinetics, the
effect of lung collapse and recruitment on respiratory
mechanics is well known.7,9 –11 Our results fit well
with these studies showing that the elastic properties
of the respiratory system increase and airway flow
resistance decreases when lung volumes are restored
by RMs and PEEP.
These changes in lung mechanics can be explained
Figure 3. Time constants for CO2 elimination throughout the
study protocol. Tau-CO2 ⫽ CDYN ⫻ RAW ⫻ VTCO2,br, where by the concept of expiratory time constant (ETC). The
CDYN is the dynamic compliance, RAW is the expiratory ETC describes how fast the passive respiratory system
airway resistance, and VTCO2,br is the amount of CO2 responds to an external mechanical perturbation dur-
eliminated in one breath. CO2flow ⫽ CDYN ⫻ VTCO2,br/RAW, ing expiration. Classically, a short ETC goes along with
using the same variables as above but in a modified arrange-
ment, with RAW in the denominator. IndexARS ⫽ level of a fast response, whereas a long ETC indicates a delayed
positive end-expiratory pressure (PEEP) after an alveolar attainment of a new equilibrium within the respira-
recruitment strategy. Data are presented as mean ⫾ sd. * Vs tory system. At ZEEP, the lower CDYN caused by
15ARS, P ⬍ 0.05. atelectasis allows the equilibrium to be reached very
quickly while the VT is distributed within a smaller
lung. This leads to decreased global compliance and
already been shown in normal weight patients with increased PEF. The opposite mechanism is observed
anesthesia-induced atelectasis11,12 and is now con- after lung recruitment: higher CDYN allows a slower
firmed in these morbidly obese patients. In contrast to and more homogeneous expiratory flow. ETC in-
these results, Blanch et al.37 found no differences in
creases despite a decrement in RAW mainly because of
VC indices when PEEP levels of 0, 5, 10, and 15 cm
a more-than-proportional increase in CDYN. VT is now
H2O were applied; however, this was without prior
distributed within more functional lung tissue, the
recruitment in patients with normal lungs or in pa-
characteristic of an “open-lung” condition. Increased
tients with acute lung injury or acute respiratory
compliance and decreased PEF are the consequences.
distress syndrome. Their results are almost identical to
The relationship between the effects of ARS and
our data obtained before the lung RM, in which
PEEP on lung mechanics and CO2 kinetics is repre-
changes were small and nonsignificant. The physi-
sented by the Tau-CO2 concept (Fig. 3). Despite stable
ologic differences between the effect of PEEP with and
without lung recruitment are well known7,8,13,32 and hemodynamics (Table 4) and strict protocol timing,
can easily explain such contradictory results. this composite variable showed that the amount of
CO2 eliminated within 1 ETC was highest at 15ARS, a
Mechanism Behind Change in SIII Induced by condition related to the most favorable lung condition,
Lung Recruitment as reflected by oxygenation and dead space. The
As we have already pointed out, mechanisms af- CO2flow variable showed an even better profile than
fecting the CO2 transport within the lungs during the Tau-CO2 concept for monitoring the recruitment
expiration are the main determinants of SIII. Expira- effect, because the lower RAW seen after recruitment,
tory flow is a passive process that depends exclusively which is now in the denominator, will augment the

Table 4. Hemodynamic Data

PEEP (cm H2O) ARS

0 5 10 15 20ARS 15ARS 10ARS 5ARS 0ARS

HR (beats/min) 69 ⫾ 4 67 ⫾ 5 66 ⫾ 6 64 ⫾ 6 78 ⫾ 11 68 ⫾ 7 65 ⫾ 8 67 ⫾ 8 68 ⫾ 8
MAP (mm Hg) 81 ⫾ 14 75 ⫾ 10 78 ⫾ 11 79 ⫾ 10 80 ⫾ 19 80 ⫾ 13 83 ⫾ 14 85 ⫾ 15 84 ⫾ 16
MPAP (mm Hg) 23 ⫾ 7 24 ⫾ 6 26 ⫾ 6 26 ⫾ 5 28 ⫾ 4 25 ⫾ 5 24 ⫾ 6 25 ⫾ 7 25 ⫾ 5
PCWP (mm Hg) 14 ⫾ 3 14 ⫾ 2 16 ⫾ 3 16 ⫾ 4 19 ⫾ 4 17 ⫾ 3 16 ⫾ 3 16 ⫾ 3 15 ⫾ 2
CVP (mm Hg) 14 ⫾ 4 14 ⫾ 4 13 ⫾ 3 15 ⫾ 4 18 ⫾ 3 15 ⫾ 3 14 ⫾ 4 14 ⫾ 3 14 ⫾ 4
CO (L/min) 6.7 ⫾ 1.5 6.5 ⫾ 1.5 6.4 ⫾ 1.6 6.3 ⫾ 1.6 5.4 ⫾ 1.1 6.1 ⫾ 1.6 6.1 ⫾ 1.5 6.4 ⫾ 1.6 6.5 ⫾ 1.7
Shunt (%) 0.14 ⫾ 0.07*†‡ 0.14 ⫾ 0.06*†‡ 0.13 ⫾ 0.06*† 0.11 ⫾ 0.05* — 0.06 ⫾ 0.03 0.08 ⫾ 0.04 0.10 ⫾ 0.04 0.10 ⫾ 0.03
Data are presented as mean ⫾ SD.
IndexARS ⫽ level of positive end-expiratory pressure (PEEP) after an alveolar recruitment strategy; MAP ⫽ mean systemic arterial blood pressure; MPAP ⫽ mean pulmonary arterial pressure;
PCWP ⫽ pulmonary capillary wedge pressure; CVP ⫽ central venous pressure; CO ⫽ cardiac output; HR ⫽ heart rate.
* Vs 15ARS, P ⬍ 0.05.
† Vs 10ARS, P ⬍ 0.05.
‡ Vs 5ARS, P ⬍ 0.05.

Vol. 109, No. 1, July 2009 © 2009 International Anesthesia Research Society 157
signal of lung improvement (Fig. 3). The physiological Note added in proof: See also Böhm et al.40 in this
rationale of CO2flow goes beyond this simple mathemati- issue which reports different findings in the same
cal association and is related to the above explanations. patients.
RAW to expiratory flow has an indirect correlation with
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