Determinants of pulmonary blood volume

Computer simulated modeling of healthy and diseased right ventricular and pulmonary circulation

We have previously developed a simulated cardiovascular physiology model for in-silico testing and validation of novel closed-loop controllers. To date, a detailed model of the right heart and pulmonary circulation was not needed, as previous controllers were not intended for use in patients with cardiac or pulmonary pathology. With new development of controllers for vasopressors, and looking forward, for combined vasopressor-fluid controllers, modeling of right-sided and pulmonary pathology is now relevant to further in-silico validation, so we aimed to expand our existing simulation platform to include these elements. Our hypothesis was that the completed platform could be tuned and stabilized such that the distributions of a randomized sample of simulated patients' baseline characteristics would be similar to reported population values. Our secondary outcomes were to further test the system in representing acute right heart failure and pulmonary artery hypertension. After development and tuning of the right-sided circulation, the model was validated against clinical data from multiple previously published articles. The model was considered 'tuned' when 100% of generated randomized patients converged to stability (steady, physiologically-plausible compartmental volumes, flows, and pressures) and 'valid' when the means for the model data in each health condition were contained within the standard deviations for the published data for the condition. A fully described right heart and pulmonary circulation model including non-linear pressure/volume relationships and pressure dependent flows was created over a 6-month span. The model was successfully tuned such that 100% of simulated patients converged into a steady state within 30 s. Simulation results in the healthy state for central venous volume (3350 ± 132 ml) pulmonary blood volume (405 ± 39 ml), pulmonary artery pressures (systolic 20.8 ± 4.1 mmHg and diastolic 9.4 ± 1.8 mmHg), left atrial pressure (4.6 ± 0.8 mmHg), PVR (1.0 ± 0.2 wood units), and CI (3.8 ± 0.5 l/min/m²) all met criteria for acceptance of the model, though the standard deviations of LAP and CI were somewhat narrower than published comparators. The simulation results for right ventricular infarction also fell within the published ranges: pulmonary blood volume (727 ± 102 ml), pulmonary arterial pressures (30 ± 4 mmHg systolic, 12 ± 2 mmHg diastolic), left atrial pressure (13 ± 2 mmHg), PVR (1.6 ± 0.3 wood units), and CI (2.0 ± 0.4 l/min/m²) all fell within one standard deviation of the reported population values and vice-versa. In the pulmonary hypertension model, pulmonary blood volume of 615 ± 90 ml, pulmonary arterial pressures of 80 ± 14 mmHg systolic, 36 ± 7 mmHg diastolic, and the left atrial pressure of 11 ± 2 mmHg all met criteria for acceptance. For CI, the simulated value of 2.8 ± 0.4 l/min/m² once again had a narrower spread than most of the published data, but fell inside of the SD of all published data, and the PVR value of 7.5 ± 1.6 wood units fell in the middle of the four published studies. The right-ventricular and pulmonary circulation simulation appears to be a reasonable approximation of the right-sided circulation for healthy physiology as well as the pathologic conditions tested.

Influence of Thoracic Fluid Compartments on Pulmonary Congestion in Chronic Heart Failure

INTRODUCTION

Pulmonary congestion is a common finding of heart failure (HF), but it remains unclear how pulmonary and heart blood volumes (V_p and V_h, respectively) and extravascular lung water (EVLW) change in stable HF and affect lung function.

METHODS

Fourteen patients with HF (age 68 ± 11 y, LVEF 33 ± 8%) and 12 control subjects (age 65 ± 9 y) were recruited. A pulmonary function test, thoracic computerized tomographic (CT) scan, and contrast perfusion scan were performed. From the thoracic scan, a histogram of CT attenuation of lung tissue was generated and skew, kurtosis, and full-width half-max (FWHM) calculated as surrogates of EVLW. Blood volumes were calculated from the transit time of the contrast through the great vessels of the heart.

