Assessment of cardiac output with transpulmonary thermodilution during exercise in humans

Assessing Leg Blood Flow and Cardiac Output During Running Using Thermodilution

Cardiac output (Q̇C) and leg blood flow (Q̇LEG) can be measured simultaneously with high accuracy using transpulmonary and femoral vein thermodilution with a single-bolus injection. The invasive measure has offered important insight into leg hemodynamics and blood flow distribution during exercise. Despite being the natural modality of exercise in humans, there has been no direct measure of Q̇LEG while running in humans. We sought to determine the feasibility of the thermodilution technique for measuring Q̇LEG and conductance during high-intensity running, in an exploratory case study. A trained runner (30 years male) completed two maximal incremental tests on a cycle ergometer and motorized treadmill. Q̇LEG and Q̇C were determined using the single-bolus thermodilution technique. Arterial and venous blood were sampled throughout exercise, with continuous monitoring of metabolism, intra-arterial and venous pressure, and temperature. The participant reached a greater peak oxygen uptake (V̇O2peak) during running relative to cycling (74 vs. 68 mL/kg/min) with comparable Q̇LEG (19.0 vs. 19.5 L/min) and Q̇C (27.4 vs. 26.2 L/min). Leg vascular conductance was greater during high-intensity running relative to cycling (82 vs. 70 mL/min/mmHg @ ~80% V̇O2peak). The "beat phenomenon" was apparent in femoral flow while running, producing large gradients in conductance (62-90 mL/min/mmHg @ 70% V̇O2peak). In summary, we present the first direct measure of Q̇LEG and conductance in a running human. Our findings corroborate several assumptions about Q̇LEG during running compared with cycling. Importantly, we demonstrate that using thermodilution in running exercise can be completed effectively and safely.

The dependence of maximum oxygen uptake and utilization (V̇O2 max) on hemoglobin-oxygen affinity and altitude

Oxygen transport from the lungs to peripheral tissue is dependent on the affinity of hemoglobin for oxygen. Recent experimental data have suggested that the maximum human capacity for oxygen uptake and utilization (V̇O2 max) at sea level and altitude (~3000 m) is sensitive to alterations in hemoglobin-oxygen affinity. However, the effect of such alterations on V̇O2 max at extreme altitudes remains largely unknown due to the rarity of mutations affecting hemoglobin-oxygen affinity. This work uses a mathematical model that couples pulmonary oxygen uptake with systemic oxygen utilization under conditions of high metabolic demand to investigate the effect of hemoglobin-oxygen affinity on V̇O2 max as a function of altitude. The model includes the effects of both diffusive and convective limitations on oxygen transport. Pulmonary oxygen uptake is calculated using a spatially-distributed model that accounts for the effects of hematocrit and hemoglobin-oxygen affinity. Systemic oxygen utilization is calculated assuming Michaelis-Menten kinetics. The pulmonary and systemic model components are solved iteratively to compute predicted arterial and venous oxygen levels. Values of V̇O2 max are predicted for several values of hemoglobin-oxygen affinity and hemoglobin concentration based on data from humans with hemoglobin mutations. The model predicts that increased hemoglobin-oxygen affinity leads to increased V̇O2 max at altitudes above ~4500 m.

Comparison of global end-diastolic volume index derived from jugular and femoral indicator injection: a prospective observational study in patients equipped with both a PiCCO-2 and an EV-1000-device

