Haemodynamics and oxygen delivery
cardiac outputMAP and SVRFick principleoxygen contenthaemodynamic formulas
Cardiac output, mean arterial pressure, vascular resistance, oxygen content, the Fick principle and minute ventilation as one chain.
Cardiac Output (Heart Rate × Stroke Volume)
Cardiac output: the volume the left ventricle ejects each beat, multiplied by the number of beats each minute.
Mean Arterial Pressure (MAP)
The time-weighted average arterial pressure over a cardiac cycle, estimated as the diastolic pressure plus one third of the pulse pressure.
Systemic Vascular Resistance (SVR)
Systemic vascular resistance in dyn·s·cm⁻⁵: the pressure drop across the systemic circuit divided by the flow through it, with the 80 that converts the units.
Arterial Oxygen Content (CaO₂)
Oxygen carried per decilitre of arterial blood: the large amount bound to haemoglobin, plus the small amount dissolved in plasma.
Fick Principle for Oxygen Consumption
Adolf Fick's 1870 principle: oxygen consumption equals blood flow times the oxygen the blood gives up crossing the tissues.
Minute Ventilation (Tidal Volume × Respiratory Rate)
The total volume of gas moved into the lungs each minute: the volume of one breath multiplied by the number of breaths.
How they fit together
Read these as one delivery chain rather than six separate calculations. The heart moves a volume, the circulation carries a pressure, the blood holds oxygen, and the tissues take what they need. Cardiac output is the first link and the simplest: stroke volume times heart rate. It also explains why a fast heart is not always a productive one, since filling time shortens as rate climbs and stroke volume falls with it.
Mean arterial pressure weights diastole twice as heavily as systole, and that one third weighting is not a fudge. The heart spends roughly twice as long filling as ejecting, so MAP is the time-weighted average and the number that actually drives organ perfusion. Systemic vascular resistance then completes the analogy to Ohm's law, with pressure drop as voltage and cardiac output as current. It is what tells you whether a low pressure is a pump problem or a plumbing problem, and the two are treated very differently.
The last three are about the cargo rather than the flow. Arterial oxygen content makes the point that saturation alone is misleading, because almost all of the oxygen rides on haemoglobin and only a trickle dissolves in plasma. A patient at 100% saturation with half the normal haemoglobin is carrying half the oxygen. The Fick principle multiplies flow by the oxygen the blood gives up on the way round, and rearranged it is how cardiac output was measured for a century. Minute ventilation is the same idea one organ upstream. These are teaching and estimating formulas, and clinical decisions belong to clinicians with the patient in front of them.