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Department of Physiology, Temple University School of Medicine, Philadelphia, Pennsylvania 19140
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
ACKNOWLEDGEMENTS
FOOTNOTES
REFERENCES
ABSTRACT 
Shaffer, Thomas H., Raymond Foust IIII, Marla R. Wolfson,
and Thomas F. Miller, Jr. Analysis of perfluorochemical
elimination from the respiratory system. J. Appl.
Physiol. 83(3): 1033-1040, 1997.
We describe a
simple apparatus for analysis of perfluorochemicals (PFC) in expired
gas and thus a means for determining PFC vapor and liquid elimination
from the respiratory system. The apparatus and data analysis are based
on thermal conduction and mass transfer principles of gases. In vitro
studies were conducted with the PFC vapor analyzer to determine
calibration curves for output voltage as a function of individual
respiratory gases, respiratory gases saturated with PFC vapor, and
volume percent standards for percent PFC saturation (%PFC-Sat) in air.
Voltage-concentration data for %PFC-Sat of the vapor from the in vitro
tests were accurate to within 2.0% from 0 to 100% PFC-Sat, linear
(r = 0.99, P < 0.001), and highly reproducible.
Calculated volume loss of PFC liquid over time correlated well with
actual loss by weight (r = 0.99, P < 0.001). In vivo studies with
neonatal lambs demonstrated that PFC volume loss and evaporation rates
decreased nonlinearly as a function of time. These relationships were
modulated by changes in PFC physical properties, minute ventilation,
and postural repositioning. The results of this study demonstrate the
sensitivity and accuracy of an on-line method for PFC analysis of
expired gas and describe how it may be useful in liquid-assisted
ventilation procedures for determining PFC volume loss, evaporation
rate, and optimum dosing and ventilation strategy.
thermal detectors; premature lambs; liquid ventilation; evaporation
rate of perfluorochemicals
INTRODUCTION IT HAS BEEN EXTENSIVELY reported that liquid-assisted
ventilation, tidal and partial liquid ventilation utilizing
perfluorochemical (PFC) liquids, can reduce interfacial surface tension
and provide improved ventilation at low insufflation and alveolar
pressures in cases of restrictive lung disease (20, 22). Liquid
ventilation supports effective arterial oxygenation,
CO2 elimination, homogeneous distribution of ventilation, and pulmonary perfusion (15, 17). In
addition to the fact that PFC liquids are bioinert, it has been shown
that they are minimally absorbed, eliminated from the lungs primarily
by evaporation, and have no deleterious histological, cellular, or
biochemical effects (10, 13, 17, 24). The potential usefulness of PFC
liquid has been investigated in animal models and in humans with
respiratory distress syndrome (2, 4, 6, 8, 9, 11, 21, 22), pulmonary
administration of drugs (14, 23), radiologic imaging and diagnosis (5, 19, 25), and artificial red blood cell substitution (1, 12). In all
these modalities, it is important to evaluate PFC vapor elimination via
respiration to monitor PFC distribution and supplemental dosing with
respect to ventilatory management. In this study we characterize PFC
elimination from the respiratory system utilizing a simple, on-line
thermal detector analyzer. The conceptual design and basic operative
considerations of the detector and representative in vitro and in vivo
data acquired by use of this technique are also presented.
MATERIALS AND METHODS
) represents the thermal detector output
with thermal compensation. By maintaining the sample gas flow constant (500 ml/min), the reference gas flow equal to zero (sealed chamber), and the reference gas composition constant during normal operation, the
voltage output of the detector will change as a function of the
composition and physical thermal properties of the sample gas.
Therefore, the detector can be calibrated for respiratory gases
saturated or partially saturated with PFC vapor.
Fig. 1.
A: schematic of thermal detector cell
configuration. T1 and T2, thermistors.
