The body's resistance is generally not what matters, it's generally the resistance of the connection between the body and the power source. If the connection is

The body's resistance is generally not what matters, it's generally the resistance of the connection between the body and the power source. If the connection is made via an air-gap spark you can need thousands of volts to ionize the air to allow the spark to form which allows the current to enter the body (corresponding to tens of thousands of ohms at a killing current, although that resistance is highly non-linear and drops suddenly after the initial spark connection ionizes the air). If the connection is made via EEG or EKG leads attached with conductive paste, the connection resistance can be a fraction of an ohm and the killing voltage can be well under 100 volts. The highly variable nature of the connection resistance is just one reason why the killing threshold is generally described in terms of amps rather than volts. If you're worried about whether a circuit could kill a patient and you must think in terms of voltage, you're not worried about whether you know the body's resistance, you're worried about whether you know the connection resistance. In practice pretty much everyone who designs safety critical systems where electrocution is a concern focuses on current because it's a much better way to model and analyze the system risks than voltage.