Hemodynamic evaluation of anesthetized baboons and piglets by transpulmonary thermodilution: Normal values and interspecies differences with respect to xenotransplantation

Transpulmonary thermodilution is well established as a tool for in‐depth hemodynamic monitoring of critically ill patients during surgical procedures and intensive care. It permits easy assessment of graft function following cardiac transplantation and guides post‐operative volume and catecholamine therapy. Since no pulmonary catheter is needed, transpulmonary thermodilution could be useful in experimental cardiac pig‐to‐baboon xenotransplantation. However, normal values for healthy animals have not yet been reported. Here, we present data from piglets and baboons before xenotransplantation experiments and highlight differences between the two species and human reference values.


| INTRODUC TI ON
Minimally invasive hemodynamic monitoring has become the standard procedure for critically ill patients undergoing surgery or requiring intensive care. Pulmonary artery catheterization for thermodilution measurement of cardiac output (CO) is still considered the gold standard, but its use has been declining in recent years. 1 Less invasive methods of transpulmonary thermodilution (TPTD), such as the PiCCO system (Pulsion Medical Systems SE), have become an important alternative, especially for children where the use of Swan Ganz catheters is limited by vessel size. 2  is indexed to total body weight (TBW). A combined interpretation of these indexed parameters allows a detailed assessment of preload, afterload, contractility, and volume status of critically ill patients, which are pre-requisites for goal-directed volume and catecholamine therapy. The reader is referred to two excellent reviews for further details of clinical and technical aspects of this topic. 6,7 In xenotransplantation, TPTD has been used to assess CO during pig-to-primate kidney 8 and heart transplantation. [9][10][11] As for human allotransplantation, comprehensive hemodynamic monitoring and careful therapeutic management are key to a favorable outcome after cardiac xenotransplantation. However, little is known about the normal values in baboons and piglets of the sizes used for these experiments. To our knowledge, no reference tables are as yet available.

| Animals
Hemodynamic data sets from pigs and baboons obtained be-

Conclusions:
Parameters of preload, afterload, and contractility differ between baboons and piglets. In particular, baboons have a much higher afterload than piglets, which might be instrumental in causing perioperative xenograft dysfunction and post-operative myocardial hypertrophy after orthotopic pig-to-baboon cardiac xenotransplantation. Most transpulmonary thermodilution-derived parameters obtained from healthy piglets and baboons lie outside the reference ranges for humans, so human normal values should not be used to guide treatment in those animals. Our data provide reference values as a basis for developing algorithms for perioperative hemodynamic management in pig-to-baboon cardiac xenotransplantation.

K E Y W O R D S
baboon, cardiac transplantation, hemodynamic monitoring, perioperative management, reference range, transpulmonary thermodilution, xenotransplantation

| Anesthesia
All animals were fasted for 12 hours prior to anesthesia. General anesthesia of both baboons and pigs was induced with intravenous bolus administrations of propofol and fentanyl. Anesthesia was maintained with either continuous infusions of propofol (0.1-0.2 mg/kg/ min; baboons and pigs) or inhalation of isoflurane (0.8-1.2 vol%) and sevoflurane (1-2 vol% end-expiratory concentration) for baboons only. Analgesia was maintained with repetitive bolus administrations of fentanyl (2.5-8 µg/kg every 30-45 minutes). After endotracheal intubation, the animals were ventilated mechanically. During anesthesia, ventilation was adjusted to end-tidal CO 2 as necessary, and ECG, blood pressure, and peripheral oxygen saturation were monitored.  Table 1.

| Offline analysis and statistics
Recorded data were visualized with PiCCOWin 6.0 software.
Low-quality measurements were excluded depending on the ther-  14,15 Data are presented as mean ± standard deviation, median, and 95% interval. Hemodynamic measurements from pigs and baboons were compared using the Mann-Whitney rank sum test.
Exact p-values are given for each test; for correlations, Pearson's r is indicated. Statistical significance was assumed when P < .05.

| RE SULTS
Results from hemodynamic monitoring and TPTD measurements are summarized in Tables 2 and 3 Table 2 shows the results from hemodynamic monitoring. MAP was 60% higher in baboons than piglets ( Figure 2). Systolic (SAP, 49%) and diastolic arterial pressures (DAP, 68%) were also higher in baboons.

| Hemodynamic monitoring
Baboons had a 14% slower heart rate than pigs. Central venous pressure (CVP) was not significantly different between the two species.

| Transpulmonary thermodilution measurements
CI was 20% lower in baboons than pigs (Table 3, Figure 2). Since SVI was similar in both species, the increased CI must be due to an

