Cardiophysiology Illustrated by Comparing Ventricular Volumes in Healthy Adult Males and Females

Various Approaches to Define the Volume Intercept of the Ventricular End-Systolic Pressure-Volume Relationship: Implications for Statistical Analysis

Ventricular pump function is often characterized by the (non)linear end-systolic pressure-volume relationship (ESPVR). For each working point on that curve the tangent along with the intercept (Vo) reflect contractile state. Vo on the abscissa is an extrapolated point without physiological meaning, and may be negative. To obtain positive values for the intercept, investigators often choose a non-zero pressure level. Although this preference is mathematically sound, we demonstrate that statistical evaluations may yield different results, depending on the pressure level selected. Published data on 17 cardiac patients representing three diagnostic groups were analyzed, showing dicrotic notch pressure based values -14<Vo<119 mL/m²• At 20, 40, 60, 80 and 100 mmHg levels all intercepts were positive, but statistical outcomes varied when comparing the 3 groups, depending on the pressure level selected, e.g. ranging from P=0.03 to 0.28. Also, we found that the correlation between intercept value Vo and end-systolic volume varies with the choice made for the pressure level. However, as there is no need to explore alternatives for the intercept, we conclude that the traditional route for the estimation of Vo (i.e. extrapolated to zero pressure) is to be preferred and this practice warrants adequate comparison of outcomes generated by all studies. Clinical Relevance- Personal preference for an alternative intercept definition for the ESPVR does not serve any relevant goal but rather implies that the statistical outcomes are altered thus hampering interpretation of data when comparing patient groups or assessing the effect of clinical intervention.

Case report on the importance of longitudinal analysis of left ventricular end-systolic volume, rather than ejection fraction, in a heart transplant patient

Background

Sequential determinations of left ventricular (LV) volume constitute a cornerstone in the mechanical performance evaluation of any heart transplant (HTX) patient. A comprehensive analysis of volumetric data offers unique insight into adaptation and pathophysiology.

Case summary

With a focus on eight sequential biplane angiocardiographic LV end-systolic volume (ESV) determinations, we evaluate the clinical course of a male patient following HTX (female donor) at the age of 61 years. This former smoker had a history of chronic obstructive pulmonary disease, hypertension, and hypercholesterolaemia refractory to treatment, and presented with multivessel coronary artery disease. The later course was complicated by pulmonary hypertension, an abdominal aortic aneurysm, and secondary chronic kidney disease. After an additional episode of pulmonary embolism, the patient died at the age of 79. At one point, the ESV was > 700% higher than the starting value, and actually by far exceeded the relative change of any other volume-based metric evaluated, including ejection fraction (EF).

Discussion

The longitudinal study of LV volumetric data in HTX patients offers a unique window to the pathophysiology of remodelling and sex-specific adaptation processes. The present case documents that proper analysis of serial findings form a rich source of clinically relevant information regarding disease progression. End-systolic volume is the primary indicator, in contrast to the popular metric EF. This finding is supported by population-based studies reported in the literature. We conclude that comprehensive analysis of volumetric data, particularly ESV, contributes to personalized medicine and enhances insight into LV (reverse) remodelling, while also informing about prognosis.

Ratiology and a Complementary Class of Metrics for Cardiovascular Investigations

Cardiovascular investigations often involve ratio-based metrics or differences: ejection fraction, arterial pressure augmentation index, coronary fractional flow reserve, pulse pressure. Focusing on a single number (ratio or difference) implies that information is lost. The lost companions constitute a well-defined but thus far unrecognized class, having additive value, a physical dimension, and often a physiological meaning. Physiologists should play a prominent role in exploring these complementary avenues and also define alternatives.