Transpulmonary thermodilution (TPTD)-derived global end-diastolic volume index (GEDVI) is a static marker of preload which better predicted volume responsiveness compared to filling pressures in several studies. GEDVI can be generated with at least two devices: PiCCO and EV-1000. Several studies showed that uncorrected indicator injection into a femoral central venous catheter (CVC) results in a significant overestimation of GEDVI by the PiCCO-device. Therefore, the most recent PiCCO-algorithm corrects for femoral indicator injection. However, there are no systematic data on the impact of femoral indicator injection for the EV-1000 device. Furthermore, the correction algorithm of the PiCCO is poorly validated. Therefore, we prospectively analyzed 14 datasets from 10 patients with TPTD-monitoring undergoing central venous catheter (CVC)- and arterial line exchange. PiCCO was replaced by EV-1000, femoral CVCs were replaced by jugular/subclavian CVCs and vice-versa. For PiCCO, jugular and femoral indicator injection derived GEDVI was comparable when the correct information about femoral catheter site was given (p = 0.251). By contrast, GEDVI derived from femoral indicator injection using the EV-1000 was obviously not corrected and was substantially higher than jugular GEDVI measured by the EV-1000 (846 ± 250 vs. 712 ± 227 ml/m²; p = 0.001). Furthermore, measurements of GEDVI were not comparable between PiCCO and EV-1000 even in case of jugular indicator injection (p = 0.003). This is most probably due to different indexations of the raw value GEDV. EV-1000 could not be recommended to measure GEDVI in case of a femoral CVC. Furthermore, different indexations used by EV-1000 and PiCCO should be considered even in case of a jugular CVC when comparing GEDVI derived from PiCCO and EV-1000.

Constant-rate intravenous infusion of indocyanine green leading to high fluorescence intensity in infrared thoracoscopic segmentectomy

Objectives

The purpose of this study was to determine whether or not fluorescence could be increased by administering indocyanine green at a constant rate, thus stabilizing its blood concentration.

Methods

In 20 consecutive patients undergoing segmentectomy, the dominant pulmonary arteries were ligated, blocking blood in the target segment. Fluorescence intensity was then observed using different indocyanine green administration methods under infrared thoracoscopy. Intravenous administration of indocyanine green, via a syringe pump at a rate of 12.5 mg/min, was defined as the constant rate group. The bolus group was defined by a 5-mg indocyanine green rapid intravenous injection. The fluorescence intensity was compared at the time of maximum fluorescence and 2 minutes after fluorescence initiation.

Results

At maximum staining, the fluorescence intensity of the normal blood flow area was brighter in the constant rate group (median, 184.2; interquartile range, 170.2-200.1) compared with the bolus group (median, 122.3; interquartile range, 87.3-144.7; P = .0003). The fluorescence of the normal blood flow was retained even after 2 minutes. There was no difference in the fluorescence intensity of the ischemic segments.

Conclusions

The constant rate method showed brighter and better fluorescence than the bolus injection, without an increase in the dose. The contrast between adjacent segments was clear, facilitating the differentiation of the areas.

Unexplained exertional intolerance associated with impaired systemic oxygen extraction

PURPOSE

The clinical investigation of exertional intolerance generally focuses on cardiopulmonary diseases, while peripheral factors are often overlooked. We hypothesize that a subset of patients exists whose predominant exercise limitation is due to abnormal systemic oxygen extraction (SOE).

METHODS

We reviewed invasive cardiopulmonary exercise test (iCPET) results of 313 consecutive patients presenting with unexplained exertional intolerance. An exercise limit due to poor SOE was defined as peak exercise (Ca-vO₂)/[Hb] ≤ 0.8 and VO_2max < 80% predicted in the absence of a cardiac or pulmonary mechanical limit. Those with peak (Ca-vO₂)/[Hb] > 0.8, VO_2max ≥ 80%, and no cardiac or pulmonary limit were considered otherwise normal. The otherwise normal group was divided into hyperventilators (HV) and normals (NL). Hyperventilation was defined as peak PaCO₂ < [1.5 × HCO₃ + 6].

RESULTS

Prevalence of impaired SOE as the sole cause of exertional intolerance was 12.5% (32/257). At peak exercise, poor SOE and HV had less acidemic arterial blood compared to NL (pHa = 7.39 ± 0.05 vs. 7.38 ± 0.05 vs. 7.32 ± 0.02, p < 0.001), which was explained by relative hypocapnia (PaCO₂ = 29.9 ± 5.4 mmHg vs. 31.6 ± 5.4 vs. 37.5 ± 3.4, p < 0.001). For a subset of poor SOE, this relative alkalemia, also seen in mixed venous blood, was associated with a normal PvO₂ nadir (28 ± 2 mmHg vs. 26 ± 4, p = 0.627) but increased SvO₂ at peak exercise (44.1 ± 5.2% vs. 31.4 ± 7.0, p < 0.001).