RESULTS

Patients with HF had greater V_p and V_h (V_p 0.55 ± 0.21 L vs 0.41 ± 0.13 L; V_h 0.53 ± 0.33 L vs 0.40 ± 0.15 L) and EVLW (skew 3.2 ± 0.5 vs 3.7 ± 0.7; kurtosis 19.4 ± 6.6 vs 25.9 ± 9.4; FWHM 73 ± 13 HU vs 59 ± 9 HU). Spirometric measures were decreased in HF (percentage of predicted: forced vital capacity 86 ± 17% vs 104 ± 9%; forced expiratory volume in 1 second 83 ± 20% vs 105 ± 11%; maximal mid-expiratory flow 82 ± 42% vs 115 ± 43%). V_p was associated with decreased expiratory flows, and EVLW was associated with decreased lung volumes.

CONCLUSIONS

Congestion in stable patients with HF includes expanded V_p and V_h and increased EVLW associated with reductions in lung volumes and expiratory flows.

Optimal Control of Inspired Perfluorocarbon Temperature for Ultrafast Hypothermia Induction by Total Liquid Ventilation in an Adult Patient Model

GOAL

Recent preclinical studies have shown that therapeutic hypothermia induced in less than 30 min by total liquid ventilation (TLV) strongly improves the survival rate after cardiac arrest. When the lung is ventilated with a breathable perfluorocarbon liquid, the inspired perfluorocarbon allows us to control efficiently the cooling process of the organs. While TLV can rapidly cool animals, the cooling speed in humans remains unknown. The objective is to predict the efficiency and safety of ultrafast cooling by TLV in adult humans.

METHODS

It is based on a previously published thermal model of ovines in TLV and the design of a direct optimal controller to compute the inspired perfluorocarbon temperature profile. The experimental results in an adult sheep are presented. The thermal model of sheep is subsequently projected to a human model to simulate the optimal hypothermia induction and its sensitivity to physiological parameter uncertainties.

RESULTS

The results in the sheep showed that the computed inspired perfluorocarbon temperature command can avoid arterial temperature undershoot. The projection to humans revealed that mild hypothermia should be ultrafast (reached in fewer than 3 min (-72 °C/h) for the brain and 20 min (-10 °C/h) for the entire body).

CONCLUSION

The projection to human model allows concluding that therapeutic hypothermia induction by TLV can be ultrafast and safe.

SIGNIFICANCE

This study is the first to simulate ultrafast cooling by TLV in a human model and is a strong motivation to translate TLV to humans to improve the quality of life of postcardiac arrest patients.

Targeting inhaled therapy beyond the lungs

Pulmonary disease has been the primary target of inhaled therapeutics for over 50 years. During that period, increasing interest has arisen in the use of this route of administration to gain access to the systemic circulation for the treatment of a number of diseases beyond the airways. In order to effectively employ this route, the barriers to transport from the lungs following deposition of aerosols must be considered, including the nature of the disease (whether proximal, as in pulmonary hypertension, or distal, as in diabetes). Delivery to the systemic circulation begins with the efficiency of aerosol generation and subsequent deposition in the airways and proceeds to the influence of mechanisms of clearance, including absorption, metabolism, and mucociliary and cell-mediated transport, on the residence time of the drugs in the lungs. The nature of the drug (small or large molecules/low or high molecular weight), susceptibility to degradation and general physicochemical properties play a role in the chemistry of its formulation, physics of aerosol delivery and biology of disposition.

Pavane for a pulse pressure variation defunct

Hemodynamic management of critically ill patients in the ICU or high-risk patients in the operating room has paradoxically shown progress in terms of outcome after the systematic application of volume responsiveness/flow optimization based on pulse pressure variation and/or stroke volume variation during controlled, positive-pressure ventilation in patients without spontaneous respiratory efforts. This assessment of circulatory optimization should ideally be based on an exhaustive, predictive and coherent physiological understanding of the cardiovascular system model. This paper sketches the extremely complex physiological background of the concept of volume responsiveness, concluding that it is not a reliable means of guiding hemodynamic optimization because it is based on a nonexhaustive, nonpredictive and incoherent physiological model.

