Interaction between afterload and contractility in the newborn heart: evidence of homeometric autoregulation in the intact circulation

OBJECTIVES

We undertook the present study to determine whether afterload and contractility interact in the hearts of newborn lambs. We specifically investigated whether stepwise increases in afterload increase contractility.

BACKGROUND

Several studies in the isolated and intact adult dog heart have shown that afterload and contractility are not independent determinants of cardiac performance; rather, they interact. Afterload and contractility are unlikely to interact in the newborn heart because the factors that may mediate the interaction in the adult are missing in the newborn.

METHODS

We measured contractility at different steady state levels of afterload in seven newborn lambs under complete anesthesia. Contractility was measured by three different indexes: end-systolic pressure-volume relations (slope and volume position); preload-corrected first derivative of left ventricular pressure (dP/dtmax); and preload-corrected stroke work. Left ventricular pressure and volume were measured with a micromanometer and conductance catheter, respectively. Preload and afterload were manipulated by inflating or deflating a balloon catheter in the inferior vena cava and descending thoracic aorta, respectively. Data are expressed as mean value +/- 1 SD.

RESULTS

Stepwise increases in afterload increased contractility, independent of which of the three indexes was used. The slope of the end-systolic pressure-volume relation increased from a mean baseline value of 4.44 +/- 2.43 to 6.69 +/- 2.89 kPa/ml at the highest level of afterload. Concomitantly, volume at 14 kPa of the end-systolic pressure-volume relation decreased from 3.34 +/- 1.52 ml at baseline to 1.12 +/- 0.83 ml at the highest afterload. The other two indexes showed qualitatively similar changes. Beats selected from unloading interventions on the basis of the same end-diastolic volume for each level of afterload showed no difference in stroke volume.

CONCLUSIONS

This study in newborn lambs demonstrates that stepwise increases in afterload increase contractility considerably and that this enables the heart to maintain stroke volume at different levels of afterload. This forms direct evidence for the existence of homeometric autoregulation in the intact newborn heart.

Determinants of end-systolic pressure during different load alterations in the in situ left ventricle

Because of the strong dependency of the end-systolic pressure-volume relation on the type of transient loading intervention in the in situ left ventricle (LV), experiments in the basal inotropic state in 16 open-chest anesthetized dogs were reanalyzed to find additional variables to model and predict end-systolic pressure (ESP) of both afterloading and preloading interventions by a single equation. Random-coefficients regression analysis was performed on 22 experiments in the basal inotropic state simultaneously, yielding an overall R2 of 0.97. The major part of total variance of ESP was due to linear terms of end-systolic volume (ESV) (74%) and stroke volume (SV) (19%). The SV effect was consistently negative and quantitatively quite important. An average load-independent end-systolic elastance of 6.7 mmHg/ml and an average SV effect of -5.7 mmHg/ml ejected were estimated, separating the "force-length" property from shortening effects in the in situ LV. History-related effects appeared to be only minor.

Dependence of anisotropic myocardial electrical resistivity on cardiac phase and excitation frequency

Knowledge of myocardial electrical resistivity is of interest because passive electrical properties govern the electrotonic spread of current through the myocardium and influence the shape and velocity of the excitation wave. In addition, measurements of myocardial resistivity may provide information about tissue structure and components. The aim of the present study was to determine the excitation frequency dependence and the changes during the cardiac cycle of anisotropic myocardial electrical resistivity. Longitudinal and transverse myocardial resistivity were measured using an epicardial sensor in four open-chest dogs with excitation frequencies in the range of 5-60 kHz. Mean longitudinal resistivity gradually decreased from 313 +/- 49 omega.cm at 5 kHz to 212 +/- 32 omega.cm at 60 kHz, transverse resistivity decreased from 487 +/- 49 to 378 +/- 53 omega.cm. To analyze the phasic changes, we compared mean resistivity (averaged over the full cardiac cycle) with resistivity during four cardiac phases: pre-ejection, ejection, early diastole and late diastole. Longitudinal resistivity was significantly higher during the ejection phase (+9.6 +/- 4.1 omega.cm) and lower during late diastole (-6.9 +/- 2.9 omega.cm). Transverse resistivity was significantly higher during late diastole (+4.0 +/- 2.3 omega.m). The values during the other cardiac phases were not significantly different from mean resistivity. The phasic changes in longitudinal and transverse resistivity during the cardiac cycle were independent of the excitation frequency. We speculate that these changes are related to geometrical changes, especially to changes in myocardial blood volume.

Cardiovascular MR imaging: pressure-gating using the arterial pressure signal from a conventional ferromagnetic micromanometer-tip catheter

We used the arterial pressure wave as a reliable trigger to synchronize MR imaging to the cardiac cycle. Pressure readings and MR images remained undistorted using a conventional ferromagnetic micromanometer-tip catheter. As a safe alternative to triggering to ventricular pressure, this approach allows varying the time relation between the trigger and the cardiac cycle while maintaining the advantages of pressure-gating.

Acute effects of balloon valvuloplasty and pacing on left ventricular performance in children with moderate pulmonary valve stenosis, analysed by systolic and diastolic pressure-volume relationships

Right ventricular overload of volume and/or pressure type may affect left ventricular systolic and diastolic function. This has been shown in animal studies and has been suggested in non-invasive studies in man. Altered geometry of the left ventricle, myocardial hypertrophy and changes in contractile state may be responsible for the change in function. Balloon valvuloplasty is an effective treatment for isolated valvular pulmonary stenosis in children, and results in an immediate decrease of right ventricular systolic pressure. Whether this results in immediate changes in left ventricular performance is unknown. Eight children (age 5.2 to 13.9 years) with moderate pulmonary valve stenosis underwent pulmonary balloon valvuloplasty under general anaesthesia. Left ventricular function measurements before and after valvuloplasty were performed using a combined micromanometer-conductance catheter to obtain end-systolic (ESPVR) and end-diastolic (EDPVR) pressure-volume relationships employing inferior vena cava occlusion both at normal and pacing-induced increased heart rates. Pulmonary valvuloplasty resulted in a decrease in peak systolic right ventricular pressure from 62.8 +/- 13.5 to 34.4 +/- 7.3 mmHg (P < 0.001), without significant changes in left ventricular systolic and end-diastolic pressure, or in cardiac index. The ESPVR was fitted to a linear function to obtain the slope (Ees) and the volume intercept at 75 mmHg (V75). The EDPVR was fitted to an exponential function. At baseline, Ees was 1.68 +/- 0.99 mmHg.ml-1 and V75 was 33.6 +/- 21.8 ml. Neither valvuloplasty nor pacing, which increased mean heart rate from 81 to 112 beats.min-1 (P < 0.001), resulted in significant changes of the parameters Ees or V75.(ABSTRACT TRUNCATED AT 250 WORDS)

