Frank–starling mechanism

Clinical relevance (anaesthesia/ICU)

Explains why increasing venous return (e.g. fluid, leg raise) can increase stroke volume up to a limit; beyond this, extra preload gives little benefit and may cause congestion.
- Used in fluid responsiveness: a patient on the steep part of the curve is more likely to increase stroke volume with preload augmentation.
Helps match right and left ventricular outputs beat-to-beat; prevents pulmonary/systemic congestion when venous return changes.
- If RV output rises, LV filling rises → LV stroke volume rises on subsequent beats.
Interpreting haemodynamics: CVP/PAOP are imperfect surrogates for preload; the key is position on the Starling curve and ventricular compliance.
- A high filling pressure can mean high preload with poor compliance (stiff ventricle) rather than adequate volume.
Ventilation effects: positive pressure ventilation/PEEP reduces venous return and RV preload; may reduce LV preload and stroke volume, especially if hypovolaemic.
- Conversely, reducing LV transmural pressure can reduce LV afterload and sometimes increase LV stroke volume in LV failure.
Heart failure: curve is flatter and shifted down; fluid may mainly raise filling pressures (pulmonary oedema) rather than stroke volume.
- Inotropes shift curve up (greater stroke volume at any given preload).

Definition and core statement

The Frank–Starling mechanism is the intrinsic property of cardiac muscle whereby increased end-diastolic fibre length (preload) leads to increased force of contraction and thus increased stroke volume, within physiological limits.
It is heterometric autoregulation (change in force due to change in fibre length), distinct from changes in contractility (inotropy).

What is “preload” (exam definitions)

Best physiological definition: end-diastolic fibre length (or sarcomere length) just before contraction.
Clinical surrogates: LVEDV (volume) and LVEDP/PAOP (pressure) but these depend on ventricular compliance and intrathoracic pressure.
- Transmural filling pressure = intracavitary pressure − surrounding pressure (e.g. pleural/pericardial).

Mechanistic basis (cellular and sarcomere)

As sarcomere length increases toward an optimal range (~2.0–2.2 μm), actin–myosin overlap becomes more favourable and active tension rises.
Length-dependent activation: stretch increases myofilament Ca2+ sensitivity (more force for the same intracellular Ca2+ transient).
Reduced lattice spacing with stretch (titin-based effects) brings filaments closer, facilitating cross-bridge formation.
Frank–Starling is rapid (beat-to-beat) and does not require neural/humoral input (though these modulate the curve).

Starling curves and how to interpret them

Typical plot: stroke volume (or cardiac output) vs preload (LVEDV or LVEDP). The curve rises then plateaus; at high filling pressures, further preload gives minimal SV increase.
Different curves represent different contractility states: increased inotropy shifts the curve up/left; decreased inotropy shifts down/right.
- At a given preload, higher contractility → higher SV; at a given SV, higher contractility requires lower preload.
Steep portion: preload-responsive; flat portion: preload-unresponsive (risk of congestion with fluids).

Determinants that move you along vs shift the curve

Move along the same curve (change preload): venous return changes (blood volume, venous tone, posture), intrathoracic pressure, ventricular filling time (HR), atrial contraction, pericardial constraint.
- Tachycardia reduces diastolic filling time → reduced preload and SV, especially with impaired relaxation.
Shift the curve (change contractility/afterload/compliance): inotropes, ischaemia, acidosis, β-blockade; afterload changes alter SV at a given preload; compliance changes alter pressure–volume relationship and apparent preload if using pressure surrogates.
- Increased afterload tends to reduce SV for a given preload and contractility (more end-systolic volume).
- Reduced compliance (e.g. LVH, tamponade) means higher LVEDP for the same LVEDV; pressure-based preload estimates can mislead.

Right vs left ventricle and interaction

Both ventricles exhibit Frank–Starling behaviour; the RV is more sensitive to afterload (pulmonary vascular resistance) and can fail with acute increases in PVR despite adequate preload.
Ventricular interdependence: RV dilation can shift the septum and impair LV filling (reducing LV preload) even if total cardiac volume is high.