A model of perfusion of the healthy human lung

This study presents a model that simulates the pulmonary capillary perfusion. The model describes the lungs as divided into horizontal layers and includes: capillary geometry; capillary wall elasticity; pressure at the pulmonary artery; blood viscosity; the effect of the chest wall; the change in lung height and hydrostatic effects of the lung tissue and of the blood during breathing. The model simulates pulsatile blood perfusion with an increasing blood distribution down the lungs, in agreement with previous experimental studies. Moreover the model is in agreement with experimentally measured total capillary perfusion, total capillary volume, total capillary surface area and transition time of red blood cells passing through the pulmonary capillary network. The presented model is the first to be validated against the mentioned experimental data and to model the link between airway pressure, lung volume and perfusion.

Central venous pressure and pulmonary capillary wedge pressure: fresh clinical perspectives from a new model of discordant and concordant heart failure

Heart-failure phenotypes include pulmonary and systemic venous congestion. Traditional heart-failure classification systems include the Forrester hemodynamic subsets, which use 2 indices: pulmonary capillary wedge pressure (PCWP) and cardiac index. We hypothesized that changes in PCWP and central venous pressure (CVP), and in the phenotypes of heart failure, might be better evaluated by cardiovascular modeling. Therefore, we developed a lumped-parameter cardiovascular model and analyzed forms of heart failure in which the right and left ventricles failed disproportionately (discordant ventricular failure) versus equally (concordant failure). At least 10 modeling analyses were carried out to the equilibrium state. Acute discordant pump failure was characterized by a "passive" volume movement, with fluid accumulation and pressure elevation in the circuit upstream of the failed pump. In biventricular failure, less volume was mobilized. These findings negate the prevalent teaching that pulmonary congestion in left ventricular failure results primarily from the "backing up" of elevated left ventricular filling pressure. They also reveal a limitation of the Forrester classification: that PCWP and cardiac index are not independent indices of circulation. Herein, we propose a system for classifying heart-failure phenotypes on the basis of discordant or concordant heart failure. A surrogate marker, PCWP-CVP separation, in a simplified situation without complex valvular or pulmonary disease, shows that discordant left and right ventricular failures are characterized by differences of ≥ 4 and ≤ 0 mmHg, respectively. We validated the proposed model and classification system by using published data on patients with acute and chronic heart failure.

Increased Pulmonary Transit Times in Asymptomatic Dogs with Mitral Regurgitation

Pulmonary transit time (PTT) normalized to heart rate (nPTT) is a measure of the pulmonary blood volume (PBV) to stroke volume ratio (PBV/SV). It is an index of cardiac performance. To determine the effect of compensated mitral regurgitation (CMR) and decompensated mitral regurgitation (DMR) caused by valvular endocardiosis on the index nPTT, we measured nPTT by first-pass radionuclide angiocardiography and ECG in 13 normal dogs, 18 dogs with CMR, and 13 dogs with DMR. PTT was measured as time between onset of appearance of activity at the pulmonary trunk and the left atrium. In the normal dogs, the relationship between PTT and mean R-R interval (mRR) was PTT = 4.08 x mRR + 0.15 (R2 = 0.71). Normal nPTT was 4.4 +/- 0.6 (SD) (range. 3.6-5.3). in CMR, 6.3 +/- 1.6 (SD) (range, 4.0-9.7). and in DMR, 11.9 +/- 3.4 (SD) (range, 8.0-18.8). The differences among all groups were significant. Heart rates were 110 +/- 22 bpm in normal dogs, 111 +/- 20 in dogs with CMR, and 144 +/- 18 in dogs with DMR (P < .001 for difference between DMR group and normal and CMR groups). Increased nPTT in CMR indicates preclinical heart pump dysfunction. Heart rate-normalized pulmonary transit times may be a useful index of heart function in mitral regurgitation.

