Cardiac Output in Critical Care Echocardiography
It represents the volume of blood pumped by the heart each minute and reflects the adequacy of systemic perfusion.
Unlike pulmonary artery catheter measurements, echocardiography estimates CO non-invasively by measuring LVOT diameter and LVOT Velocity Time Integral (VTI).
Introduction
Cardiac Output = Amount of blood ejected by the ventricle each minute.
CO = Stroke Volume × Heart Rate
Where
Stroke Volume (SV)= LVOT Cross-sectional Area × LVOT VTI
Therefore,
CO = LVOT Area × LVOT VTI × HR
Why Measure Cardiac Output?
CO tells you whether oxygen delivery is adequate.
Remember
Oxygen Delivery (DO₂)
DO₂ = CO × CaO₂ × 10
Normal Values
Parameter | Normal |
Cardiac Output | 4–8 L/min |
Cardiac Index | 2.5–4.0 L/min/m² |
Stroke Volume | 60–100 mL |
LVOT VTI | 18–22 cm |
Heart Rate | 60–100/min |
Components of Cardiac Output
CO depends upon two variables
1. Stroke Volume
Determined by
- Preload
- Contractility
- Afterload
2. Heart Rate
CO increases with HR only until diastolic filling becomes impaired.
Which is at Very high HR (>150–170/min)
Determinant of Cardiac Output (CO) | Explanation |
Preload | ↑ Preload → ↑ End-diastolic volume (EDV) → ↑ Stroke volume (SV) → ↑ CO until the Frank–Starling curve plateaus. Examples of increased preload: Fluid bolus, passive leg raise, pregnancy. Reduced preload (hemorrhage, dehydration, vasodilation, positive-pressure ventilation) → ↓ EDV → ↓ SV → ↓ CO. |
Contractility | ↑ Contractility → ↓ End-systolic volume (ESV) → ↑ Stroke volume → ↑ CO. Examples: Dobutamine, epinephrine, milrinone. Reduced contractility (myocardial infarction, dilated cardiomyopathy, septic cardiomyopathy, myocarditis) → ↑ ESV → ↓ SV → ↓ CO. |
Afterload | ↑ Afterload → LV ejects less blood → ↑ ESV → ↓ SV → ↓ CO. Examples: Severe hypertension, aortic stenosis, vasopressors. ↓ Afterload → ↑ SV → ↑ CO. Examples: Sepsis (vasodilation), vasodilator therapy. |
Heart Rate | Bradycardia → fewer cardiac cycles per minute → ↓ CO. Examples: Complete heart block, severe β-blocker toxicity. Extreme tachycardia → reduced diastolic filling time → ↓ SV → ↓ CO. Examples:Atrial fibrillation with rapid ventricular response (AF with RVR), supraventricular tachycardia (SVT), ventricular tachycardia (VT). |
How To Measures Cardiac Output
Echo calculates Stroke Volume where Volume = Area × Distance
The “distance” is represented by LVOT VTI.
Step 1 Measure LVOT Diameter
- View—Parasternal Long Axis
- Timing—Mid-systole
- Measure—Inner edge to inner edge,At the insertion of the aortic valve leaflets.
- Normal LVOT diameter-1.8–2.4 cm(Average adult≈2.0 cm)
Step 2 Calculate LVOT Area
Area = πr² or Area = 0.785 × Diameter²
Step 3 Measure LVOT VTI
- View—Apical Five Chamber or Apical Three Chamber
- Use-Pulse Wave Doppler in which Sample volume Placed 0.5–1 cm proximal to the aortic valve
- Avoid placing it inside the valve because turbulence causes error.
- Keep Doppler beam parallel to blood flow.
- Angle error should be <20°.
- Trace the Doppler envelope carefully.
Average
- 3 beats (sinus rhythm)
- 5–10 beats (atrial fibrillation)
Normal LVOT VTI
VTI | Interpretation |
>18 cm | Normal |
15–18 cm | Borderline |
<15 cm | Low stroke volume |
<10 cm | Severe reduction in forward flow |
Step 4 Calculate Stroke Volume
SV = Area × VTI
Example Area =3.14 cm²,VTI=20 cm
SV=63 mL
Cardiac Index
- CO varies with body size. Therefore
- Cardiac Index (CI)= CO / Body Surface Area
- Normal—2.5–4.0 L/min/m²
- CI is preferred over CO in ICU because it accounts for patient size.
Limitations
Limitation | Effect |
Poor acoustic window | Inaccurate measurement |
Incorrect LVOT diameter | Largest source of error |
Doppler angle >20° | Underestimates VTI and CO |
Aortic regurgitation | Forward stroke volume may be overestimated relative to effective systemic flow |
Subaortic obstruction (e.g., HCM) | Turbulent flow affects measurement |
Irregular rhythm (AF) | Beat-to-beat variability |
Mechanical ventilation | Respiratory variation |
Tachycardia | Difficult Doppler tracing |
Severe obesity/COPD | Poor image quality |
Common Sources of Error
Error | Consequence |
Measuring LVOT in diastole | Wrong diameter |
Measuring outer-to-outer | Overestimates area |
Sample gate inside valve | Turbulent signal |
Sample gate too proximal | Underestimates VTI |
Misaligned Doppler beam | Underestimates VTI |
Not tracing the full Doppler envelope | Underestimates SV and CO |
Using only one beat in AF | Inaccurate CO |
Response to Therapy
Intervention | Expected Echo Change |
Fluid bolus (fluid responsive patient) | ↑ LVOT VTI, ↑ SV, ↑ CO |
Dobutamine | ↑ Contractility, ↑ VTI, ↑ CO |
Vasopressors | May increase CO if hypotension limited coronary perfusion; excessive afterload can reduce CO |
Mechanical circulatory support | ↑ Forward flow, ↑ CO |
Successful thrombolysis for PE | ↓ RV afterload, ↑ LV filling, ↑ CO |
Dynamic Assessment
Because CO changes continuously, serial measurements are more valuable than a single value. In CCE, clinicians often reassess LVOT VTI before and after interventions.
Typical thresholds:
- Increase in LVOT VTI ≥10–15% after passive leg raise or fluid challenge → suggests fluid responsiveness.
- Increase in CO after starting an inotrope → indicates improved contractility.
- Falling CO despite normal blood pressure → may indicate occult shock and inadequate tissue perfusion.
Trend analysis is therefore preferred over isolated measurements in critically ill patients.