B: schematic of in-line vapor analyzer
circuit for determination of perfluorochemical (PFC) vapor
concentration in expired gas. Circuit consists of an in-line sampling
chamber (1.5-ml dead space), 2 solenoid valves (V1 and V2) with 4 positions (common port always open and positions
1-3 open or closed), a pump (P), and a thermal
detector analyzer (D). Valve positions 1-3 are closed (no-flow condition). In standby
mode, valve positions 1 and
2 are closed, valve
position 3 is open, and resident gas is recirculated within analyzer circuit. In calibration or
continuous-sampling mode, valve positions
2 and 3 are closed as
calibration gas is aspirated through V1 (position
1 to C) and circulated through analyzer and through V2
(C to position 1), where it is vented. Finally, in discrete
expiratory sampling mode, valve positions 1 and 3 are closed as
gas is sampled from sampling chamber (endotracheal tube side) through
V1 (position 2 to C), pumped through
analyzer circuit, and returned through V2 (C to
position 2) to sampling chamber
(ventilator side). 
, gas flow.
[View Larger Versions of these Images (15 + 17K GIF file)]
. In
addition, respiratory gases saturated with the vapor from several PFC
liquids were sampled (see Table 1 for PFC
specifications: vapor pressures were calculated on the basis of
software provided by the National Institute of Standards and
Technology, "Structures and Properties," version 2.0). PFC liquids studied in this investigation included perflubron (LiquiVent, Alliance Pharmaceutical), perfluoroalkylfuran (Rimar 101, Miteni, represented in the US by Mercantile Development), perfluorodecalin (APF-140, Air Products and Chemicals), and
perfluorodimethyl-cyclohexane (APF-125, Air Products and Chemicals).
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(1) |
is density,
Cp is specific
heat, and k is thermal conductivity of
the medium. For mixtures of gases (i.e., He-air), the calculated value
of each physical property (,
Cp,
k) is based on its volumetric
proportion of the mixture.
The voltage output of the detector () was then assessed for
linearity and sensitivity as a nondimensional function of Nu for
various respiratory gases combined with various saturations of PFC
vapors (0-100%). The volume percentage of PFC vapor saturated in
test gas (%PFC-Sat) was determined as follows
|
(2) |
|
(3) |
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![= [<A><AC>V</AC><AC>˙</AC></A> × %PFC-Sat × (ml PFC fluid/ml vapor) × <IT>t</IT>]](/content/vol83/issue3/fulltext/1033/img005.gif)
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(4) |
is the gas flow or minute
ventilation (ml/min), %PFC-Sat is the measured volumetric percentage
of PFC vapor in the sample gas, (ml PFC fluid/ml vapor) is based on
molecular weight and density of the PFC liquid and temperature, and
t is time (min). Calculated volume
loss at room temperature conditions (25-26.5°C) was then
compared with actual PFC fluid loss from a 200-ml glass flask for a
calibrated, constant gas flow (medical-grade air, 500 ml/min;
Cole-Palmer rotameter, 150-mm variable-area flowmeter, Niles, IL) that
was bubbled under the surface of the liquid PFC. Flasks were initially
filled with 100 ml (high-volume condition) or 50 ml (low-volume
condition) of PFC liquid (LiquiVent or Rimar 101) and allowed to
evaporate over time while volume loss was determined gravimetrically
using a digital scale (model SL 5000, Scientech) and the appropriate
PFC specific gravities. The high initial volume condition maintained
100% saturation, while the low initial volume condition allowed a
change in gas/PFC surface area as PFC fluid evaporated from the flask.
In vivo studies.
Assessment of expired %PFC-Sat, PFC volume loss, and rate of volume
loss was conducted in five preterm lambs (125 days gestation). The
animals were managed according to National Institutes of Health regulations and the "Guiding Principles in the Care and Use of Animals" of the American Physiological Society. In addition, the study was performed with the approval of the Temple University Medical
School Animal Care Committee.
The pregnant ewe was sedated with ketamine (5.0 ml/kg im) followed by
epidural administration of 1.0 mg/kg of 0.75% bupivacaine HCl.
Cesarean section was performed while the ewe was restrained in the
prone position, as previously described (16, 22).
The uterus was exposed and opened sufficiently for the head of the
fetal lamb to emerge, and a rubber glove containing warmed saline
solution was placed over its snout to prevent inspiration. Skin and
superficial tissues were anesthetized (4.0 mg/kg of 5% lidocaine).