Determinants of pulmonary blood volume. Effects of acute changes in pulmonary vascular pressures and flow

To examine the effects of pulmonary vascular pressures and flow on pulmonary blood volume (PBV), experiments were performed at constant heart rate and zone 3 conditions (mean left atrial pressure (LAP) above airway pressure) in six anesthetized, open-chest dogs. PBV was calculated as the product of electromagnetic aortic flow and pulmonary mean transit time for ascorbate, obtained without blood withdrawal by polarographic recording of aortic ascorbate changes. In three series of experiments LAP was raised similarly in three steps, from 4.5 to 14.8 mmHg: by mitral constriction which reduced pulmonary blood flow, by blood volume expansion which more than doubled pulmonary blood flow, or by a combination of the two procedures which kept pulmonary blood flow constant. In all three series, LAP and mean pulmonary arterial pressure (PAP) rose in proportion, but PBV was better correlated to PAP (r = 0.87 +/- 0.02) than to LAP (r = 0.66 +/- 0.09). These experiments suggest that PAP is the most important factor in determining PBV under zone 3 conditions, whether PAP is raised by increasing pulmonary blood flow or by mitral constriction.

Radionuclide determined pulmonary blood volume in ischaemic heart disease

Most measurements of pulmonary blood volume have been based on the Stewart-Hamilton dye dilution principle and have required direct catheterisation of the cardiac chambers. Alternatively a precordial counter may be used to detect the composite right and left heart curves after an intravenous injection of radionuclide. We investigated the use of a gamma camera/computer system to determine the radionuclide dilution curves from individual cardiac chambers. Pulmonary transit time and pulmonary blood volume were measured in nine normal subjects, eight patients with angina pectoris but without heart failure, and 13 patients with ischaemic heart disease and left ventricular failure. Patients with heart failure had significantly greater (p less than 0.001) pulmonary blood volumes and pulmonary transit times than normal subjects or patients without heart failure. Reproducibility measurements of pulmonary blood volume, determined in 12 subjects, gave a coefficient of variation of 2.6%. The effect of posture on pulmonary blood volume was determined in six subjects lying supine and tilted at a 45 degree angle. A reduction in pulmonary blood volume in the tilted position was observed in each subject (p less than 0.005). This simple non-invasive measurement should allow more detailed assessment of physiological or pharmacological changes of the pulmonary vascular bed.

Noninvasive detection of left ventricular dysfunction with a portable electrocardiographic gated scintillation probe device

A comparison of left ventricular function data derived from a low cost, portable electrocardiographic gated scintillation probe (nuclear stethoscope) with conventional scintiangiographic data was performed in 68 patients. Ejection fraction correlation (r = 0.86, p less than 0.005) was better in patients with uniform wall motion than in those with regional asynergy (r = 0.68 p less than 0.01). Probe variables reflecting systolic emptying rates, diastolic filling rates and timing intervals, and relative volumes analyzed in combination provided 100 percent sensitivity, specificity, and predictive value in detecting abnormal left ventricular performance. The results suggest that radionuclide angiography with an electrocardiographic gated scintillation probe is a sensitive, rapid and relatively inexpensive portable method of screening for cardiac dysfunction with a yield similar to that from the more costly gamma camera derived scintiangiogram.

Behavior of the pulmonary circulation in the grossly obese patient. Pathogenesis of pulmonary arterial hypertension at an altitude of 2,240 meters

Twenty persons living at an altitude of 2,240 meters were studied in order to examine the relative roles of passive and active factors in the genesis of pulmonary arterial hypertension in obesity (overweight, 75 +/- 39 percent). Pulmonary arterial hypertension was present in 80 percent (16) of the patients (mean pulmonary arterial systolic pressure, 45 +/- 17 mm Hg). In 95 percent (19) of the 20 patients, resistance to pulmonary flow at the end of diastole was increased (estimated mean pulmonary arteriolar resistance, 210 +/- 144 dynes.sec.cm-5; mean pulmonary arterial diastolic-pulmonary wedge pressure gradient 7.86 +/- 1.40 mm Hg). The mean arterial oxygen pressure was 50 +/- 9 mm Hg, the arterial carbon dioxide tension was 37 +/- 6 mm Hg and the arterial pH was 7.42 +/- 0.08. Since the pulmonary arterial systolic pressure has been reasonably predicted (r = 0.91; P < 0.001), it would appear that the compliance of the elastic pulmonary arteries in obese patients follows a normal pattern. The behavior of the right ventricular end-diastolic pressure at rest (mean change, 4.6 mm Hg; P < 0.001) and of the pulmonary wedge pressure (mean change, 4.7 mm Hg; P < 0.001) during passive lifting of the legs was indirect evidence of the increase in pulmonary blood volume. The presence of an abnormal resistance to pulmonary blood flow at the end of diastole is suggestive of a decrease in the distention of the pulmonary microcirculation. The pulmonary arterial diastolic-pulmonary wedge pressure gradient and the pulmonary arterial diastolic pressure were related to arterial oxygen unsaturation (r = 0.70; P < 0.05) but not to the concentration of hydrogen ions; thus hypercapnic acidemia appears as a secondary factor in the genesis of pulmonary arterial hypertension at high atitudes. The explanation could be the relative hyperventilation of high altitudes, with a compensatory metabolic alkalosis. The increased pulmonary blood volume and the alveolar hypoxia are the main causes in the pathogenesis of pulmonary arterial hypertension in the grossly obese patient at this altitude.

