### Key points Echocardiography provides information regarding volumes [e.g. stroke volume (SV), ejection fraction], flows [e.g. cardiac output (CO), shunt fractions], and pressures (e.g. gradients across valves). As the only directly measurable variables are length (using 2D imaging) and velocity (using spectral Doppler), quantitative haemodynamic data is acquired using extrapolation of these two modalities. Area can also be measured directly using planimetry in most systems. This review demonstrates how the physical principles behind Doppler echocardiography can be used to calculate volumes, flows and pressures. Systolic and diastolic ventricular function, filling status, and valve pathology can all be quantified, providing depth and accuracy to decision-making processes. f = 2 v f 0 cos θ c where f is Doppler shift frequency, v velocity of red cell target, f frequency of transmitted ultrasound beam, θ angle between the ultrasound beam and vector of the red blood cell flow and c is velocity of ultrasound in blood. The Doppler effect is the change in frequency of a reflected sound wave for an observer moving relative to its source. In blood flow, the frequency shifts relate to red blood cell velocities. If the source moves towards the observer, the observed frequency increases (and thus, wavelength decreases). …