Cardiac sympathetic denervation does not change the load dependence of the left ventricular end-systolic pressure/volume relationship in dogs

It has been shown that in the intact canine heart the left-ventricular end-systolic pressure/volume relation (ESPVR) depends on loading conditions: an increase in arterial vascular resistance causes a leftwards shift and a steeper slope of the ESPVR, suggesting an increased inotropic state. Our purpose was to investigate the possible contribution of the sympathetic nervous system to this load sensitivity of the ESPVR, using intact, but denervated, hearts with normal coronary perfusion and afterload. We used two types of loading intervention: venous volume infusion and gradual occlusion of the descending aorta. ESPVRs were obtained in six anaesthetized open-chest dogs, both before and after bilateral ablation of the stellate ganglia. To exclude the influence of heart rate changes, bilateral vagotomy was performed and the heart was paced. The absence of (unpaced) heart rate changes in response to pressure alterations was used to confirm total denervation. Left ventricular pressure was measured with a micromanometer and volume with a conductance catheter. ESPVRs were essentially linear and characterized by their slope (Ees) and volume intercept at 12 kPa (V12). We found that Ees (P < 0.0001) and V12 (P < 0.05) were both significantly different during pressure and volume interventions (0.67 +/- 0.29 and 0.41 +/- 0.18 kPa/ml for Ees and 16.2 +/- 8.2 and 18.2 +/- 8.4 ml for V12 respectively). Denervation did not significantly affect the parameters of the ESPVR obtained by either volume infusion or aortic occlusion. Two-way analysis of variance revealed no significant interactive effect between denervation and intervention, indicating that the sympathetic nervous system does not influence the load dependency of the ESPVR.(ABSTRACT TRUNCATED AT 250 WORDS)

The four-electrode resistivity technique in anisotropic media: theoretical analysis and application on myocardial tissue in vivo

Several aspects of the four-electrode resistivity technique were studied with special emphasis on the theoretical determination of penetration depth and sample volume in anisotropic media such as (cardiac) muscle. Moreover, the presence of a thin disturbing layer on top of the medium under study was analyzed. A four-electrode sensor was developed for the measurement of local myocardial resistivity in two orthogonal directions. The sensor was applied to the epicardium of anesthetized open chest dogs and, as an example, results are given on the frequency dependence (5-60 kHz) and the changes during the cardiac cycle of longitudinal and transverse myocardial resistivity.

The influence of indomethacin on the autoregulatory ability of the cerebral vascular bed in the newborn lamb

Prevention of hyperperfusion of the brain in the perinatal period has been thought to be an important mechanism by which indomethacin reduces the risk for severe periventricular-intraventricular hemorrhage. The present study investigated whether an indomethacin-induced enhancement of the upper limit of cerebral vascular autoregulatory ability in the neonate contributed to this reduction in cerebral blood flow. In seven anesthetized newborn lambs, we measured temporal blood flow velocity (TMFV) in the carotid artery over a wide range of mean aortic blood pressures (MABP) before and 30 min after an i.v. dose of 1 mg/kg indomethacin. TMFV in the carotid artery was used as an estimate for changes in cerebral blood flow. Stepwise changes in MABP of approximately 10 mm Hg were achieved by progressive balloon occlusion of the thoracic aorta or by progressive bleeding. Multiple linear regression analysis of TMFV versus MABP, indomethacin, and the possible interactive effects confirmed that, at MABP values up to 86 mm Hg, indomethacin lowered TMFV of the carotid artery. Above 86 mm Hg, indomethacin reduced the slope of the TMFV-MABP relationship, indicating an improvement of the autoregulatory ability of the cerebral vascular bed. There was a significant interanimal variability. Thus, indomethacin may reduce the risk for PIVH by limiting cerebral blood flow, especially during increased cerebral perfusion pressures, which often occur after birth asphyxia.

Left ventricular stroke volume by single and dual excitation of conductance catheter in dogs

The conductance method employs a multielectrode catheter to measure intracavitary electric conductance from which left ventricular volume is estimated. A dual-excitation method introduced by us uses a more homogeneous electric field and thereby should enable more accurate volume estimation. In six anesthetized open-chest dogs we compared stroke volume obtained from electromagnetic flow probes with the conventional single-excitation method and with the new dual-excitation conductance method. Caval occlusion and left atrial hemorrhage were used to obtain a wide range of stroke volumes. The slope of the relation between stroke volume calculated from the flow probes and from the conductance catheter increased significantly (P < 0.001) from 0.635 with single excitation to 0.835 with dual excitation, but the interanimal variability was not reduced. The linearity of the relation was substantially improved.

Myocardial perfusion and performance after indomethacin administration in newborn lambs

Indomethacin is a drug widely used to achieve pharmacologic closure of a patent ductus arteriosus in the premature infant. In several vascular beds (brain, kidney, intestine), indomethacin has been shown to cause vasoconstriction. Possible negative effects on myocardial blood flow and performance could be deleterious in premature infants with limited cardiac reserve. Before, during, and 30 and 60 min after administration of 1 mg.kg-1 of indomethacin in nine newborn lambs, we measured coronary blood flow velocity (Doppler flow probe around the left circumflex coronary artery), left ventricular (LV) pressure (by tip manometer) and volume (by conductance catheter technique), cardiac output, arterial pressure, arterial and venous saturations and calculated systemic and coronary vascular resistance, LV systolic function by the end-systolic pressure-volume relationship, and myocardial oxygen extraction. To investigate the effect of indomethacin on the flow regulation of the coronary vascular bed, we measured coronary flow and LV function under different levels of myocardial demand, achieved by stepwise occluding of the descending aorta. During indomethacin infusion, coronary and systemic vascular resistance increased significantly (by 43 and 76%, respectively), resulting in an increase in arterial pressure from 10.2 to 16.9 kPa, whereas neither coronary flow nor LV systolic function changed despite the increase in afterload. Thirty and 60 min after indomethacin, coronary and systemic vascular resistance had returned to baseline levels and LV systolic function remained unchanged. The relationship between coronary flow and cardiac demand was not different before or after indomethacin.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of coronary occlusion and reperfusion on local electrical resistivity of myocardium in dogs