Ketamine (1 mg/kg im) was given, and catheters were placed in a jugular
vein and carotid artery. A tracheotomy was performed, and a Hi-Lo jet
endotracheal tube (Mallinckrodt) was introduced, with the tip inserted
proximal to the carina. Pancuronium bromide (0.10 mg/kg) and sodium
bicarbonate (2.50 meq/kg) were administered intravenously before the
rubber glove was removed, and the lamb was then delivered. Animals were
ventilated using an InfantStar (Infrasonics) pressure ventilator, using
conventional management [tidal volume
(VT) = 6-10 ml/kg,
respiratory rate = 30 breaths/min, inspiratory
O2 fraction = 1 saturated with
water at 37°C]. Respiratory parameters of
VT, breathing frequency, and minute ventilation were determined by computed pneumotachography (PEDS,
MAS).
After tracheal administration of PFC liquid (10 ml/kg, LiquiVent) over
5 min, the PFC analyzer circuit was placed in-line. Mixed expired gas
was collected by means of a low dead space
(VD) sample chamber (1.5 ml)
placed in series with the ventilator (Fig. 1B). A circulating pump (model KNF,
Neuberger, Trenton, NJ), calibrated to a known flow rate, was
synchronized with the ventilator by means of an electronic trigger and
solenoid valves that allowed sampling of expired gas and assessment of
%PFC-Sat. Thus a synchronized sampling system was created, such that
respiratory gas samples were removed from the airways, analyzed, and
reintroduced into the breathing circuit. Hence, no net gas was added or
removed from the system.
In preliminary studies with partial liquid ventilation (PLV),
simultaneous mixed expired gas and arterial blood-gas samples were
collected after the intratracheal instillation of PFC and analyzed
(Stat Profile 3, NOVA Biomedical) for
CO2. These studies were conducted
to test a simple algorithm utilizing arterial
PCO2 (PaCO2) as an indirect means for in vivo
correction of %PFC due to small alterations in expiratory
CO2 fraction
(FECO2) of carrier gas, since the thermal detector is not specific to PFC
vapor. As previously reported by Tutuncu et al. (21),
VD/VT after PLV with LiquiVent was ~0.7. Therefore, using the Bohr equation
![0.7 = [F<SC>a</SC><SUB>CO<SUB>2</SUB></SUB> − F<SC>e</SC><SUB>CO<SUB>2</SUB></SUB>]/F<SC>a</SC><SUB>CO<SUB>2</SUB></SUB>](/content/vol83/issue3/fulltext/1033/img006.gif)
|
(5) |
![P<SC>a</SC><SUB>CO<SUB>2</SUB></SUB> = [760-47] × F<SC>a</SC><SUB>CO<SUB>2</SUB></SUB>](/content/vol83/issue3/fulltext/1033/img007.gif)
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(6) |
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(7) |
RESULTS
values to establish a standard curve for the given range of the
thermal detector. As shown in Fig.
2A, a
linear correlation (r = 0.92, P < 0.001) was established between
Nu for respiratory gases and measured . Additionally, three PFC
vapors combined with respiratory gases were sampled in the in vitro
circuit, and as noted in Fig. 2B,
there was a linear correlation (r = 0.97, P < 0.001) between and
%PFC-Sat × Nu for each carrier gas
(O2, air,
N2) in combination with PFC
vapor. Each data point represents the mean of five repeated
measurements for each carrier gas/PFC concentration. These experiments
were repeated extensively and found to be reproducible within 2%
variation. It is also noteworthy that the slopes (
/%PFC-Sat × Nu) of carrier gases saturated with PFC vapor were similar;
however, their intercepts were shifted as a function of carrier gas
baseline (no PFC), as noted in Fig. 2B.
units
as a function of Nusselt number (Nu). B: units as a
function of volume percent of PFC vapor in carrier gas (%PFC) × Nu, PFC vapor, and carrier gas. C: units as a function
of %PFC saturation (%PFC-Sat) and type of PFC vapor
(C).
