Nonlinearity of the left ventricular end-systolic wall stress-velocity of fiber shortening relation in young pigs: a potential pitfall in its use as a single-beat index of contractility

Hypertension: An Important But Reversible Cause of Systolic Dysfunction in a Cohort of Pediatric Patients

Cardiac dysfunction due to hypertension (CDHTN) in pediatrics is not well described. We aimed to describe the presentation and outcomes of pediatric CDHTN and identify clinical features associated with resolution of dysfunction. A single-center retrospective cohort study of patients ≤ 21 years with CDHTN from January 2005-September 2020 was performed. Patients with systolic dysfunction without another cause, blood pressure > 95th percentile, and physician judgment that dysfunction was secondary to hypertension were included. Demographics, clinical characteristics, echocardiographic findings, and outcomes were examined using Fisher's exact and Mann-Whitney U tests. Multiple correspondence analysis was used to explore the relationship of resolution of dysfunction to clinical features. Thirty-four patients were analyzed at a median age of 10.9 (IQR 0.3-16.9) years. Patients were divided into groups < 1 year (n = 12) and ≥ 1 year (n = 22). Causes of hypertension were varied by age, with renovascular disease most common in infants (42%) and medical renal disease most common in older patients (77%). Echocardiography demonstrated mild LV dilation (median LV end-diastolic z-score 2.6) and mild LV hypertrophy (median LV mass z-score 2.4). Most patients (81%) had resolution of dysfunction, particularly infants (92%). One patient died and one patient was listed for heart transplant. None required mechanical circulatory support (MCS). No clinical features were statistically associated with resolution of dysfunction. Hypertension is an important but reversible cause of systolic dysfunction in children. Patients are likely to recover with low mortality and low utilization of MCS or transplantation. Further studies are needed to confirm features associated with resolution of dysfunction.

Operative contractility: A functional concept of the inotropic state

1. Initial unsuccessful attempts to evaluate ventricular function in terms of the 'heart as a pump' led to focusing on the 'heart as a muscle' and to the concept of myocardial contractility. However, no clinically ideal index exists to assess the contractile state. The aim of the present study was to develop a mathematical model to assess cardiac contractility. 2. A tri-axial system was conceived for preload (PL), afterload (AL) and contractility, where stroke volume (SV) was represented as the volume of the tetrahedron. Based on this model, 'operative' contractility ('OperCon') was calculated from the readily measured values of PL, AL and SV. The model was tested retrospectively under a variety of different experimental and clinical conditions, in 71 studies in humans and 29 studies in dogs. A prospective echocardiographic study was performed in 143 consecutive subjects to evaluate the ability of the model to assess contractility when SV and PL were measured volumetrically (mL) or dimensionally (cm). 3. With inotropic interventions, OperCon changes were comparable to those of ejection fraction (EF), velocity of shortening (Vcf) and dP/dt-max. Only with positive inotropic interventions did elastance (Ees) show significantly larger changes. With load manipulations, OperCon showed significantly smaller changes than EF and Ees and comparable changes to Vcf and dP/dt-max. Values of OperCon were similar when AL was represented by systolic blood pressure or wall stress and when volumetric or dimensional values were used. 4. Operative contractility is a reliable, simple and versatile method to assess cardiac contractility.

Cardiovascular physiology during supine cycle ergometry and dobutamine stress

PURPOSE

This study compared cardiac hemodynamics during supine cycle ergometry and dobutamine stress.

METHODS

Thirty-two healthy volunteers (19 female, 13 male, 23.5 +/- 3.5 yr old) completed respective tests on separate days and in random order. Heart rate, blood pressure, and cardiac output were recorded at baseline and peak stress. Echocardiographic measures included left ventricular end-diastolic dimension, fractional shortening, heart rate corrected velocity of circumferential fiber shortening, end-systolic wall stress, and the difference between measured and predicted fiber shortening for measured wall stress.