The effect of coronary occlusion and reperfusion on myocardial electrical resistivity was studied in nine anesthetized open-chest dogs. Anisotropic resistivity was measured on the anterior free wall of the left ventricle (LV) before (control) and during transient occlusion of the left anterior descending (LAD) coronary artery, and during reperfusion. To measure local resistivity longitudinal (RL) and transverse (RT) to epicardial muscle fiber direction, a sensor was developed based on the four electrode (FE) technique with an electrode distance of 1 mm. Previous calculations showed that measurements with this system were confined to a 2-mm-thick epicardial layer. Control values for RL and RT were 243 +/- 32 ohms.cm and 358 +/- 45 ohms.cm (mean +/- SD, n = 9) respectively. During a 2-min LAD occlusion, RL increased gradually by 12.4% (p < 0.05) and RT by 7.8% (p < 0.05) above the preceding control values. During a 5-min reperfusion period resistivities returned towards control values, but tended to remain elevated. RL showed a slight initial further increase during the first min of reperfusion and remained significantly above control values during 3 min of reperfusion. RT returned to values not significantly different from control after about 1 min of reperfusion.

Heart rate fall during acute hypoxemia: a measure of chemoreceptor response in fetal sheep

The peripheral arterial chemoreceptor response to hypoxemia in the fetus is predominantly cardiovascular, invoking a fall in heart rate and a variable change in blood pressure. No quantifiable measure of chemoreceptor activity has yet been described in the intact fetus. We described the course, quantified the overall response, and defined the reproducibility of the heart rate response to acute hypoxemia in 22 late-term unanesthetized fetal sheep. Fetuses were chronically instrumented and studied between 1-6 days postoperatively. Acute hypoxemia was induced by occluding a balloon cuff around the common hypogastric artery. We performed 151 occlusions, starting at an initial saturation of 66 +/- 11%, decreasing saturation by 8-50%. Soon after balloon inflation, arterial oxygen saturation fell, followed by a decrease in heart rate. We calculated delta HR/delta sat, the fall in heart rate divided by the fall in saturation. Multiple linear regression analysis showed a sensitive chemoreflex, delta HR/delta sat averaging 2.5 +/- 1.2 bpm.%saturation-1. Initial saturation did not alter the first phase of the response (from the onset of the decrease in oxygen saturation to the onset of the decrease in oxygen saturation to the onset of the decrease in heart rate), but it did increase the overall response (delta HR/delta sat) when saturation was less than 65%. After adjusting in the lower range of initial saturations to that predicted at 65%, delta HR/delta sat was very reproducible within animals, with intra-animal variance being only 7% of inter-animal variance.(ABSTRACT TRUNCATED AT 250 WORDS)

Left ventricular filling after long-term angiotensin converting enzyme inhibition in congestive heart failure

As a rule, left ventricular relaxation is impaired in patients with coronary artery disease and congestive heart failure. In addition, the passive elastic properties in early and late diastole change when the ventricle dilates. Diastolic properties of the left ventricle were studied in 11 patients with congestive heart failure class II-IV (NYHA) before and 3 months after 10-20 mg enalapril was added to their regimen of salt restriction, a diuretic and occasionally digitalis. Haemodynamic studies were performed using radionuclide angiography and simultaneous pressure-volume measurements. Systemic vascular resistance decreased from 1479 to 1182 dynes.s.-1 cm-5 (P < 0.05) and left ventricular end-diastolic pressure from 19.2 to 15.9 mmHg (P < 0.05). Left ventricular end-diastolic volume index decreased from 130 +/- 22 to 81 +/- 22 ml (P < 0.01). Indices of early diastolic relaxation, such as peak filling rate (1.43 +/- 0.46 to 1.49 +/- 0.84 EDV/s), time to peak filling rate (460 +/- 70 to 490 +/- 70 ms), peak negative dP/dt (-903 +/- 190 to -891 +/- 190 mmHg/s) and tau, the time constant of isovolumic pressure decay (58.7 +/- 14.4 to 48.4 +/- 15.2 ms) did not change significantly. In nine patients pressure-volume loops shifted to the left in all patients but one due to reduction in end-systolic and end-diastolic volume. The steepness of the diastolic part of the pressure-volume relationship increased, indicating an increase in chamber stiffness. The stiffness constant increased about 25% towards a more normal value. The alteration in stiffness seemed to be mainly due to the change of the geometry of the ventricle and not to a major change in the visco-elastic properties of the ventricular wall. In conclusion, regression of remodelling induced by enalapril does not change diastolic function parameters in patients with chronic congestive heart failure beyond the changes caused by regression of ventricular dilation.

Left ventricular segmental volume by conductance catheter and Cine-CT

The ability of the conductance catheter method to measure left ventricular segmental and total volume was evaluated by comparison with the Cine-CT technique. In the seven dogs studied, 19 conductance catheter and simultaneous Cine-CT runs were obtained. High correlation coefficients were found for total volume and segmental volumes, except in the basal segment. However, in most cases there was a significant variability in slope and intercept between animals. Both methods are promising tools for estimating dynamic segmental left ventricular volume, each having specific advantages such as a continuous signal (conductance catheter) or anatomic detail (Cine-CT). However, the results also show the need for further improvement of both methods.

The end-systolic pressure-volume relationship in young animals using the conductance technique

Evaluation of ventricular performance by the end-systolic pressure-volume relationship (ESPVR) has been extensively performed in the adult heart using the conductance technique. We undertook this study to validate the conductance technique and to generate ESPVRs in the small heart. To validate the technique, we simultaneously measured left ventricular volume by the conductance catheter and biplane cineangiography in nine piglets during changes in volume and contractility. Raw conductance volumes correlated highly with cineangiographic volumes (R = 0.97), and the slope was near identity (1.11 +/- 0.04). However, 'alpha Vc-corrected' volumes correlated less well (R = 0.85), probably because of errors induced by the saline technique for alpha Vc. We evaluated the ESPVR in nine lambs by inferior vena cava (IVC) occlusion, aortic occlusion, and volume infusion at rest and during changes in contractility. Reliable and linear ESPVRs were obtained in almost all IVC and aortic occlusions but not in volume infusions. Neither slope (Ees) nor position (V14) significantly changed over time or with dobutamine, but both changed after propranolol, supporting studies showing a limited contractile reserve in the newborn. However, Ees was 25% less steep when generated by IVC occlusion as compared to aortic occlusion. We conclude that the ESPVR can be reliably generated in the small heart using the conductance technique, but that it is sensitive to the loading technique.

