Sample · CICM
Venous return: mean systemic filling pressure, PEEP and noradrenaline
Cardiovascular and shock · 3 parts · 10 marks. Decide what you would write, then compare with the rubric and model answer. In the app you write it under time and it comes back marked.
A 70 kg man is being ventilated in the intensive care unit (ICU) after an emergency laparotomy. His mean arterial pressure is 62 mmHg and his central venous pressure (CVP) is 12 mmHg. The team is considering a fluid bolus, an increase in noradrenaline, and an increase in positive end-expiratory pressure (PEEP) from 8 to 14 cmH2O for worsening oxygenation.
This question is about venous return, not cardiac output or oxygen delivery.
(a) Define mean systemic filling pressure and outline the determinants of venous return. 4 marks
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- Mean systemic filling pressure (MSFP): the pressure throughout the systemic circulation when flow stops and pressures equalise (zero flow); it is the upstream driving pressure for venous return (1)
- MSFP = stressed volume / systemic vascular compliance; unstressed volume (about 70% of blood volume, mostly venous) fills vessels without generating pressure, stressed volume (about 30%) stretches them (1)
- Guyton relationship: venous return = (MSFP - right atrial pressure) / resistance to venous return; at steady state venous return equals cardiac output (1)
- Determinants named with direction: stressed volume (blood volume, venous tone), venous compliance, right atrial pressure (downstream pressure), resistance to venous return (venous calibre, posture, intra-abdominal pressure) (1)
- Mechanism linked to effect, for example a rise in right atrial pressure reduces the pressure gradient and therefore venous return even if MSFP is unchanged (1)
Mean systemic filling pressure (MSFP) is the pressure that would be measured everywhere in the systemic circulation if the heart stopped and pressures equalised. It is the upstream pressure that drives blood back to the heart.
MSFP depends on the stressed volume and the compliance of the system: MSFP = stressed volume / compliance. Of a normal blood volume (about 70 mL/kg), roughly 70% is unstressed volume, which fills the vessels without raising pressure, and about 30% is stressed volume, which stretches the vessel walls and generates pressure. Most of this sits in the compliant venous system.
Guyton's relationship: venous return = (MSFP - right atrial pressure) / resistance to venous return. At steady state venous return equals cardiac output.
Determinants:
- Stressed volume: increased by fluid or blood, and by venoconstriction, which recruits unstressed volume into stressed volume.
- Venous compliance: lower compliance (venoconstriction) raises MSFP for the same volume.
- Right atrial pressure: the downstream pressure. A higher right atrial pressure narrows the gradient and reduces venous return.
- Resistance to venous return: venous calibre, external compression (raised intra-abdominal pressure), posture.
(b) Describe the venous return curve and how it is altered by a fluid bolus and by haemorrhage. 3 marks
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- Axes: venous return (y) against right atrial pressure (x); linear fall as right atrial pressure rises; x-intercept at MSFP where flow is zero; slope = -1/resistance to venous return (1)
- Plateau when right atrial pressure is below about 0 mmHg: great veins collapse at the thoracic inlet, so lowering right atrial pressure further does not increase flow (1)
- Fluid: parallel shift to the right (higher MSFP, same slope); haemorrhage: parallel shift to the left; the operating point is the intersection with the cardiac function (Frank-Starling) curve (1)
The venous return curve plots venous return (y-axis) against right atrial pressure (x-axis).
- It crosses the x-axis at MSFP: when right atrial pressure equals MSFP there is no gradient and flow stops.
- As right atrial pressure falls, venous return rises linearly, with a slope of -1/resistance to venous return.
- Below a right atrial pressure of about 0 mmHg the curve plateaus, because the great veins collapse where they enter the thorax; making right atrial pressure more negative does not increase flow.
A fluid bolus raises stressed volume and MSFP, shifting the curve to the right in parallel (same slope). Haemorrhage does the opposite, a parallel shift to the left.
The actual cardiac output and right atrial pressure are where the venous return curve intersects the cardiac function (Frank-Starling) curve. A fluid bolus therefore increases cardiac output only if the heart is on the steep part of its function curve; on the flat part, right atrial pressure rises with little gain in flow.
(c) Explain the expected effects on venous return of increasing PEEP and of increasing noradrenaline in this patient. 3 marks
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- PEEP raises intrathoracic and right atrial pressure, reducing the MSFP - right atrial pressure gradient and venous return, most marked when hypovolaemic (1)
- PEEP partly offsets this by diaphragmatic descent raising intra-abdominal pressure and MSFP; net effect depends on volume status and lung compliance (less transmission in stiff lungs) (1)
- Noradrenaline (alpha-1 venoconstriction) recruits unstressed to stressed volume, raising MSFP and venous return; may also increase resistance to venous return; net effect usually increased venous return at moderate doses (1)
- Applied to this patient: CVP 12 with low pressure after laparotomy; check fluid responsiveness before the bolus, anticipate a fall in output with the PEEP increase (1)
PEEP from 8 to 14 cmH2O:
- Raises intrathoracic pressure, which is transmitted to the right atrium. Right atrial pressure rises, so the gradient (MSFP - right atrial pressure) falls and venous return falls. This is most marked if he is hypovolaemic.
- PEEP also pushes the diaphragm down and raises intra-abdominal pressure, which increases MSFP and partly preserves the gradient.
- In stiff lungs less pressure is transmitted to the pleural space, so the effect is smaller.
- Net: expect some fall in venous return and blood pressure after the change, so assess volume status first and watch the arterial trace as PEEP is increased.
Noradrenaline:
- Alpha-1 venoconstriction reduces venous capacitance, converting unstressed volume into stressed volume. MSFP rises, so venous return rises without any fluid given (an "auto-transfusion").
- It may also raise resistance to venous return, but at usual doses the rise in MSFP dominates and venous return increases.
For this patient, with a CVP of 12 mmHg after a laparotomy (likely raised intra-abdominal pressure), I would test fluid responsiveness (for example a passive leg raise with a cardiac output measure) rather than giving fluid empirically, and expect the PEEP increase to need more vasopressor.
Where candidates lose marks here
- Answering about cardiac output or oxygen delivery (the neighbouring topic) instead of venous return: the 2026.1 First Part report flagged exactly this.
- Stating that venous return is 'driven by CVP': CVP is the downstream pressure that opposes venous return, not the driving pressure.
- Drawing the curve without labelled axes, the MSFP intercept or the plateau below 0 mmHg.
- Listing effects of PEEP and noradrenaline without linking each mechanism to the gradient in the Guyton equation.
- Spending most of the time on part (a) definitions and leaving part (c), which carries the applied marks, as one line.
Original content written for Fellowship Ready, for exam preparation only, not clinical advice. Not affiliated with the CICM.