RESULTS

Compared with peak exercise, dobutamine infusion resulted in lower cardiac output (12 +/- 2 vs 16 +/- 4 l x min(-1), P < 0.0001), heart rates (163 +/- 7 vs 175 +/- 12 beats x min(-1), P < 0.0001), and systolic blood pressure (160 +/- 22 vs 185 +/- 20 mm Hg, P < or = 0.0001). Echocardiography demonstrated smaller left ventricular end-diastolic dimension (4.2 +/- 0.7 vs 4.5 +/- 0.7 cm, P = 0.013), higher fractional shortening (0.55 +/- 0.07 vs 0.50 +/- 0.06%, P < 0.001), higher VCFc (2.07 +/- 0.36 vs 1.54 +/- 0.20 circs x s(-1), P < 0.001) higher VCFdiff (0.94 +/- 0.35 vs 0.48 +/- 0.20 circs x s(-1), P < 0.001), and lower end-systolic wall stress (25 +/- 11 vs 42 +/- 16 g x cm(-2), P < 0.001). The stress-velocity relationship during dobutamine demonstrated higher y-intercept and steeper slope, indicating greater load-independent contractility.

CONCLUSION

The cardiovascular adaptation to exercise and dobutamine stress differ significantly. Cardiac output during peak exercise is greater than during peak dobutamine secondary to increased heart rate and stroke volume. Despite a greater increase in contractility and decrease in afterload, a smaller increase in cardiac output during dobutamine stress may be secondary to limited ventricular preload.

Moderate-dose dobutamine maximizes left ventricular contractile response during dobutamine stress echocardiography in children

BACKGROUND

Assessment of ventricular contractile reserve by dobutamine stress echocardiography (DSE) may be a powerful tool for detection of subclinical ventricular dysfunction, however, the hemodynamic dose-response relationship during DSE in children has not been established.

METHODS

To characterize changes in hemodynamics and ventricular contractility during DSE in children, 26 participants (age 8.3 +/- 4.8 years; 17 male/9 female) with normal resting left-ventricular function underwent DSE. Participants with abnormal wall motion at rest or during DSE, or rejection were excluded. Left ventricular M-mode echocardiography and carotid pulse tracings were obtained at each stage for calculation of shortening fraction, velocity of circumferential fiber shortening (VCFc), and end-systolic wall stress (WS). Contractility was expressed as the difference between actual and predicted VCFc for measured WS. Dose-response curves for shortening fraction, VCFc, WS, and contractility (the difference between actual and predicted VCFc for measured WS) were obtained.

RESULTS

Stepwise changes in contractility, systolic blood pressure, WS, and left ventricular shortening fraction were observed at doses up to, but not beyond, 20 microg/kg/min. Increases in double product were observed at doses up to 30 microg/kg/min.

CONCLUSIONS

DSE at a dobutamine dose of 20 microg/kg/min is optimal to fully assess contractile reserve in children. Lesser doses may provide insufficient stress, whereas higher doses may incur unnecessary increases in myocardial oxygen consumption and side effects.

Monitoring cardiac function in intensive care

Systolic cardiac function results from the interaction of four interdependent factors: heart rate, preload, contractility, and afterload. Heart rate can be quantified easily at the bedside, while preload estimation has traditionally relied on invasive pressure measurements, both central venous and pulmonary artery wedge. These have significant clinical limitations; however, adult literature has highlighted the superiority of several novel preload measures. Measurement of contractility and afterload is difficult; thus in clinical practice the bedside assessment of cardiac function is represented by cardiac output. A variety of techniques are now available for cardiac output measurement in the paediatric patient. This review summarises cardiac function and cardiac output measurement in terms of methodology, interpretation, and their contribution to the concepts of oxygen delivery and consumption in the critically ill child.

Effect of extracorporeal membrane oxygenation on left ventricular function of swine

BACKGROUND

Previous clinical and experimental investigations have produced inconsistent data describing the effects of veno-arterial extracorporeal membrane oxygenation (VA ECMO) on intrinsic left ventricular (LV) function. We report an animal model that allows investigation of the effects of VA ECMO on the mechanics of the LV using two load-insensitive indices: end-systolic pressure-minor axis dimension relationship (ESPDR) and preload recruitable dimensional stroke work (PRDSW).

