Numerous factors influence heart rate variability, including age, exercise and posture. The Cardio Stress Index (CSI) is a transformed measure of heart rate variability that is determined via a miniature digital multi-channel electrocardiogram system. Although the CSI and heart rate variability are reportedly analogous, little is known about how the two concepts compare in peer-reviewed research. The aim of this study was to examine the differences between CSI and heart rate as measured on a mini- electrocardiogram device when subjects were sitting upright and when they were lying down (in supine position). This is a case-series study with no intervention or follow-up. Sitting and supine CSI and heart rate readings were compared in a random sample of 55 women volunteers recruited through advertising in Pretoria, South Africa. The mean age of the sample was 25.01 years (SD = 7.56). After completing a biographical questionnaire, subjects’ CSI and heart rates were evaluated using a digital medical device, the ViportTM. The combined CSI for the group was elevated above the normative value of 20% (31.00%; SD = 14.03). The seated, supine and combined CSI all differed significantly from one another (p<0.05) and the CSI was significantly correlated with heart rate (p=0.41). In conclusion, the CSI readings, like heart rate variability measurements taken in different postures, cannot be used interchangeably in clinical practice or in research. The CSI appears to mirror existing research evidence on heart rate variability and posture.