plotted as a function of PFC vapor concentration was linear for
each PFC studied (Fig. 2C). As
summarized in Table 3, all -concentration curves were linear over the concentration range 0-100% PFC-Sat, and the slope of each line was PFC dependent. When the physical properties of the various PFC liquids were considered (Table 1), it was noted that the slope [thermal detector
sensitivity (TDS)] of the -%PFC-Sat curves increased as a
function of PFC liquid vapor pressure. Regression analysis of these
data (Table 3) showed that TDS = 
0.097Pv + 8.43 (r = 0.99, P < 0.01), thus demonstrating that
the sensitivity of the PFC analyzer increased in proportion to PFC
vapor pressure. Although not shown in Fig. 2, the presence of
O2,
CO2, and 100% saturated water
vapor shifted each curve (-%PFC-Sat) upward 0.02 V/1% increase in
O2 concentration, 0.146 V/1%
decrease in CO2 concentration, and
0.039 V/10% increase in water vapor concentration. Therefore, the
intercept of the -%PFC-Sat curve was shifted as noted above;
however, the TDS (slope) for each PFC was independent of carrier gas
concentration. On the basis of these experimental data, we calculated
that the combined effect of 2.79%
CO2 and 100% saturated water
vapor in mixed expired gas would essentially nullify any individual
offsets in thermal detector signal. However, when mixed expired
CO2 exceeds the normal range of
2.5% < CO2 < 4%, must be
appropriately corrected for CO2.
Moreover, as shown in Fig. 2C, the
significance of water vapor, CO2,
and O2 compensation is less
critical for higher vapor pressure liquids (APF-125 and Rimar 101) than
for low vapor pressure liquids (APF-140 and LiquiVent).
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In vivo data. All preterm lambs were ventilated with conventional mechanical gas ventilation while PFC (LiquiVent) was slowly infused. After a functional residual capacity (10 ml/kg) volume was introduced, the initial PFC liquid lung volume and rate of PFC volume loss were 10 ml/kg and 0.055 ± 0.009 (SE) ml · kg
1 · min1,
respectively (Fig. 4). There was a
nonlinear decrement in these parameters over 2 h of gas
ventilation, which was experimentally derived by direct
%PFC-Sat determinations and calculations utilizing Eq. 4. As noted above, changes in
%PFC-Sat for water vapor and CO2
were taken into consideration.
During this 2-h study, arterial oxygenation, PCO2, and pH were effectively maintained [arterial PO2 = 136 ± 53 (SE) Torr, PaCO2 = 48 ± 5 Torr, pH = 7.31 ± 0.03]. As expected from Eq. 4, it was experimentally determined that the PFC lung volume and PFC rate of loss were related to time (Fig. 4), such that both decreased over the 2 h of ventilation. In addition, as shown in Fig. 5A, in two lambs that required very different minute ventilation to maintain normocapnia, volume loss (3.8 vs. 7.5 ml/kg) increased in approximate proportion to minute ventilation (209.9 vs. 409.3 ml/min); other respiratory gas conditions (saturated water vapor and inspiratory O2 fraction = 1.0) were maintained. Finally, as shown in Fig. 5B, %PFC-Sat decreased over the 2 h of ventilation, and a change in animal posture at 140 min with no instillation of PFC fluid resulted in a transient increase of ~20% in %PFC-Sat (supine to prone). Also, a later position change (prone to supine) at 200 min resulted in a smaller increase of 5% in %PFC-Sat (prone to supine).
DISCUSSION
The process reported here relates to a method of evaluating liquid PFC vapor elimination via the respiratory system. PFC elimination analysis utilizing an on-line thermal detector provides a useful and convenient method for continuous determination of %PFC-Sat in expired gas and a means of accurately determining PFC volume loss and rate of volume loss from the lungs.
As previously discussed, to maintain effective ventilation during liquid-assisted ventilation, it is of value to have a continuous assessment of the amount of PFC in the lung as well as the relative distribution of PFC throughout the lung. However, because of the inevitable evaporation of PFC from the respiratory system, liquid volume and distribution of PFC throughout the lungs vary over the duration of gas ventilation. This evaporative process and redistribution have been documented in short- and long-term radiographic studies in experimental animals and humans after administration of various amounts of PFC to the lungs (4-6, 8, 19, 25).
Previous methods for assessment of expired PFC vapor and liquid volume loss during liquid-assisted ventilation have been problematic. The approaches have been encumbered by inconvenient sampling techniques as well as the expense of gas chromatography or mass spectroscopy technology required to obtain quantitative gas values (10, 14, 16). Furthermore, radiographic evaluation or visual observation of a "meniscus" of PFC liquid in the endotracheal tube is qualitative at best (3, 5, 6, 8). None of these methods has provided a convenient and effective means for continuously and accurately assessing the amount PFC vapor in expired gas, PFC liquid in the lungs, or the volume of PFC loss over time. In addition, over the course of treatment, inaccurate evaluation of PFC lung volume could result in over- or underexpansion of the lung, thus resulting in suboptimal ventilation strategy.