CONCLUSIONS

We identified a cohort of patients whose exercise limitation is due only to systemic oxygen extraction, due to either an intrinsic abnormality of skeletal muscle mitochondrion, limb muscle microcirculatory dysregulation, or hyperventilation and left shift the oxyhemoglobin dissociation curve.

Muscle mass and inspired oxygen influence oxygen extraction at maximal exercise: Role of mitochondrial oxygen affinity

AIM

We examined the Fick components together with mitochondrial O₂ affinity (p50_mito ) in defining O₂ extraction and O₂ uptake during exercise with large and small muscle mass during normoxia (NORM) and hyperoxia (HYPER).

METHODS

Seven individuals performed 2 incremental exercise tests to exhaustion on a bicycle ergometer (BIKE) and 2 on a 1-legged knee extension ergometer (KE) in NORM or HYPER. Leg blood flow and VO₂ were determined by thermodilution and the Fick method. Maximal ADP-stimulated mitochondrial respiration (OXPHOS) and p50_mito were measured ex vivo in isolated mitochondria. Mitochondrial excess capacity in the leg was determined from OXPHOS in permeabilized fibres and muscle mass measured with magnetic resonance imaging in relation to peak leg O₂ delivery.

RESULTS

The ex vivo p50_mito increased from 0.06 ± 0.02 to 0.17 ± 0.04 kPa with varying substrate supply and O₂ flux rates from 9.84 ± 2.91 to 16.34 ± 4.07 pmol O₂ ·s^-1 ·μg^-1 respectively. O₂ extraction decreased from 83% in BIKE to 67% in KE as a function of a higher O₂ delivery and lower mitochondrial excess capacity. There was a significant relationship between O₂ extraction and mitochondrial excess capacity and p50_mito that was unrelated to blood flow and mean transit time.

CONCLUSION

O₂ extraction varies with mitochondrial respiration rate, p50_mito and O₂ delivery. Mitochondrial excess capacity maintains a low p50_mito which enhances O₂ diffusion from microvessels to mitochondria during exercise.

Leveraging Engineering of Indocyanine Green-Encapsulated Polymeric Nanocomposites for Biomedical Applications

In recent times, photo-induced therapeutics have attracted enormous interest from researchers due to such attractive properties as preferential localization, excellent tissue penetration, high therapeutic efficacy, and minimal invasiveness, among others. Numerous photosensitizers have been considered in combination with light to realize significant progress in therapeutics. Along this line, indocyanine green (ICG), a Food and Drug Administration (FDA)-approved near-infrared (NIR, >750 nm) fluorescent dye, has been utilized in various biomedical applications such as drug delivery, imaging, and diagnosis, due to its attractive physicochemical properties, high sensitivity, and better imaging view field. However, ICG still suffers from certain limitations for its utilization as a molecular imaging probe in vivo, such as concentration-dependent aggregation, poor in vitro aqueous stability and photodegradation due to various physicochemical attributes. To overcome these limitations, much research has been dedicated to engineering numerous multifunctional polymeric composites for potential biomedical applications. In this review, we aim to discuss ICG-encapsulated polymeric nanoconstructs, which are of particular interest in various biomedical applications. First, we emphasize some attractive properties of ICG (including physicochemical characteristics, optical properties, metabolic features, and other aspects) and some of its current limitations. Next, we aim to provide a comprehensive overview highlighting recent reports on various polymeric nanoparticles that carry ICG for light-induced therapeutics with a set of examples. Finally, we summarize with perspectives highlighting the significant outcome, and current challenges of these nanocomposites.

Femoral indicator injection for transpulmonary thermodilution using the EV1000/VolumeView(®): do the same criteria apply as for the PiCCO(®)?

