Cardiac Catheterisation and Haemodynamics
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Direct answer
Cardiac catheterisation and haemodynamics means measuring pressures, oxygen saturations and cardiac output in the heart and great vessels to define disease. Normal mean right atrial pressure is about 2 to 6 mmHg, and a saturation step-up of about 5 per cent or more between the right atrium and pulmonary artery signals a left-to-right shunt. The shunt ratio Qp/Qs grades the shunt, and valve areas come from the Gorlin formula.
What you must remember
- Right atrium: mean pressure of about 2 to 6 mmHg, with a and v waves that reflect atrial contraction and venous filling.
- Right ventricle: systolic pressure of about 15 to 25 mmHg and an end-diastolic pressure near 0 to 8 mmHg.
- Pulmonary artery: systolic pressure of about 15 to 30 mmHg, with a mean below 20 mmHg counted as normal.
- Wedge pressure: about 4 to 12 mmHg, used as the estimate of left atrial pressure.
- Saturation step-up: a rise of about 5 per cent or more at ventricular or pulmonary artery level (about 7 per cent at atrial level) suggests a left-to-right shunt at that level.
- Shunt ratio: Qp/Qs near 1 is normal, while a value of 1.5 or more usually makes a shunt worth closing.
- Cardiac index: normally about 2.5 to 4 L/min/m², calculated from output and body surface area.
Working out a shunt from catheter data
A 35-year-old woman has a loud fixed split of the second heart sound and a systolic flow murmur, and she is referred for catheter assessment before closure. The steps below show how the saturation data are turned into a shunt size.
- Sample the chambers. Record oxygen saturation from the venae cavae, right atrium, right ventricle, pulmonary artery and a systemic artery in an oximetry run.
- Look for a step-up. A rise of about 5 per cent or more between the venae cavae and pulmonary artery indicates a left-to-right shunt at that level.
- Calculate the shunt ratio. Qp/Qs equals aortic saturation minus mixed venous saturation, divided by pulmonary venous saturation minus pulmonary arterial saturation.
- Estimate the output. The Fick method divides oxygen consumption by the arteriovenous oxygen difference, while thermodilution is used in many laboratories.
- Assess pulmonary resistance. Pulmonary vascular resistance equals mean pulmonary artery pressure minus wedge pressure, divided by cardiac output, in Wood units, with values under about 3 traditionally taken as normal (current pulmonary hypertension guidance uses above 2 as abnormal).
Qp/Qs near 1 suggests no significant shunt, while a ratio of 1.5 or more usually prompts closure of an atrial septal defect. In valve disease, the Gorlin formula converts the gradient and output into a valve area, and an aortic valve area below 1 cm² indicates severe stenosis.
Where the exam tries to catch you
Catheter questions reward a clean reading of the saturation sample and the pressure pattern, so the traps below repeat the common misreads.
- Trap: a saturation step-up is read as proof of pulmonary hypertension. Correct: a step-up points to a shunt, while pulmonary hypertension is judged from the pressures and resistance.
- Trap: a peripheral arterial sample is used alone for a shunt study. Correct: shunt calculation needs samples from the right heart chambers and pulmonary artery as well.
- Trap: a high right atrial pressure with equal diastolic pressures is read as simple heart failure. Correct: equalised diastolic pressures suggest constriction or tamponade, and a dip-and-plateau (square root) ventricular pressure trace points to constriction; both need pericardial assessment rather than diuretics.
Frequently asked questions
What are the normal right atrial and pulmonary artery pressures?
Normal mean right atrial pressure is about 2 to 6 mmHg, and the pulmonary artery pressure is about 15 to 30 mmHg systolic with a mean below 20 mmHg. Wedge pressure is typically 4 to 12 mmHg and is used to estimate left atrial pressure.
What step-up in oxygen saturation suggests a left-to-right shunt?
A step-up of about 5 per cent or more in oxygen saturation between the venae cavae or right atrium and the pulmonary artery suggests a left-to-right shunt. An atrial septal defect is a classic cause, and the shunt ratio Qp/Qs is then calculated to grade its size.
What does Qp/Qs mean and when does a shunt need closure?
Qp/Qs is the ratio of pulmonary to systemic blood flow, and its normal value is close to 1. A ratio of 1.5 or more usually makes closure of an atrial septal defect worth considering, with the final decision depending on symptoms and chamber size.
How is aortic valve area calculated at catheterisation?
The Gorlin formula calculates aortic valve area from cardiac output, heart rate, systolic ejection period and the mean transvalvular gradient. An area below 1 cm² indicates severe aortic stenosis.
What is pulmonary vascular resistance and what is normal?
Pulmonary vascular resistance is the mean pulmonary artery pressure minus wedge pressure, divided by cardiac output and expressed in Wood units. A value below about 3 Wood units is normal, and higher values point to pulmonary arterial disease.