METHODS

Eight piglets (5 to 11 kg) were anesthetized, instrumented, and placed on VA ECMO. Throughout the experiment, systemic and left atrial partial pressure of oxygen were maintained between 100 to 200 mm Hg. At ECMO flow rate of 50% of baseline cardiac output, data were collected prior to ECMO, at 4 and 6 hours during ECMO, and after weaning from ECMO. Data measured or calculated for each time point included heart rate, LV pressures and minor axis dimensions at different pre-loads, first derivative of LV pressure with respect to time, velocity of circumferential fiber length shortening (VCF), LV shortening fraction (LVSF), ESPDR, and PRDSW.

RESULTS

A significant (p < 0.05) decrease in LVSF and VCF was seen at 4 and 6 hours during ECMO when compared to baseline, but the ESPDR and PRDSW did not change during ECMO.

CONCLUSIONS

VA ECMO alone changes some of the load-dependent parameters of contractility, but intrinsic function of the heart is not significantly affected as measured by load-insensitive indices of LV performance.

Single-beat determination of preload recruitable stroke work relationship: derivation and evaluation in conscious dogs

OBJECTIVES

To derive and evaluate a method of estimating the slope (Mw) of the preload recruitable stroke work (PRSW) relationship between left ventricular stroke work (SW) and end-diastolic volume (EDV) from a single beat.

BACKGROUND

Mw is a load-insensitive index of contractile function, but its clinical application has been limited by the need to record multiple beats over a wide volume range.

METHODS

Pressure-volume loops were recorded over a variable preload and afterload range by vena caval and aortic constrictions in 12 conscious dogs instrumented with epicardial dimension transducers and micromanometers. Single-beat Mw (SBMw) was determined as the ratio SW/(EDV-Vw), where the volume-axis intercept of the PRSW relationship (Vw)(EDV at zero SW) was estimated as k x EDVB + (k - 1)LVwall, k is the ratio of the epicardial shell volumes corresponding to Vw and baseline EDV (EDVB) and LVwall is wall volume.

RESULTS

In the first six dogs, k was found to be essentially constant at 0.7, SBMw estimates were insensitive to wide preload variation, and the relationship between SBMw and multibeat Mw determined during caval and aortic constrictions did not differ significantly from the line of identity. When the same constant k value was applied to SBMw estimation in a different group of six dogs, SBMw did not differ significantly from multibeat Mw (83 +/- 12 erg x cm(-3) x 10(3) and 77 +/- 12 erg x cm(-3) x 10(3), respectively), neither changed significantly during aortic constriction and both increased significantly with calcium infusion (107 +/- 18 erg x cm(-3) x 10(3) and 95 +/- 19 erg x cm(-3) x 10(3), respectively, both p < 0.05). Single-beat Mw was less load-dependent, more reproducible and a more sensitive index of inotropic state than two previously described single-beat indexes, single-beat elastance and maximum power divided by EDV2.

CONCLUSIONS

Mw can be determined accurately from a single, steady-state beat in the normal canine heart and is sensitive to inotropic alterations while being insensitive to wide variations in preload and afterload. Single-beat Mw estimation should facilitate noninvasive, load-independent assessment of contractile function.

The influence of low afterload on the nature of the stress-velocity relationship

OBJECTIVES

Correct assessment of contractility by conventional methods during manipulation of afterload is often disappointing. To this purpose, the stress-velocity relationship offers assessment of contractility at different levels of afterload. We decided to study the influence of afterload on the nature of the stress-velocity relation.

BACKGROUND

Although linear at baseline conditions in a population older than two years, data in newborns or after administration of low-dose dobutamine suggest a different nature of this relationship at low afterload.

METHODS

Ten healthy piglets (five to six weeks; 11 to 13 kg) were studied. End-systolic meridional wall stress (ESWS) and rate-corrected velocity of circumferential fiber shortening (VcFc) were measured in these piglets at baseline, after balloon occlusion of the descending aorta, and at nitroprusside infusion rates of 1, 2 and 5 microg/kg/min. To eliminate inotropic influences mediated by reflex tachycardia, we subsequently studied five piglets and six adult pigs after bilateral cervical vagotomy.