As shown in the present study, %PFC-Sat, PFC lung volume, and PFC loss rate from the lungs of premature lambs decrease as a function of time during conventional gas ventilation. These parameters can be modulated by changes in minute ventilation and repositioning (Fig. 5). Therefore, %PFC-Sat is associated with the amount as well as the distribution of PFC liquid in the lung. It is also noteworthy that additional dosing coupled with postural repositioning and redistribution of PFC liquid in the lung, while providing improved PFC protection of the lung surface from barotrauma, may also promote increased respiratory PFC fluid loss due to an increase in %PFC-Sat. Thus maintenance of a high %PFC-Sat in the lungs might suggest better protection of the respiratory system during mechanical ventilation. As indicated here, high %PFC-Sat in expired respiratory gas immediately after filling may be associated with an adequate volume and uniform distribution of PFC liquid in the lungs, both of which appear to correlate with good physiological function. The maintenance of a stable volume (>10 ml/kg) and uniform distribution of PFC in the lungs could perhaps be best facilitated by frequent or continuous dosing and/or postural repositioning.
The thermal detector utilized for PFC vapor analysis in this study would be best categorized as a universal (nonspecific) detector, in that it is sensitive in various degrees to respiratory gases and water vapor as well as to PFC vapors. Thermal detector technology has been used extensively in gas chromatography because of its simplicity, low cost, and universality. However, the nonspecific nature of thermal detectors, while beneficial at times, requires additional consideration for the possible presence of other gases. All in vitro studies described here were conducted under controlled conditions where only one PFC was under analysis at any one time and calibrations were determined for specific carrier gas concentrations. Under similar conditions, in all in vivo studies only one PFC (LiquiVent) was tested; calibrations were determined for constant temperature, O2, and saturated water vapor conditions, and small variations in expired CO2 were corrected as described in RESULTS (In vitro data). For future applications, thermal detector compensation and improved gas specificity are being explored with the addition of miniature in-line O2, CO2, and water vapor sensors to electronically correct for background carrier gas concentrations. In addition, a triggering system has been designed, such that a pressure threshold (e.g., 3 cmH2O) can be electronically set so that the solenoids are activated when ventilator pressure is less than threshold pressure. This signal indicates the start of expiration. The expiratory time (TE) is set on the analyzer to match that of the ventilator (e.g., 2.0 s), and sampling time can be adjusted between 20 and 80% of TE, such that gas sampling can be varied during expiration. This new system configuration is more user friendly and reduces operator error in valve triggering; however, on the basis of pilot study results of various expiratory sampling times between 20 and 80% of TE, this feature does not appear to significantly vary our calculation of %PFC compared with the results reported here.
In conclusion, this investigation has described a convenient and inexpensive means for continuous evaluation of PFC vapor in expired gas samples. The results of this study 1) demonstrate the sensitivity, specificity, range, and accuracy of an on-line method for in vitro analysis of several PFC vapors, 2) characterize the changes in %PFC-Sat in expired gas, PFC volume loss, and rate of volume loss from the lungs of preterm lambs during gas ventilation, and 3) describe the implications of this technique in liquid-assisted ventilation applications. The analyzer described in this report may also be useful for several other PFC biomedical applications, such as in liquid ventilation, drug delivery, radiologic imaging, and blood substitution.
ACKNOWLEDGEMENTS
The authors thank Miteni, Air Products and Chemicals, and Alliance Pharmaceutical for generously supplying the perfluorochemicals used in the study and Infrasonics for the use of the InfantStar ventilators. The authors also acknowledge the technical assistance of Charles Philips and Robert Roache.
FOOTNOTES
Address for reprint requests: T. H. Shaffer, Dept. of Physiology, Temple University School of Medicine, 3420 N. Broad St., Philadelphia, PA 19140.
Received 6 January 1997; accepted in final form 29 April 1997.
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 Y. FUJINO, S. GODDON, J.-D. CHICHE, J. HROMI, and R. M. KACMAREK Partial Liquid Ventilation Ventilates Better than Gas Ventilation Am. J. Respir. Crit. Care Med., August 1, 2000; 162(2): 650 - 657. [Abstract] [Full Text] [PDF]
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