OBJECTIVE

Comparison of global end-diastolic volume index (GEDVI) obtained by femoral and jugular transpulmonary thermodilution (TPTD) indicator injections using the EV1000/VolumnView(®) device (Edwards Lifesciences, Irvine, USA).

METHODS

In an 87-year-old woman with hypovolemic shock and equipped with both jugular and femoral vein access and monitored with the EV1000/VolumeView(®) device, we recorded 10 datasets, each comprising duplicate TPTD via femoral access and duplicate TPTD (20 ml cold saline) via jugular access.

RESULTS

Mean femoral GEDVI ((674.6±52.3) ml/m(2)) was significantly higher than jugular GEDVI ((552.3±69.7) ml/m(2)), with P=0.003. Bland-Altman analysis demonstrated a bias of (+122±61) ml/m(2), limits of agreement of -16 and +260 ml/m(2), and a percentage error of 22%. Use of the correction-formula recently suggested for the PiCCO(®) device significantly reduced bias and percentage error. Similarly, mean values of parameters derived from GEDVI such as pulmonary vascular permeability index (PVPI; 1.244±0.101 vs. 1.522±0.139; P<0.001) and global ejection fraction (GEF; (24.7±1.6)% vs. (28.1±1.8)%; P<0.001) were significantly different in the case of femoral compared to jugular indicator injection. Furthermore, the mean cardiac index derived from femoral indicator injection ((4.50±0.36) L/(min·m²)) was significantly higher (P=0.02) than that derived from jugular indicator injection ((4.12±0.44) L/(min·m²)), resulting in a bias of (+0.38±0.37) L/(min·m²) and a percentage error of 19.4%.

CONCLUSIONS

Femoral access for indicator injection results in markedly altered values provided by the EV1000/VolumeView(®), particularly for GEDVI, PVPI, and GEF.

Central and peripheral hemodynamics in exercising humans: leg vs arm exercise

In humans, arm exercise is known to elicit larger increases in arterial blood pressure (BP) than leg exercise. However, the precise regulation of regional vascular conductances (VC) for the distribution of cardiac output with exercise intensity remains unknown. Hemodynamic responses were assessed during incremental upright arm cranking (AC) and leg pedalling (LP) to exhaustion (Wmax) in nine males. Systemic VC, peak cardiac output (Qpeak) (indocyanine green) and stroke volume (SV) were 18%, 23%, and 20% lower during AC than LP. The mean BP, the rate-pressure product and the associated myocardial oxygen demand were 22%, 12%, and 14% higher, respectively, during maximal AC than LP. Trunk VC was reduced to similar values at Wmax. At Wmax, muscle mass-normalized VC and fractional O2 extraction were lower in the arm than the leg muscles. However, this was compensated for during AC by raising perfusion pressure to increase O2 delivery, allowing a similar peak VO2 per kg of muscle mass in both extremities. In summary, despite a lower Qpeak during arm cranking the cardiovascular strain is much higher than during leg pedalling. The adjustments of regional conductances during incremental exercise to exhaustion depend mostly on the relative intensity of exercise and are limb-specific.

Consistency of cardiac function index and global ejection fraction with global end-diastolic volume in patients with femoral central venous access for transpulmonary thermodilution: a prospective observational study

Global ejection fraction (GEF) and cardiac function index (CFI) are transpulmonary thermodilution (TPTD)-derived indices of the systolic function. Their validity relies on an accurate determination of the global end-diastolic volume (GEDV). Due to an overestimation of GEDV using a femoral central venous catheter (CVC) a correction formula for indexed GEDV (GEDVI) has been implemented in the latest PiCCO™-algorithm. However, a recent study demonstrated that correction for femoral CVC does not pertain to pulmonary vascular permeability index PVPI, which is calculated of extravascular lung water EVLW and GEDV. Therefore, it was the aim of our study to evaluate, if GEF and CFI are corrected for femoral CVC. In ten adult ICU-patients with PiCCO™-monitoring, ten triplicate TPTDs were performed within 30 h. 95 complete data sets were analyzed, if a GEDV corrected for CVC site was applied to derive CFI and GEF. Therefore, we compared displayed values CFI_displayed and GEF_displayed to CFI_calculated and GEF_calculated, which were calculated from displayed GEDV, cardiac output and stroke volume. GEDV_calculated derived from division of GEDVI by predicted body surface area did not substantially differ from GEDV_displayed (1448 ± 414 ml vs. 1447 ± 416 ml), which suggests a correction of GEDV for CVC site. However, CFI_displayed was significantly lower than CFI_calculated (3.8 ± 1.6/min vs. 5.1 ± 1. 8/min: p < 0.001), suggesting that CFI_displayed is based on an uncorrected GEDV. By contrast, GEF_calculated (23.1 ± 8.7 %) was not substantially different from GEF_displayed (22.4 ± 8.6 %). Although GEDV and GEF are corrected for femoral CVC site, this does not apply to CFI. However, all indices derived from GEDV should be calculated consistently.