Ventricular pressure-volume relations in vivo

A number of fundamental mechanical properties of cardiac muscle as well as of the total ventricle are discussed. These include: Starling's law, shortening deactivation, homeometric autoregulation, and so-called hyperactivation associated with a small amount of ejection. Whenever appropriate, muscle properties are related to those of the intact ventricle. The phenomenon of load-dependence of the end-systolic pressure-volume relationship (ESPVR) receives particular attention and is placed within the framework of shortening deactivation and homeometric autoregulation. However, this effect also leads to a more basic definition of myocardial contractility, based on the observed, rather parallel shift of the ESPVR with different afterload conditions. Using this definition, the conclusion is drawn that an increase in afterload is, in fact, met by the left ventricle increasing its inotropic state. Finally, within this approach, it is proposed that the value of end-systolic volume at a chosen constant end-systolic pressure (e.g. 13 kPa or 100 mmHg) be used to characterize changes in myocardial contractility in patients after an intervention. The way of obtaining this parameter is explained.

Single and dual excitation of the conductance-volume catheter analysed in a spheroidal mathematical model of the canine left ventricle

The conductance method employs a multi-electrode catheter to generate an electrical field and measure intracavitary segmental conductances. Left ventricular (LV) volumes are calculated using an algorithm which assumes the electrical field to be homogeneous. This assumption may be violated leading to a non-linear relation between conductance-derived and true volumes. In addition, this relation may vary between segments. A new method is introduced which uses a more homogeneous field. Volume estimates using the conventional single excitation and the new dual excitation method were compared in a mathematical model of a canine LV, which was varied over a large volume range. With single excitation the slope factors, relating conductance-derived and true volumes, varied from 0.50 to 0.76 between segments and was 0.65 for total LV volume. Using dual excitation the segmental slope variability was reduced (range: 0.74-0.77) and the slope factor for total volume increased to 0.76. The linearity of the relation between conductance-derived and true volume was improved with dual excitation and extended over a larger range.

Chemoreceptor responsiveness in fetal sheep

Fetal peripheral chemoreceptor responses to arterial O2 saturation and changes in PCO2 have not yet been quantitated. In 24 late-term chronically instrumented fetal sheep, we measured the heart rate response to acute hypoxemia induced by uterine arterial occlusion at various resting O2 saturations (25-86%) and at induced reductions and increases in baseline O2 saturation. As an index of fetal chemoreceptor responsiveness we calculated the fall in heart rate divided by the fall in arterial O2 saturation (delta HR/delta sat). delta HR/delta sat was inversely related to resting O2 saturation at levels less than 65%, but greater than 65% this relationship was no longer present. However, an induced increase in baseline O2 saturation from 66 +/- 12 to 76 +/- 10% decreased delta HR/delta sat from 2.6 +/- 1.6 to 1.8 +/- 1.0, indicating that when resting O2 saturation is greater than 65% there may be adaptation of peripheral chemoreceptors. Below 65%, an induced decrease in baseline O2 saturation increased delta HR/delta sat (to 3.8 +/- 1.8), suggesting a lack of adaptation to lower O2 saturations. Concomitant changes in PCO2, or differences in baseline PCO2, did not affect delta HR/delta sat during uterine arterial occlusion, which suggests that there is no interdependence between O2 and CO2 as a stimulus for the fetal peripheral chemoreceptor. However, acute hypercapnia (n = 24 in 8 fetal sheep) induced bradycardia. Furthermore, this bradycardia was related to the increase in fetal arterial PCO2. We conclude that the fetal peripheral chemoreceptor is sensitive to hypoxemia and hypercapnia and that the hypoxemia response is accentuated with decreases in initial O2 saturation.

Systolic and diastolic pressure-volume relationships during cardiac surgery

Seven patients undergoing elective coronary artery bypass surgery were studied to assess left ventricular (LV) performance by pressure-volume loops. LV pressure was measured by micromanometry and instantaneous LV volume by a conductance catheter. Continuous pressure-volume relationships were determined during preload reduction before and after cardiopulmonary bypass (CPB). End-systolic elastance (Ees), as the slope of the end-systolic pressure-volume relationship (ESPVR), and diastolic elastance (Ed) were calculated from these interventions. Changes in position of the Ees were assessed at V75, the value of LV end-systolic volume at 75 mm Hg of LV pressure. From pre-CPB to post-CPB, Ees increased in three patients with a decrease of V75 in two patients, and Ees decreased in four patients with a concomitant increase in V75. Ed increased significantly (P less than 0.01) following CPB, demonstrating a decrease of ventricular distensibility. It is concluded that continuous measurement of LV pressure-volume relationships using the conductance catheter is feasible and may be a useful tool to estimate LV performance during cardiac surgery.

Acceleration of blood flow velocity in the carotid artery and myocardial contractility in the newborn lamb

We investigated the influence of quantitative changes in myocardial contractile state, reflected by changes in the end-systolic pressure-volume relationship (its slope and volume intercept) and by changes in the slope of the relationship between change in pressure per unit time and end-diastolic volume induced by beta-adrenergic stimulation or inhibition, on the Doppler derived blood flow velocity wave form of the carotid artery, using a newborn lamb model. Acceleration time of the velocity wave form was investigated during control state I, during 4 and 8 micrograms/kg/min dobutamine infusion, during control state II, and during 0.5 mg/kg propranolol infusion, respectively. Using multiple linear regression analysis with dummy variables, confounding effects such as interanimal variability were removed. Acceleration time showed a strong relationship to both the slope and the volume intercept of the end-systolic pressure-volume relationship and to the change in pressure per unit time-end-diastolic volume relationship. The relations appeared to be independent of aortic pressure and relative resistance in the vascular bed of the carotid artery. These results indicate that acceleration of cerebral blood velocity may prove to be useful in assessing changes in myocardial contractile state of the newborn.