Heart Failure Phenotypes Require Sex-Specific Criteria Which Are Based on Ventricular Dimensions

Ejection fraction (EF) is often used as a criterion to establish diagnostic phenotypes of heart failure (HF). Because EF is a derived metric based on end-systolic volume (ESV) and end-diastolic volume (EDV), it is more logical to consider ESV or EDV as cut-off marker. We concentrate on the impact of ESV, which has the advantage of being linearly related to EDV and nonlinearly with EF, both with highly significant correlations. In particular we also analyze if HF classification should distinguish between females and males.ESV and EDV were determined by biplane angiography in 197 HF patients (67 women). As body surface indexed (i) ESVi values for adult healthy females are smaller than for males, we employ classes of ESVi (bins of 10 mL/m²) to group preserved and reduced EF's (cut-off at 50%) for HF. Reference values regarding mean and standard deviation for ESVi are based on a control group (N=155, 65 women) without HF. For interpretation of the findings we use the documented universal relationship connecting EF to ESV: EF = 1 + c₁ {ESV / (c₂ - ESV)}, where c₁ and c₂ are population-based sex-independent constants. In the reference group ESVi (mL/m²) in women (27.4 ± 27.6) is smaller (P=0.0026) than in their male counterparts (43.6 ± 37.5). Similarly, for HF the ESVi in women (45.7 ± 41.4) is smaller (P=0.0033) than in men (64.2 ± 41.4). This signifies (see formula above) that women have higher values for EF, primarily resulting from smaller ventricular size related to their sex, and not exclusively reflecting disease state. Current phenotype classification based on pooled data for males and females may be inappropriate for either sex.The significantly smaller ESVi observed in women has direct consequences for the traditional classification based on EF cutoff values for HF. Sex-specific criteria (regarding ESVi or EF) for HF phenotypes are warranted, and expectedly have substantial consequences for identification, classification, and management of HF patients.

Real-world heart rate norms in the Health eHeart study

Emerging technology allows patients to measure and record their heart rate (HR) remotely by photoplethysmography (PPG) using smart devices like smartphones. However, the validity and expected distribution of such measurements are unclear, making it difficult for physicians to help patients interpret real-world, remote and on-demand HR measurements. Our goal was to validate HR-PPG, measured using a smartphone app, against HR-electrocardiogram (ECG) measurements and describe out-of-clinic, real-world, HR-PPG values according to age, demographics, body mass index, physical activity level, and disease. To validate the measurements, we obtained simultaneous HR-PPG and HR-ECG in 50 consecutive patients at our cardiology clinic. We then used data from participants enrolled in the Health eHeart cohort between 1 April 2014 and 30 April 2018 to derive real-world norms of HR-PPG according to demographics and medical conditions. HR-PPG and HR-ECG were highly correlated (Intraclass correlation = 0.90). A total of 66,788 Health eHeart Study participants contributed 3,144,332 HR-PPG measurements. The mean real-world HR was 79.1 bpm ± 14.5. The 95th percentile of real-world HR was ≤110 in individuals aged 18–45, ≤100 in those aged 45–60 and ≤95 bpm in individuals older than 60 years old. In multivariable linear regression, the number of medical conditions, female gender, increasing body mass index, and being Hispanic was associated with an increased HR, whereas increasing age was associated with a reduced HR. Our study provides the largest real-world norms for remotely obtained, real-world HR according to various strata and they may help physicians interpret and engage with patients presenting such data.

Sex- and Age-Related Reference Values in Cardiology, with Annotations and Guidelines for Interpretation

The definition of "abnormal" in clinical sciences is often based on so-called reference values which point to a range that experts by some sort of consensus consider as normal when looking at biological variables. Such a level is commonly calculated by taking (twice) the standard deviation from the mean, or considering certain percentiles. The suspicion or even confirmation of a disease is then established by demonstrating that the value measured exceeds the upper or lower reference value. As is often the case, the measurement accuracy may depend on the conditions and specific method employed to collect and analyze data. This implies that, for example, data assessed by 2D echocardiography possibly differ from those obtained by MRI and therefore require modality-specific reference values. In this review we summarize reference values for the electrocardiogram, cardiac compartmental volumes, and arterial vessel size in males and females for various age groups. These values may further depend on other variables such as body size, physical training status, and ethnicity. Additional variables relevant for cardiology such as those referring to the microcirculation and biomarkers are only mentioned with reference to the pertinent literature. In general, the sex- and age-specific differences observed are often remarkable and warrant consideration in clinical practice and basic biomedical sciences.