Arterial to end-tidal Pco2 difference during exercise in normoxia and severe acute hypoxia: importance of blood temperature correction

Negative arterial to end-tidal Pco₂ differences ((a-ET)Pco₂) have been reported in normoxia. To determine the influence of blood temperature on (a-ET)Pco₂, 11 volunteers (21 ± 2 years) performed incremental exercise to exhaustion in normoxia (Nx, P_Io₂: 143 mmHg) and hypoxia (Hyp, P_Io₂: 73 mmHg), while arterial blood gases and temperature (ABT) were simultaneously measured together with end-tidal Pco₂ (P_ETco₂). After accounting for blood temperature, the (a-ET) Pco₂ was reduced (in absolute values) from −4.2 ± 1.6 to −1.1 ± 1.5 mmHg in normoxia and from −1.7 ± 1.6 to 0.9 ± 0.9 mmHg in hypoxia (both P < 0.05). The temperature corrected (a-ET)Pco₂ was linearly related with absolute and relative exercise intensity, VO₂, VCO₂, and respiratory rate (RR) in normoxia and hypoxia (R²: 0.52–0.59). Exercise CO₂ production and P_ETco₂ values were lower in hypoxia than normoxia, likely explaining the greater (less negative) (a-ET)Pco₂ difference in hypoxia than normoxia (P < 0.05). At near-maximal exercise intensity the (a-ET)Pco₂ lies close to 0 mmHg, that is, the mean P_aco₂ and the mean P_ETco₂ are similar. The mean exercise (a-ET)Pco₂ difference is closely related to the mean A-aDO₂ difference (r = 0.90, P < 0.001), as would be expected if similar mechanisms perturb the gas exchange of O₂ and CO₂ during exercise. In summary, most of the negative (a-ET)Pco₂ values observed in previous studies are due to lack of correction of P_aco₂ for blood temperature. The absolute magnitude of the (a-ET)Pco₂ difference is lower during exercise in hypoxia than normoxia.

Limitations to oxygen transport and utilization during sprint exercise in humans: evidence for a functional reserve in muscle O2 diffusing capacity

To determine the contribution of convective and diffusive limitations to V̇(O2peak) during exercise in humans, oxygen transport and haemodynamics were measured in 11 men (22 ± 2 years) during incremental (IE) and 30 s all-out cycling sprints (Wingate test, WgT), in normoxia (Nx, P(IO2): 143 mmHg) and hypoxia (Hyp, P(IO2): 73 mmHg). Carboxyhaemoglobin (COHb) was increased to 6-7% before both WgTs to left-shift the oxyhaemoglobin dissociation curve. Leg V̇(O2) was measured by the Fick method and leg blood flow (BF) with thermodilution, and muscle O2 diffusing capacity (D(MO2)) was calculated. In the WgT mean power output, leg BF, leg O2 delivery and leg V̇(O2) were 7, 5, 28 and 23% lower in Hyp than Nx (P < 0.05); however, peak WgT D(MO2) was higher in Hyp (51.5 ± 9.7) than Nx (20.5 ± 3.0 ml min(-1) mmHg(-1), P < 0.05). Despite a similar P(aO2) (33.3 ± 2.4 and 34.1 ± 3.3 mmHg), mean capillary P(O2) (16.7 ± 1.2 and 17.1 ± 1.6 mmHg), and peak perfusion during IE and WgT in Hyp, D(MO2) and leg V̇(O2) were 12 and 14% higher, respectively, during WgT than IE in Hyp (both P < 0.05). D(MO2) was insensitive to COHb (COHb: 0.7 vs. 7%, in IE Hyp and WgT Hyp). At exhaustion, the Y equilibration index was well above 1.0 in both conditions, reflecting greater convective than diffusive limitation to the O2 transfer in both Nx and Hyp. In conclusion, muscle V̇(O2) during sprint exercise is not limited by O2 delivery, O2 offloading from haemoglobin or structure-dependent diffusion constraints in the skeletal muscle. These findings reveal a remarkable functional reserve in muscle O2 diffusing capacity.