The end-systolic pressure-volume relationship in the newborn lamb: effects of loading and inotropic interventions

Indices of global systolic performance of the newborn left ventricle exceed those of the adult, despite isolated tissue studies showing immature contractile mechanisms. To evaluate contractility in situ, we investigated the end-systolic pressure-volume relationship (ESPVR) by the conductance technique in nine newborn lambs. After percutaneous placement of catheters, we generated ESPVR by inferior vena cava occlusion, aortic occlusion, and volume infusion in two control states, during three levels of dobutamine infusion, and after propranolol. We performed linear and nonlinear regression analyses of the end-systolic points and derived the slope (Ees) and volume at 14 kPa pressure. We found that reliable ESPVR could be obtained in almost all inferior vena cava and aortic occlusions (50 of 51 in each), but in only 18 of 27 volume infusions. Overall, linear regressions adequately defined the ESPVR (75 of 102 were not statistically different than nonlinear regressions; of those different, the mean linear R2 was 0.934 +/- 0.048). By multiple regression analysis, neither Ees nor volume at 14 kPa significantly changed with dobutamine, but both changed after propranolol (23% less than control and 54% greater, respectively), supporting previous studies showing a limited contractile reserve in the newborn secondary to high resting beta-adrenergic tone. Neither Ees nor volume at 14 kPa was different between control states. However, Ees was 25% less steep when generated by inferior vena cava than by aortic occlusion. We conclude that the ESPVR can be generated reliably and reproducibly in the newborn lamb and is relatively linear and sensitive to changes in contractility, but that it is also sensitive to the technique of load intervention.

Indomethacin-induced changes in renal blood flow velocity waveform in premature infants investigated with color Doppler imaging

Renal dysfunction has been recognized as an adverse effect of indomethacin treatment and is probably secondary to impairment of renal blood flow. We therefore evaluated renal artery blood flow velocity in 15 premature infants with a symptomatic ductus arteriosus before and during the first 12 hours after a single intravenous dose of 0.1 mg/kg of indomethacin. Renal artery blood flow velocity was measured serially by color-Doppler flow imaging and used as a qualitative measure of true renal blood flow. Indomethacin administration led to a sharp decrease in peak systolic flow velocity and temporal mean flow velocity of the renal artery. This effect was maximal at 10 minutes after indomethacin dosing; the flow velocities showed a slow recovery, reaching baseline values again at 2 hours after indomethacin dosing. We conclude that indomethacin can affect renal blood supply in the premature infant for a period of at least 1 hour after indomethacin treatment.

Influence of angiotensin converting enzyme inhibition on pump function and cardiac contractility in patients with chronic congestive heart failure

Eleven patients with coronary artery disease and chronic heart failure were studied before and three months after the angiotensin converting enzyme inhibitor enalapril was added to their frusemide medication. The following were measured: left ventricular pressure and volume with transient occlusion of the inferior vena cava, radionuclide angiography, and hormone concentrations in plasma. As in other reported studies, the clinical condition of the patients improved and their exercise tolerance increased moderately. Addition of enalapril reduced end diastolic and systolic pressure, reduced ventricular volume, and concomitantly increased the ejection fraction. The end systolic pressure-volume relation shifted to the left as it did in a similar animal study. In the animal study unloading by a vasodilator did not induce a leftward shift, so it can be inferred that in the present study unloading combined with a decrease in the angiotensin concentration was instrumental in remodelling the heart. Though unloading was expected to have a beneficial effect on the oxygen supply/demand ratio of the heart, the patients still showed the same drop in the ejection fraction during exercise as they did before treatment with enalapril, and early diastolic filling did not improve. Normally, regression of cardiac dilatation is only found if pump function improves; the present study showed that unloading in combination with angiotensin converting enzyme inhibition reshapes the ventricle without improving intrinsic pump function.

Nonlinearity and load sensitivity of end-systolic pressure-volume relation of canine left ventricle in vivo

The effects of mechanical changes in loading conditions on the left ventricular end-systolic pressure-volume relation (ESPVR) were studied in nine open-chest dogs, including three dogs studied before and after beta-adrenergic blockade. Left ventricular pressure was measured with a micromanometer, and left ventricular volume was measured with a conductance catheter. ESPVRs were obtained by increasing left atrial inflow over wide volume ranges (as much as threefold) under three different conditions: control or high or low aortic impedance. High impedance was obtained by occlusion of the descending aorta, and low impedance was obtained by a shunt between the subclavian artery and the left atrium. In the unblocked animals in 21 of 28 runs, a second-order polynomial equation gave a better fit for the ESPVR than a linear relation. To quantify the effects of the changes in aortic impedance on the ESPVR, we calculated from the quadratic equation its volume intercept (V18) and its local slope (E18) at an end-systolic pressure (Pes) of 18 kPa. In the unblocked animals, a statistically significant difference was found in V18 between low impedance (21.50 +/- 6.27 ml) and high impedance (14.10 +/- 8.98 ml; p less than 0.005) and between control (19.14 +/- 9.58 ml) and high impedance (p less than 0.05). In most dogs, E18 was increased at high and decreased at low impedance, but not significantly. In the additional experiments with beta-blockade, the nonlinearity diminished somewhat, but the load dependency of the ESPVR remained present after beta-blockade because the same leftward shift of the ESPVR with high aortic impedance was found. Two other relations, namely, of dP/dtmax and of stroke work versus end-diastolic volume, were also investigated, which on the whole showed the same behavior as the ESPVR. These results indicate that the ESPVR and dP/dtmax-Ved and stroke work-end-diastolic volume relations, when studied over a wide volume range, are nonlinear and that changes in loading conditions influence indexes of contractility derived from these relations, especially the volume intercepts, in such a way that an increase in aortic impedance may be interpreted as an increase in contractility. Blocking the beta-adrenergic receptors did not influence the load dependency of the ESPVR but, in some cases, tended to decrease the nonlinearity in concordance with the relation between contractility and nonlinearity in isolated hearts.

