Mass-specific myocardial oxygen delivery

Mass-specific myocardial oxygen delivery expresses oxygen inflow relative to the mass of the represented myocardial tissue. If total coronary oxygen delivery is $D_{{\mathrm{O}}_2}$ to mass $m$, the per-mass quantity is $D_{{\mathrm{O}}_2}/m$. Reporting per $100\mathrm{g}$ givesThe mass and inflow must refer to compatible territories. Normalizing a branch flow by an unrelated whole-heart mass changes the quantity rather than simply changing its units.

An original example has two regions: $50\mathrm{g}$ receiving $15\mathrm{mL}{\mathrm{O}}_2\mathrm{mi}{\mathrm{n}}^{-1}$, and $150\mathrm{g}$ also receiving $15$. Their mass-specific deliveries are $30$ and $10\mathrm{mL}{\mathrm{O}}_2\mathrm{mi}{\mathrm{n}}^{-1}$ per $100\mathrm{g}$. The combined value isIt is the mass-weighted mean $0.25(30)+0.75(10)$, not the unweighted mean $20$. Equal total inflows do not imply equal delivery per tissue mass.

Normalization does not remove measurement uncertainty. For small independent input errors,Relative uncertainties $5\%$ in delivery and $10\%$ in mass combine to about $11.2\%$. With correlated errors, the relative covariance term $-2\mathrm{Cov}(D,m)/(Dm)$ must also be included.^[ref]

An experimental flow measurement still needs separate oxygen-content and mass information. The primary coronary thermodilution apparatus estimated volumetric flow in a selected artery using infusion and temperature sensing.^[ref] A mass-normalized oxygen-delivery estimate therefore combines several observations and a territory model.

Finally, a regional mean can conceal spatial variation. Two regions can share the same average per-mass delivery while distributing flow differently within them. The denominator makes comparisons explicit; it does not establish uniform perfusion or equal coronary oxygen uptake.