A new equation to estimate temperature-corrected PaCO2 from PET CO2 during exercise in normoxia and hypoxia

End-tidal PCO2 (PET CO2 ) has been used to estimate arterial pressure CO2 (Pa CO2 ). However, the influence of blood temperature on the Pa CO2 has not been taken into account. Moreover, there is no equation validated to predict Pa CO2 during exercise in severe acute hypoxia. To develop a new equation to predict temperature-corrected Pa CO2 values during exercise in normoxia and severe acute hypoxia, 11 volunteers (21.2 ± 2.1 years) performed incremental exercise to exhaustion in normoxia (Nox, PI O2 : 143 mmHg) and hypoxia (Hyp, PI O2 : 73 mmHg), while arterial blood gases and temperature (ABT) were simultaneously measured together with end-tidal PCO2 (PET CO2 ). The Jones et al. equation tended to underestimate the temperature corrected (tc) Pa CO2 during exercise in hypoxia, with greater deviation the lower the Pa CO2 tc (r = 0.39, P < 0.05). The new equation has been developed using a random-effects regression analysis model, which allows predicting Pa CO2 tc both in normoxia and hypoxia: Pa CO2 tc = 8.607 + 0.716 × PET CO2 [R(2)  = 0.91; intercept SE = 1.022 (P < 0.001) and slope SE = 0.027 (P < 0.001)]. This equation may prove useful in noninvasive studies of brain hemodynamics, where an accurate estimation of Pa CO2 is needed to calculate the end-tidal-to-arterial PCO2 difference, which can be used as an index of pulmonary gas exchange efficiency.

Constant infusion transpulmonary thermodilution for the assessment of cardiac output in exercising humans

To determine the accuracy and precision of constant infusion transpulmonary thermodilution cardiac output (CITT-Q) assessment during exercise in humans, using indocyanine green (ICG) dilution and bolus transpulmonary thermodilution (BTD) as reference methods, cardiac output (Q) was determined at rest and during incremental one- and two-legged pedaling on a cycle ergometer, and combined arm cranking with leg pedaling to exhaustion in 15 healthy men. Continuous infusions of iced saline in the femoral vein (n = 41) or simultaneously in the femoral and axillary (n = 66) veins with determination of temperature in the femoral artery were used for CITT-Q assessment. CITT-Q was linearly related to ICG-Q (r = 0.82, CITT-Q = 0.876 × ICG-Q + 3.638, P < 0.001; limits of agreement ranging from -1.43 to 3.07 L/min) and BTD-Q (r = 0.91, CITT-Q = 0.822 × BTD + 4.481 L/min, P < 0.001; limits of agreement ranging from -1.01 to 2.63 L/min). Compared with ICG-Q and BTD-Q, CITT-Q overestimated cardiac output by 1.6 L/min (≈ 10% of the mean ICG and BTD-Q values, P < 0.05). For Q between 20 and 28 L/min, we estimated an overestimation < 5%. The coefficient of variation of 23 repeated CITT-Q measurements was 6.0% (CI: 6.1-11.1%). In conclusion, cardiac output can be precisely and accurately determined with constant infusion transpulmonary thermodilution in exercising humans.