Comparison between force-velocity and end-systolic pressure-volume characterization of intrinsic LV function

We reanalyzed experiments in in situ hearts of 16 open-chest anesthetized dogs, in which two different loading interventions were performed, i.e., an occlusion of the descending aorta (InP) and a rapid volume infusion (InV). Previous studies had demonstrated that the end-systolic elastance (Ees) of the InP was substantially larger than the Ees of the InV suggesting either a load dependency of Ees as such, or an increase in contractility during InP. The data were reanalyzed in the light of the muscular pump concept by plotting peak normalized velocity of circumferential shortening versus a global representative force approximating the left ventricle by a sphere. In all but one experiment the points of the two interventions are located on a single relationship over a very broad range of forces (from 397 to 2,461 g between the control states of experiments and from 602 to 3,278 g difference between control and highest load within experiments). The virtual independence of the force-velocity relation (FVR) and the dependence of the end-systolic pressure-volume relation (ESPVR) on the type of loading intervention can be ascribed to the fact that the former is assessed early during ejection and is therefore less influenced by shortening deactivation and internal resistance than the ESPVR. We conclude that the FVR offers a more consistent characterization of intrinsic LV function than the ESPVR.

Effect of indomethacin on superior mesenteric artery blood flow velocity in preterm infants

In 15 preterm infants with symptomatic patent ductus arteriosus, blood flow velocity changes in the superior mesenteric artery were investigated with Doppler ultrasound just before and during the first 12 hours after a single dose of indomethacin. Indomethacin administration led to an instantaneous decrease in all infants of temporal mean flow velocity in the superior mesenteric artery, which was maximal 10 minutes after administration of indomethacin, followed by a more sustained recovery, slightly greater than baseline values, 12 hours after indomethacin treatment. Simultaneously determined temporal mean flow velocity of the anterior cerebral artery, used as an indicator of changes in cerebral blood flow, had a similar pattern as in the mean flow velocity in the superior mesenteric artery (r = 0.49; p less than 0.001). Our data suggest that indomethacin lowered blood supply to the bowel, similar to its action on cerebral blood flow.

Cerebral blood flow velocity changes in preterm infants after a single dose of indomethacin: duration of its effect

Indomethacin decreases cerebral blood flow velocity and blood flow in the preterm infant. The duration of this negative effect has not been established. Cerebral blood flow velocity was evaluated in 24 preterm infants with symptomatic patent ductus arteriosus before and during the first 12 hours after a single intravenous dose of indomethacin, 0.1 mg/kg. Cerebral blood flow velocity was estimated by serial Doppler investigations of the anterior cerebral arteries. Indomethacin administration led to an instantaneous decrease of peak systolic flow velocity, temporal mean flow velocity, and end-diastolic flow velocity of the anterior cerebral arteries in all infants, which was maximal between 2 and 40 minutes after indomethacin administration and was followed by a more sustained recovery of all velocities to baseline values. Temporal mean flow velocity was not different from pre-indomethacin values at 3 hours after the administration. It is concluded that indomethacin can impact the cerebral circulation of the preterm infant for at least 2 hours. This may have consequences in preterm infants with unstable hemodynamics and pulmonary function.

Cerebral blood-flow velocity during the first week of life of preterm infants and neurodevelopment at two years

Disturbances in perinatal cerebral perfusion appear to be associated with unfavourable neurodevelopmental outcome. Using transcutaneous Doppler technique, the authors investigated cerebral blood-flow velocity patterns in the anterior cerebral artery (ACA) of an intensive care-unit population of preterm infants during the first week of life. The results were correlated with neurodevelopmental outcome at two years of age. Children with major disability at two years of age had significantly higher pulsatility index (PI) values, mainly increased peak systolic flow velocity (PSFV), compared with children with normal or more favourable outcome. End diastolic flow velocity and area under the velocity curve values of the ACA did not differ between the groups, indicating that cerebrovascular resistance and cerebral blood-flow were not different. It is thought that the higher PI and PSFV values were caused by increased compliance of the vascular bed supplied by the ACA, possibly induced by congestion and oedema of the periventricular white-matter due to ischaemic lesions, which also cause periventricular leukomalacia.

Does caffeine affect cerebral blood flow in the preterm infant?

Caffeine, used for treatment of idiopathic apnea in preterm infants, may have a vasoconstrictive effect on cerebral vessels. The ensuing reduction in cerebral blood flow may play a role in the pathogenesis of ischemic brain damage. In 25 preterm infants possible changes in cerebral blood flow due to caffeine administration were assessed using Doppler ultrasound. During caffeine treatment PaCO2 was reduced. However, no changes were found in cerebral blood flow velocity suggesting absence of major changes in cerebrovascular resistance and actual cerebral blood flow following caffeine medication.

Sensitivity of left ventricular end-systolic pressure-volume relation to type of loading intervention in dogs

The influence of different types of loading intervention on the resulting end-systolic pressure-volume relation (ESPVR) of the left ventricle in situ was investigated in anesthetized open-chest dogs. Left ventricular volume was measured by conductance catheter and pressure was measured by tip-micromanometer. Two loading interventions were applied: a pressure intervention (INp) by gradually occluding the descending aorta and a volume intervention (INv) by rapidly infusing 180 ml blood into the vena cava. The pressure-volume loops during an intervention always showed a linear ESPVR, the slope of which, Ees, was calculated. Possible method-related artifacts were assessed and could be excluded. Results from 16 dogs show that Ees(INp) was always larger (1.37 +/- 0.45 kPa/ml) than Ees(INv) (0.73 +/- 0.32 kPa/ml) (p less than 0.001). This difference was enhanced by beta-blockade through metoprolol. The same phenomenon was found for the slope (SdPV) of the relation between dP/dtmax and end-diastolic volume: SdPV(INp) = 45.17 +/- 22.63 kPa/ml/sec and SdPV(INv) = 20.55 +/- 11.13 kPa/ml/sec. In seven dogs, a right heart bypass was performed to study the influence of stroke volume on the ESPVR by applying a pressure intervention under three conditions: with constant end-diastolic volume (decreasing stroke volume), Ees = 2.27 +/- 0.79 kPa/ml; with constant stroke volume, Ees = 1.59 +/- 0.51 kPa/ml; and with increasing stroke volume (and increasing end-diastolic volume), Ees = 1.36 +/- 0.49 kPa/ml. Analysis of variance revealed a statistically significant relation between Ees and stroke volume (p less than 0.01). From the right heart bypass experiments, we conclude that shortening-related deactivation plays a role in the observed behavior of the ESPVR. However, the results from the series with intact circulation indicate that aortic occlusion has an additional effect on the slope of the ESPVR, leading to increased myocardial inotropism, perhaps mediated through a peripheral stimulus in response to decreased perfusion.

