11.2 Stroke Volume & Cardiac Output (Fick, Thermodilution, Angiographic)

Stroke Volume & Cardiac Output (Fick, Thermodilution, Angiographic)

Procedures 1.C Hemodynamic Calculations starts with the flow numbers. Pressures from right- and left-heart catheterization tell you whether the circuit is hypertensive or empty. Stroke volume (SV) and cardiac output (CO) tell you whether the pump is actually moving blood. Typical adult teaching ranges below (CO about 4–8 L/min, CI about 2.5–4.0 L/min/m²) are cath-lab teaching values, not unpublished ARRT cutoffs.

Quick Answer: SV = EDV − ESV. EF = SV / EDV. CO = SV × HR. CI = CO / BSA. Fick: CO (L/min) = VO2 (mL/min) / [13.6 × Hgb (g/dL) × (SaO2 − SvO2)] when saturations are decimals. Assumed VO2 ≈ 125 mL/min/m² × BSA is a teaching shortcut — assumed VO2 is not measured VO2. Thermodilution fails with severe tricuspid regurgitation (TR) and intracardiac shunts. Angiographic CO from LV volumes is invalid on a post-pigtail extrasystole.

The shared arithmetic

Four identities show up on every method. Learn them as a set, then pick how you obtained the volumes or the flow.

• Stroke volume (SV) = end-diastolic volume (EDV) − end-systolic volume (ESV). Units mL/beat.
• Ejection fraction (EF) = SV / EDV (often reported × 100 as a percent). EF is the fraction of the filling volume ejected — it is not cardiac output.
• Cardiac output (CO) = SV × heart rate (HR). Convert mL/min to L/min by dividing by 1,000.
• Cardiac index (CI) = CO / body-surface area (BSA). Indexing stops a small patient from looking like low output, or a large patient from looking fine, when the raw liters are misleading.

Worked volumes (the same arithmetic whether the volumes came from a ventriculogram, a measured Fick-derived SV, or a thermodilution-derived SV): EDV 150 mL, ESV 50 mL, HR 70, BSA 1.75 m².

• SV = 150 − 50 = 100 mL
• EF = 100 / 150 = 0.67 (67%)
• CO = 100 mL × 70 / 1,000 = 7.0 L/min
• CI = 7.0 / 1.75 = 4.0 L/min/m²

If the next sinus beat is a post-PVC beat, SV is larger (post-extrasystolic potentiation). That beat is not the patient's steady-state stroke volume.

Fick cardiac output

Oxygen in has to equal oxygen used. Rearranged, CO = VO2 / (CaO2 − CvO2) — flow equals oxygen consumption divided by the arteriovenous oxygen-content difference.

Teaching form used on calculations:

CO (L/min) = VO2 (mL/min) / [13.6 × Hgb (g/dL) × (SaO2 − SvO2)]

Saturations are decimals (0.95, not 95). The 13.6 packages 1.36 mL O2 per gram of hemoglobin (Hüfner's constant as commonly taught) times 10 (converting dL to L). Equivalent writing: 1.36 × Hgb × 10 × Δsat. If a stem gives percent saturations, convert first: 95% and 65% are 0.95 and 0.65, so Δsat = 0.30, not 30.

VO2 is oxygen consumption in mL/min.

• Measured VO2 from a metabolic cart, hood, or equivalent is the reference when it is available.
• Assumed VO2 in teaching labs is often ~125 mL/min/m² × BSA. Label it assumed. Fever, anxiety, and thyrotoxicosis raise true VO2. Sedation, hypothermia, and hypothyroidism lower it. Using 125 mL/min/m² when the patient is deeply sedated overestimates VO2 and therefore overestimates CO. The reverse (assumed VO2 too low) underestimates CO. The exam cares that you can name that error, not that you memorize every metabolic state.

Arterial saturation (SaO2) is from a left-heart or arterial sample. Mixed-venous saturation (SvO2) is from the pulmonary artery — not a random superior vena cava draw if a true mixed-venous sample is available. Anemia (low hemoglobin) widens the CO that Fick will compute for the same VO2 and Δsat, because each liter of blood carries less oxygen.

Worked Fick example

BSA 2.00 m². Assumed VO2 = 125 mL/min/m² × 2.00 = 250 mL/min. Hemoglobin 15.0 g/dL. Arterial saturation 0.95. Pulmonary-artery mixed-venous saturation 0.65.

• Δsat = 0.95 − 0.65 = 0.30
• Denominator = 13.6 × 15.0 × 0.30 = 61.2
• CO = 250 / 61.2 = 4.08 L/min (about 4.1 L/min)
• CI = 4.08 / 2.00 = 2.04 L/min/m²

If measured VO2 is actually 200 mL/min (sedated, true consumption lower), true Fick CO = 200 / 61.2 = 3.27 L/min. The assumed-VO2 number looked kinder than the patient. That is the assumed-versus-measured error in one line.

If someone plugs 95 and 65 as if they were already decimals, the denominator explodes and the CO becomes nonsense. Convert percents to decimals, or write Δsat as 0.30, not 30.

Thermodilution

A cold saline bolus is injected into the right atrium through the proximal port of a pulmonary-artery catheter. A thermistor in the pulmonary artery records the temperature–time curve. Stewart-Hamilton: cardiac output is inversely related to the area under that curve. A small, brief temperature dip means the injectate was washed away quickly — high output. A long, shallow curve means low output.

Needs: known injectate volume and temperature, the correct injection port, a competent thermistor, and a right heart that actually mixes the bolus into pulmonary-artery flow.

Fails when:

• Severe TR — indicator sloshes backward into the right atrium and the curve is not a single washout of PA flow.
• Intracardiac shunts — indicator is lost leftward (left-to-right) or extra flow appears, so pulmonary-artery temperature no longer represents systemic output.
• Wrong injectate temperature or volume, a broken thermistor, and very low output with a barely readable curve are additional practical wreckers.

Do not report a beautiful thermodilution number in torrential TR as if it were a Fick output. The method's assumptions are gone.

Angiographic cardiac output

Left-ventricular EDV and ESV from a ventriculogram (area-length or Simpson-type tracing) give SV; multiply by HR. This is the same SV arithmetic as above, but the volumes came from contrast pictures rather than from oxygen or temperature.

Needs: a calibrated image so magnification is known, a tracing of a representative sinus beat, and a ventricle you can actually see (not a wall hidden by the pigtail).

Fails when: extrasystoles after pigtail injection — the PVC beat and usually the post-PVC beat are not the patient's resting SV. Skip them and wait for a sinus cycle that is not potentiated. Atrial fibrillation makes every beat different; pick several representative cycles, not one lucky filling period. Mitral regurgitation means forward SV is less than angiographic SV, because some of EDV − ESV went back into the left atrium rather than out the aorta.

Method / needs / fails when

Exam traps

• EF is SV / EDV, not SV / ESV and not CO.
• Fick saturations are decimals in the 13.6 formula (or use the 1.36 × 10 equivalent).
• Assumed 125 mL/min/m² × BSA is not a measured VO2; the wrong VO2 scales the entire CO.
• Thermodilution is the wrong tool in severe TR and shunts.
• Do not calculate angiographic SV from a post-pigtail extrasystole.