RESULTS

The ESWS changed from a baseline mean of 50 g/cm2 to 137 g/cm2 after balloon occlusion and to 19 g/cm2 at 5 microg/kg/min of nitroprusside. The VcFc changed from 1.19 c/s (circumference/second) to values of 0.9 c/s and 1.73 c/s, respectively. The ensuing stress-velocity regression line proved to be curvilinear instead of linear. The steeper slope at low afterload could suggest enhanced contractility compared to expected values had the relationship been linear.

CONCLUSIONS

Data from young piglets and adult pigs suggest a curvilinear relationship of the stress-velocity relationship. This could probably explain some of the "hypercontractile states" encountered in conditions with low afterload.

Means for load variation during echocardiographic assessment of the Frank-Starling relationship

Because minimally invasive methods of preload variation are not validated for load-insensitive indexes of cardiac performance, intravenous nitroglycerin (NTG), phenylephrine, and saline solution (VOL) boluses were used in blocked and intact autonomic states to alter load and were compared with vena caval occlusion in the assessment of preload recruitable stroke work relationships between stroke work and left ventricular end-diastolic volume in dogs. In both autonomic states NTG and VOL produced comparable linear relationships. NTG and saline solution were combined with noninvasive measurements of left ventricular pressure and volume to construct echocardiographic relationships between stroke work and left ventricular end-diastolic cross-sectional area; NTG produced linear relationships similar to vena caval occlusion. Therefore NTG and VOL reliably alter load in constructing preload recruitable stroke work relationships, and NTG may be used with noninvasive measurements to provide load-insensitive estimates of cardiac function in a minimally invasive manner.

Influence of age and low afterload on the stress-velocity relation of the left ventricle

The relation between systolic meridional wall stress (WS) and velocity of circumferential fiber shortening (VcFc) is widely accepted as a preload-independent index of contractility, with a linear relation in most subjects older than 2 y. However, this relation seems to become different in infants and after administration of inotropic agents. We decided to study the nature of the stress-velocity relation by a cross-sectional assessment of the influence of age, low afterload, and increased contractility. Study subjects were 30 healthy infants, 32 healthy older children, and 35 asymptomatic older children after completion of anthracycline chemotherapy. WS and VcFc at rest were studied in these infants and children. WS and VcFc were also studied after dobutamine infusion in both groups of older children. Linear regression analysis of the stress-velocity relation showed parallel slopes between the older children at rest and the post anthracycline children after dobutamine. The regression lines between the infants at rest and the healthy older children after dobutamine were also parallel, but with a different and steeper slope compared with the former groups. When comparing the stress-velocity relation of the overall population at rest with the overall population after dobutamine, the resulting regression lines are curvilinear and parallel, with a steeper slope at low afterload. The stress-velocity relation in infants and after dobutamine, resulting in low afterload is different compared with the stress- velocity relation in older children at rest and at higher afterload. Data of the overall population at rest and after dobutamine suggest a curvilinear relation.

Noninvasive Assessment of Regional and Global Myocardial Contractility in Normal Control Subjects and in Patients with Dilated Cardiomyopathies

The aim of the present study was to develop a new noninvasive approach for the assessment of regional and global myocardial contractility without the need for pharmacological intervention to alter load. Thirty-four healthy adults and five adults with dilated cardiomyopathy (DCM) were studied. Patients with diabetes mellitus and hyperthyroidism were eliminated from the study. The remainder underwent echocardiography, sphygmomanometric blood pressure determination, and carotid pulse tracings. Left ventricular cross section in the parasternal long-axis four- and two-chamber views was divided into 20 segments. Associated measurements of end-systolic pressure and left ventricular ejection time enabled shortening, shortening rate, and ejection fraction/afterload relationships to be determined. A discriminant analysis showed that the ejection fraction/afterload relationship in patients with DCM differed substantially from that of control subjects and was the most sensitive in this regard. Endocardial shortening, mid-wall shortening, and ejection fraction/afterload relationships demonstrated linearity or nonlinearity for control subjects. This study thus permits the assessment of contractility in individual subjects without the need for drug interventions because load alteration stems from the variation of wall stress from base to apex in the left ventricle. More importantly, the approach may be applied to patients with segmental abnormalities of contractile function.