Identifiability of left ventricular end-systolic pressure-volume relationships

A widely accepted model of the left ventricle (LV) consisting of a time-varying elastance and a nonlinear internal resistance was investigated to make inferences about the identifiability of its parameters by means of simulated experiments. We aimed to retrieve maximum elastance (Emax) and dead volume (Vd) by the usual slope method or end-systolic pressure-volume relations (ESPVR) and by model-based parameter identification. The ESPVR deviated increasingly from the assigned values with increasing internal resistance depending on the type of loading intervention. Model-based parameter identification proved to be hampered by considerable error propagation if applied to single contractions with noise on the data. Better results were obtained by reducing the number of parameters to be estimated or by combining contractions with different loading conditions. The LV model was also matched with experimental data in three open-chest anesthetized dogs when both methods of estimation were used. The trend of ESPVR was in accordance with the model predictions, with larger Emax and larger Vd observed with arterial rather than with venous loading. Inclusion of an internal resistance in the classical elastance model can explain the dependence of the ESPVR on the type of loading intervention. However, application of model-based parameter identification indicates that the model fails to represent the entire systolic pressure-volume time course of the in situ LV.

The influence of abnormal blood gases on cerebral blood flow velocity in the preterm newborn

The influence of hypercarbia, combined hypercarbia and hypoxemia, and hypocarbia on cerebral blood flow of preterm infants with a gestational age of less than 34 weeks was investigated by measuring peak systolic flow velocity (PSFV), end diastolic flow velocity (EDFV), pulsatility index (PI) and area under the velocity curve (AUVC) of the anterior cerebral artery (ACA) using transcutaneous Doppler technique. Mild and moderate hypocarbia did not change cerebral blood flow velocity. During severe hypercarbia (PaCO2 greater than or equal to 6.7 kPa) significantly lower PI-values were detected, mainly caused by an increase of EDFV, indicating a decreased cerebrovascular resistance and increased cerebral blood flow. A highly significant decrease in PI-values during combined severe hypercarbia and hypoxemia (PaO2 less than 6.0 kPa) was found suggesting that hypoxemia superimposed on hypercarbia strengthens the increase of cerebral blood flow. While the increase of EDFV is thought to be due to carbon dioxide-induced vasodilation of cerebral arterioles, we assume that the accompanied increase of PSFV during combined hypercarbia and hypoxemia is caused by an increase in cardiac output due to hypoxemia which can alter the blood flow velocity wave form of the ACA.

Aetiological rôle of cerebral blood-flow alterations in development and extension of peri-intraventricular haemorrhage

The aetiology and extension of peri-intraventricular haemorrhage (PIVH) are thought to be related to cerebral blood-flow alterations, and especially to increased cerebral blood-flow and fluctuating velocity of blood-flow. Using transcutaneous Doppler technique, the authors investigated cerebral blood-flow in 60 infants with gestations of less than 34 weeks. Pulsatility index (PI) and area under the velocity curve (AUVC) of the anterior cerebral arteries (ACA) were used as qualitative measures of cerebral blood-flow, and the coefficient of variation of PI and AUVC as indicators of fluctuations in blood-flow velocity. A reasonable correlation was found between PI and AUVC and their coefficients of variation in the ACA. First onset of PIVH was related to fluctuating blood-flow velocity, and extension of PIVH with both increased velocity (indicating increased blood-flow) and fluctuating velocity. Increased cerebral blood-flow and its fluctuating pattern were positively correlated with arterial carbon dioxide tension.

Cerebral blood flow velocity pattern in healthy and asphyxiated newborns: a controlled study

In a controlled study serial determinations of cerebral blood flow velocity using Doppler ultrasound and repeated real-time ultrasonographic- or computerized axial tomographic studies of the brain were performed in 17 (nearly) full-term newborns who experienced perinatal asphyxia and in 17 healthy matched controls during the first week of life. A higher cerebral blood flow velocity was found during the first 4 days of life, indicating a lower cerebrovascular resistance in the asphyxiated infants compared to the control infants. These haemodynamic changes coincided with cerebral oedema and neurological abnormalities. It is speculated that the changes in the cerebral circulation in asphyxiated infants are at least partly caused by cerebral oedema-induced increase of intracranial pressure due to severe perinatal asphyxia. Serial Doppler ultrasound investigations of the brain may be a useful non-invasive method for early detection and follow-up of the consequences of severe perinatal asphyxia.

Accuracy of volume measurement by conductance catheter in isolated, ejecting canine hearts

We evaluated the accuracy of the recently reported technique of estimating intraventricular volume by measurement of intracavitary electrical conductance in six isolated, ejecting, canine left ventricles. Left ventricular volumes were measured directly by a previously validated servosystem that employed an electroconductive balloon placed in the left ventricular cavity. The volume measured continuously by the balloon method (Vbal) was compared with that estimated by the conductance method (Vcath). For this test, the hearts were made to eject and fill physiologically by the use of a previously described computer-simulated arterial loading system. Complex ejection and filling patterns were created by stimulating the atrium mechanically, which resulted in irregular arrhythmatic contractions spanning a wide range of volumes. We found that there was a highly linear relationship (r2 = .982 +/- .014) between Vbal and Vcath: Vcath = 0.82 (+/- .05) Vbal + 26.7 (+/- 11.8) ml. Despite the wide variation in the offset term of this relationship among the different hearts, the offset within a given heart was predicted within 3.5 ml by a previously detailed "dilution" method that is applicable to the heart in situ within a closed thorax. Thus, since the offset term is obtainable in situ, the conductance method provides a signal that is proportional to the actual volume. To determine whether right ventricular volume influenced the accuracy of left ventricular measurement, we compared the relationship between Vcath and Vbal obtained with right ventricular volumes of 0 and 30 ml. Increasing the right ventricular volume shifted the relationship upward by less than 3 ml in the working range.(ABSTRACT TRUNCATED AT 250 WORDS)

Continuous measurement of left ventricular volume in animals and humans by conductance catheter

