Vascular Function Curve: Confusion and Clarification
Authors: Ko Michael Y. , Yang Xiaoqi , ElSayed Suzan , Kuang Serena Y. 
Published: May 2022

Introduction

The vascular function curve (VFC), cardiac function curve (CFC), and their combination are widely introduced in medical physiology. However, teaching and learning this content is difficult because of a lack of essential information about the independent and dependent variables (IV and DV respectively) of the VFC, which often leads to contradictory interpretations of the combination of the 2 curves. Hence, the goals of this presentation are the following: 1) Describe the origin of the confusion and clarify the confusion in physiology education. 2) Show how the content is inconsistently presented in physiology textbooks and other resources.

Method

Literature review, logical reasoning, and graphical illustration.

Results and Discussion

1) The origin of the problem and its clarification.

The VFC was first plotted and combined with the CFC by Guyton et al. in the 1950s and it describes the relationship between the steady‐state blood flow (i.e., cardiac output (CO)/venous return (VR) in steady‐state) and the right atrial pressure (RAP). Guyton et al. originally plotted the RAP in the X‐axis and the CO/VR in the Y‐axis (Fig. 1A) implying that the RAP is the IV. If the RAP is the IV, the interpretation of the combination of the VFC and CFC is problematic: How does a change in the RAP result in opposite effects on the CO simultaneously (Fig. 1C)? Levy (1979) simplified Guyton et al.’s original experimental setting and argued that the CO in the Y‐axis is the true IV as the pumping rate (CO) was left as the only manipulated variable. Levy also addressed that Guyton’s presentation of the VFC with the RAP on the X‐axis goes against convention. Levy’s clarification is supported by Beard and Feigl in their 2011 article. Despite Levy’s clarification, it is still difficult to interpret the communication of the VFC and CFC: How can the RAP be the IV of the CFC and the DV of the VFC simultaneously? No textbooks provide a convincing explanation to this question and the presentation of the IV and DV of the VFC is inconsistent in the physiology textbooks. 2) The Inconsistent IV and DV of the VFC in textbooks. A total of 9 textbooks and Board review books were reviewed regarding how the IV and DV of the VFC. Eight of them introduced the VFC and CFC (Table 1), were only 2 books described that RAP is the DV.

Conclusions

1) Levy's clarification of the problem many decades ago has not been well known, so the problem is still prevalent in the circle of physiology education. 2) If the VFC must be taught, the IV and DV of the VFC should be taught consistently in the textbooks.

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Fig. 1. Vascular function curve (VFC, 1A), cardiac function curve (CFC, 1B), and their combination (1C) with the independent variable in red and dependent variable in blue in 1A and 1B