Comparison Between the Mean dP/dt During Isovolumetric Contraction and Other Echocardiographic Indexes of Ventricular Systolic Function

Echocardiographic assessments of ventricular function derived from estimates of the mean dP/dt during isovolumetric contraction (mean dP/dt(ic)) were compared with those obtained from measurements of the shortening fraction and the stress-velocity index (SVI). Mean dP/dt(ic) correlated well with the shortening fraction, r = 0.74, P < 0.0001. Furthermore, 10 out of 11 patients with mean dP/dt(ic) below 800 had a shortening fraction < 0.28, whereas all patients with a mean dP/dt(ic) > 1000 mmHg/sec had a shortening fraction > 0.28. A good correlation also existed between mean dP/dt(ic) and the SVI, r = 0.73, P < 0.0001. Nine out of 11 patients with a mean dP/dt(ic) < 800 mmHg/sec had an SVI > 2 standard deviations below normal, whereas all patients with mean dP/dt(ic) > 1000 mmHg/sec had normal or increased SVI. The correlation between mean dP/dt(ic) and the SVI was strengthened when mean dP/dt(ic) was adjusted for heart rate and preload. Hence, assessments of ventricular function derived from measurements of mean dP/dt(ic) appear to agree well with those provided by the shortening fraction and SVI. Because the determination of mean dP/dt(ic) is not hampered by unusual anatomy or wall motion (conditions which compromise the validity of the shortening fraction and SVI), mean dP/dt(ic) may be a good index of ventricular function in cases where measurements of the shortening fraction and SVI would be unreliable.

A New Approach to Assessing Regional and Global Myocardial Contractility

A number of studies that assessed myocardial contractility by noninvasive means have been conducted in the past. However, many of these studies are limited because they assessed the velocity of fiber shortening-afterload relationship at only a single location in the ventricle, thus assuming uniform contractility throughout the ventricle. This is often not the case, particularly in patients with coronary disease. The present study provides a new approach to assessing regional and global function that will be applicable to patients with nonuniform contractile function and makes use of entirely noninvasively obtained data. In addition, the method proposed will permit the question of linearity or nonlinearity of shortening rate-afterload relationship to be addressed in a more detailed, quantitative manner.

Noninvasive assessment of myocardial contractility, preload, and afterload in healthy newborn infants

Assessment of ventricular contractility in the newborn infant using standard echocardiographic indexes can result in error due to the unique physiologic state that exists in the neonatal period. It has been suggested from animal and human studies that maturational alterations in contractility occur with birth and continue throughout infancy. To further investigate these developmental changes, 41 newborn infants aged 3 to 10 days and 37 children aged 3 to 18 years were evaluated with 2-dimensional and M-mode echocardiography. The rate-corrected velocity of circumferential fiber shortening (VCFc)-end-systolic wall stress (ESWS) relation was used as a load-independent estimate of contractility. Preload, afterload, and ventricular mass were also measured. Despite similar shortening fractions, the infant group had significantly higher mean VCFc and lower ESWS than the older age group (1.28 vs 1.08 circ/s and 30.2 vs 37.3 gm/m2, respectively). An inverse linear relation between VCFc and ESWS was found in both age groups. The y-intercept was higher in the infant group (p < 0.01), and the slope of the mean regression line was steeper than in the older children (p < 0.01). Ventricular mass in relation to body surface area increased with age. We conclude that (1) newborn infants have a higher basal contractile state that cannot be accounted for by lower afterload, (2) myocardial performance is more sensitive to afterload in the immature heart, and (3) shortening fraction may underestimate ventricular function in the newborn.