An eight-electrode conductance catheter previously developed by us and used to determine stroke volume in dogs was applied in human beings and dogs to measure absolute left ventricular volume quantitatively. For calibration we developed the formula V(t) = (1/alpha)(L2/sigma b)G(t) - Vc, where V(t) is time-varying left ventricular volume, alpha is a dimensionless constant, L is the electrode separation, sigma b is the conductivity of blood obtained by a sampling cuvette, and G(t) is the measured conductance within the left ventricular cavity. Vc is a correction term caused by the parallel conductance of structures surrounding the cavity and is measured in two ways. The first method, applicable in the anesthetized animal, consists of temporary reduction of volume to zero by suction. The second method uses a transient change in sigma b by injection of a small bolus of hypertonic saline (dogs) or 10 ml of cold glucose (humans) into the pulmonary artery. The validity of the formula was previously established for the isolated postmortem canine heart. The predicted linearity, slope constant alpha, and accuracy of Vc for the left ventricle in vivo were investigated by comparing the conductance volume data with results from independent methods: electromagnetic blood flow measurement for stroke volume and indicator dilution technique for ejection fraction (dogs), thermal dilution for cardiac output (12 patients), and single-plane cineventriculography for V(t) (five patients). In all comparisons, linear regression showed high correlation (from r = .82 [n = 46] to r = .988 [n = 20]) while alpha, with one exception, ranged from 0.75 to 1.07 and the error in Vc ranged from 0.5% to 16.5% (mean 7%). After positioning of the catheter, no arrhythmias were observed. It is concluded that the conductance catheter provides a reliable and simple method to measure left ventricular volume, giving an on-line, time-varying signal that is easily calibrated. Together with left ventricular pressure obtained through the catheter lumen, the instrument may be used for instantaneous display of pressure-volume loops to facilitate assessment of left ventricular pump performance.

Relations between ejection fraction and ventricular volume, and their alteration by chronic beta-blockade

This study describes the newly discovered relation between ejection fraction and end-systolic volume index (ESVI) of the left ventricle as obtained by angiography at the time of cardiac catheterisation. Linear regression analysis shows that ejection fraction (%)=82.0-0.62 ESVI (ml/m2) but the correlation for patients receiving beta-adrenergic blocking drugs is significantly lower compared with the untreated group. Non-linear analysis, applied to cover also the asymptotic range for ejection fraction less than 20%, shows similar results. The good relation between the two indices indicates that the index ejection fraction may derive its clinical importance directly from the more fundamental index end-systolic volume index by virtue of the operation of the beta-adrenergic system on the heart.

Continuous stroke volume and cardiac output from intra-ventricular dimensions obtained with impedance catheter

To improve assessment of ventricular function during cardiac catheterisation there should be available a continuous registration of stroke volume and cardiac output in addition to ventricular pressure. To obtain the desired volumetric quantities a catheter has been developed which measures changes in intraventricular dimensions by electrical impedance. For this purpose, the catheter is equipped with eight electrodes spaced over a distance equal to the long axis of the left ventricle into which it is introduced. A constant current is imposed between the outermost electrodes while the inner six are used to measure resistance of volume segments of the blood contained within the ventricular cavity. The difference in resistance at the beginning and end of ejection is proportional to the contribution of each segment to stroke volume, which follows from addition to the segmental terms. Calibration is obtained by measuring electrical conductivity of a blood sample. The catheter was tested over a tenfold range of cardiac output, both in vitro, using an artificial heart model, while performance in vivo was evaluated in 12 dogs. In the animals study, stroke volume and cardiac output from the catheter were compared with flows obtained with an electromagnetic flowmeter. In both studies, linear regression analysis showed excellent correlation of cardiac output (r = 0.99, n = 10 in vitro, r = 0.95, n = 126 in vivo) while the regression equations were close to those of identity. Very good correlation (r = 0.98, n = 28) was also obtained for stroke volumes on a beat to beat basis during arrhythmia. It is concluded that the catheter, which has great potential for application in man, fulfills its primary aim of continuously recording stroke volume and cardiac output.

The effect of variations in airflow pattern on gas exchange. A theoretical study

It has been shown that gas exchange between the alveolar space and pulmonary capillary blood is affected by the pattern of airflow at the mouth in the non-homogeneous lung. The present theoretical study shows that even in the homogeneous lung, the pattern of airflow can affect gas exchange. When tidal volume, inspiratory and expiratory times remain constant, variations in the pattern of airflow result in significantly different values of steady state arterial PO2 and PCO2. This difference in steady state blood gases is exaggerated by low levels of minute ventilation and by long inspiratory times, but is unaffected by changes in the diffusion coefficient of the alveolar-capillary membrane.

Dynamic local distensibility of living arteries and its relation to wave transmission

The dynamic local distensibility of the abdominal aorta was measured in 11 anesthetized dogs by recording simultaneously phasic pressure and instantaneous intravascular cross-sectional area, utilizing a special transducer. Axial motion of the vessel wall was recorded using a modification of the same transducer. A nonlinear relationship was found to exist between area and pressure in most cases studied. Fourier analysis was performed on data from eight experiments in order to obtain frequency characteristics of distensibility. In roughly half of the cases, Fourier analysis revealed that pressure variations displayed a phase lead over area variations for frequencies up to 10 Hz. This phenomenon was ascribed to viscoelastic properties of the vessel wall and the magnitude of the phase leads roughly matched those found in vitro by others. The behavior of the vessel wall in these instances was correctly predicted by the dynamic formula for distensibility, derived by others from wave transmission theory in which absence of axial wall motion is assumed. In these experiments, axial motion of the wall was found to be virtually absent. In the other half of the cases, the reverse situation was obtained: a phase lead of area variations over pressure variations for frequencies up to 15 Hz. In those cases a craniocaudal axial displacement of the vessel wall was observed with each systole, amounting to around 1 mm. The finding of the phase leads was partially explained by a dynamic formula for distensibility, developed by us from the theory of wave transmission in which free axial motion of the wall is a chosen boundary condition. The sign and order of magnitude of the phase leads were correctly predicted by the theoretical formula, but there was a disagreement on the frequency range in which they occurred. We concluded that additional forces, not yet considered in theoretical treatments, are operative on the aortic wall, which account for this lack of agreement. The frequency dependent properties of distensibility in vivo cannot be compared to those obtained in vitro in those cases in which there is axial displacement of the vessel wall of the same order of magnitude as the radial extensions.