Definitions
Calculation increased heart rate in piglets. SVRI was 120% higher in baboons than in pigs. The volumetric parameter of cardiac preload GEDI was 21% higher in baboons than pigs, whereas the extravascular lung water index (ELWI) was 15% lower.

| Comparison with human reference values
With the exception of CI and SVI (Table 2, Figure 3A), adult human reference values differed in all other parameters from the TA B L E 2 Results of hemodynamic monitoring in baboons (n = 47) and piglets (n = 45), presented as mean ± SD, median and 95% interval  measurements taken from baboons and piglets. MAP and HR of baboons were higher than human reference ranges (  Figure 3B). The volumetric preload parameter GEDI was lower in both pigs and baboons than in humans (Table 3, Figure 3C), whereas the parameter of pulmonary edema (ELWI) was above human references values ( Figure 3D).
Discussion of pigs as organ donors for humans has mainly focused on adult donors and recipients. 26,27 Full-grown pigs have cardiac output and arterial blood pressure values comparable to, or even higher than, adult humans. [26][27][28] Systemic vascular resistance-as well as pulmonary vascular resistance-in adult pigs has been reported as twice that in humans, supposedly giving the transplanted pig heart an advantage in pumping against lower resistance. 26,27 In contrast, transplanting a juvenile porcine heart accustomed to low vascular resistance into an adolescent baboon with twice the resistance challenges the pig heart's ability to adapt to higher afterload. For most xenotransplantation experiments, clinically approved ischemic preservation is used for organ storage. In 40%-60% of these experiments, the grafts fail within 48 hours due to perioperative xenograft dysfunction (PCXD). 29 We hypothesize that ischemia/reperfusion injury caused by ischemic storage impairs the

| Cardiac preload and extravascular lung water
For many years, perioperative volume therapy has been guided by central venous (CVP) and pulmonary artery occlusion pressures. However, these parameters do not accurately reflect cardiac preload. 31 TPTD provides the volumetric parameter global end-diastolic volume index (GEDI), which represents the sum of end-diastolic volumes of all four heart chambers. GEDI has been shown to be superior to filling pressures for guiding cardiac preload. 7 TPDT also provides the parameter ELWI, which reflects the fluid that is contained within the perfused regions of the lungs. 7 Elevated ELWI is typically found in pulmonary edema 6 and has been used as a therapeutic guide after cardiac surgery. 32 To allow proper fluid management in xenotransplantation ex- ELWI values determined by gravimetry in newborn healthy lambs were more than twice as high as in adult sheep (13.3 vs 6.1 mL/ kg). 37 Lemson et al proposed that the lower GEDI values and higher ELWI values in younger children were a result of age-related changes in the ratio of lung weight to body weight and in the ratio of heart weight to BSA. 38 The greater lung weight in infants has consequences for calculating the (non-indexed) EVLW, for which the intrathoracic blood volume (ITBV) is needed. Historically, intrathoracic volumes were measured using the transpulmonary double-indicator (thermo-dye) dilution technique with two different indicators (cold and indocyanine green). Sakka et al empirically found ITBV, the sum of GEDV and pulmonary blood volume, to be ~1.25 × GEDV. 39 This linear relationship was incorporated into the single indicator methodology for TPTD.
In children, this multiplier varies from 1.5 in the newborns to 1.2 in adults. 33 Therefore, application of the adult formula underestimates ITBV and overestimates EVLW. Similar to human infants, Rossi et al found ITBV to be 1.52 × GEDV + 49.7 (mL) for landrace piglets (24-32 kg), thus greatly improving the accuracy of estimating EVLW. 40 For baboons, the exact relationship is unknown.
These findings emphasize that human reference values of TPTD parameters for volumetric preload and lung water are age-and species-dependent and cannot be simply adopted in pig-to-baboon xenotransplantation. The normal values provided by this study may serve as a reference for both baboons and pigs, but we caution that comparisons should be restricted to animals of the same species and similar age and body size. can help fine-tune or maintain an adequate drug therapy ( Figure 4B).

| TPTD parameters in xenotransplantation-
• Low preload (GEDI < 350 mL/m 2 ) indicates compensated hypovolemia and must be primarily treated with volume (crystalloid/ colloid infusions, blood products in the case of bleeding).
• Low preload and signs of pulmonary edema (ELWI > 15 mL/kg) are typical for inflammatory pulmonary disease and not common in xenotransplantation experiments. Volume therapy should be applied cautiously.
• Adequate preload (GEDI > 350 mL/m 2 ) and low ELWI are the aims of goal-directed therapy. Current volume and catecholamine therapy can be maintained.