Radionuclide evaluation of left ventricular function with nonimaging probes

Portable nonimaging probes have been developed that can evaluate left ventricular function using radionuclide techniques. Two modes of data acquisition are possible with these probe systems, first-pass and gated. Precordial radiocardiograms obtained after a bolus injection can be used to determine cardiac output, pulmonary transit time, pulmonary blood volume, left ventricle ejection fraction, and left-to-right shunts. Gated techniques can be used to determine left ventricular ejection fraction and sytolic time intervals. Probe-determined indices of left ventricular function agree excellently with comparable measurements determined by conventional camera-computer methods as well as by invasive techniques. These have begun to be used in a preliminary manner in a variety of clinical problems associated with left ventricular dysfunction. This review discusses the types of probe systems available, the methods used in positioning them, and details the specifics of their data acquisition and processing capacity. The major criticisms of probe methods are that they are nonimaging and that they measure global rather than regional left ventricular function. In spite of these criticisms, probe systems, because of their portability, high sensitivity, and relatively low cost are useful supplements to conventional camera-computer systems for the measurement of parameters of left ventricular performance using radionuclide techniques.

Pathophysiologic studies of the pulmonary and coronary circulations in man

Studies of the pulmonary circulation in normal man, performed with external radiation detectors, have shown that pulmonary blood volume is about 10% of total blood volume. Pulmonary blood volume was unchanged in patients with acute or chronic left atrial hypertension and in normal persons during expansion of total blood volume in spite of marked increases in pulmonary vascular pressures. However, pulmonary blood volume was greatly increased in patients with polycythemia rubra vera and a large total blood volume and in patients with a left to right shunt but normal pulmonary intravascular pressure. Studies of regional myocardial perfusion with injection of xenon-133 solution into the left coronary artery revealed localized areas of ischemia distal to stenotic lesions even when the patient was at rest. During angina produced by pacing, more severe ischemia occurred, thus suggesting that functional factors reduce local perfusion below resting levels. In patients with "variant" angina, intravenous injection of thallium-201 chloride during spontaneous attacks has revealed large cold areas in myocardial scintigrams not present under control conditions, thus suggesting severe transmural reduction of perfusion in heart muscle corresponding to S-T segment elevation in the electrocardiogram.

Experimental pulmonary embolism; effect on pulmonary blood volume and vascular compliance

Autologous blood clot was used to produce pulmonary macroembolism, and lycopodium spores to produce microembolism in normal mongrel dogs. Pressures were recorded from the pulmonary artery, left atrium and femoral artery; cardiac output and pulmonary blood volume (PBV) were determined using sequential indicator dilution curves from injections into the pulmonary artery and left atrium. Macro- and microembolism caused comparable elevations of pulmonary artery pressure and total pulmonary resistance. Macroembolism with blood clots resulted in marked decreases in PBV and pulmonary vascular compliance. However, microembolism with lycopodium spores caused only small decreases in PBV despite a large reduction in pulmonary vascular compliance. Prostaglandin E 1 infusion after microembolism had no effect on pulmonary hemodynamics, but caused significant systemic hypotension. After macroembolism PGE 1 infusion decreased PBV and decreased systemic arterial pressure.