Pressure–volume loop links (useful for viva)

Increasing preload increases EDV → wider PV loop → increased stroke volume, with the end-systolic pressure–volume relationship (ESPVR) unchanged if contractility constant.
Increased contractility steepens ESPVR and increases SV at the same EDV (curve shift).

Limits and pathological states

At very high sarcomere lengths, active tension falls (overstretch) and passive tension rises steeply; clinically manifests as rising filling pressures with little SV gain.
Systolic heart failure: reduced contractility → flatter curve; diastolic dysfunction: curve may appear shifted (pressure-based) due to reduced compliance; both increase risk of pulmonary oedema with fluids.
Sepsis: vasodilation increases venous capacitance (reduces stressed volume/venous return) moving the patient leftward; myocardial depression can shift curve down.

Define the Frank–Starling mechanism and state its physiological role.

Aim for a precise definition plus a functional consequence.

Definition: increased end-diastolic fibre (sarcomere) length leads to increased force of contraction and hence increased stroke volume, within limits, independent of extrinsic influences.
Role: matches cardiac output to venous return and helps equalise RV and LV outputs beat-to-beat, limiting blood pooling in either circulation.

What is preload? Give the best definition and two clinical surrogates, and explain why surrogates can mislead.

Examiners like the distinction between fibre length, volume, and pressure.

Best definition: end-diastolic myocardial fibre length (sarcomere length).
Surrogates: LVEDV (volume) and LVEDP/PAOP (pressure).
Why misleading: pressure depends on compliance and transmural pressure (affected by pleural/pericardial pressure, PEEP); the same LVEDP can correspond to different LVEDV and fibre length.
- Example: tamponade/PEEP → high measured CVP/PAOP but low transmural filling and low true preload.

Describe the cellular mechanisms underlying the Frank–Starling relationship.

Mention overlap and Ca2+ sensitivity (length-dependent activation).

Improved actin–myosin overlap as sarcomere length increases toward optimal range → increased cross-bridge formation and active tension.
Stretch increases myofilament Ca2+ sensitivity (length-dependent activation) so more force is generated for the same Ca2+ transient.
Reduced interfilament lattice spacing and titin-based effects facilitate cross-bridge cycling.

Sketch and interpret a Starling curve. What does it mean to be on the steep vs flat portion?

Use words if you can’t draw: axes, shape, and clinical meaning.

Axes: y = stroke volume/cardiac output; x = preload (LVEDV or LVEDP). Curve rises then plateaus.
Steep portion: small increases in preload → large increases in SV (more likely fluid responsive).
Flat portion: increases in preload → minimal SV gain; filling pressures rise → risk of pulmonary oedema/venous congestion.

How do changes in contractility alter the Starling curve? Give examples of causes.

Describe curve shift and name common perioperative causes.

Increased contractility shifts curve up/left (higher SV at any preload).
- Examples: dobutamine/adrenaline, reduced afterload (indirectly improving SV), improved coronary perfusion.
Decreased contractility shifts curve down/right (lower SV at any preload).
- Examples: myocardial ischaemia/infarction, acidosis, hypoxia, hyperkalaemia, β-blockade, cardiomyopathy, sepsis-related myocardial depression.

Differentiate Frank–Starling mechanism from Bowditch (force–frequency) and Anrep effect.

A common FRCA viva comparison question.

Frank–Starling: length-dependent increase in force with increased preload; immediate/beat-to-beat.
Bowditch (treppe): increased heart rate → increased contractility via increased intracellular Ca2+ availability (force–frequency relationship).
Anrep effect: afterload-induced increase in contractility (minutes) following a sudden rise in arterial pressure; involves intracellular Na+/Ca2+ handling and neurohumoral factors.

Explain how positive pressure ventilation and PEEP can change stroke volume using Frank–Starling principles.

Cover venous return, RV preload, and LV afterload/transmural pressure.

Increased intrathoracic pressure reduces venous return → reduced RV preload → reduced RV SV → reduced LV filling after a delay → reduced LV SV (especially if on steep part of curve).
PEEP can increase PVR (alveolar vessel compression) → increased RV afterload → RV dilation → septal shift → impaired LV filling (ventricular interdependence).
In LV failure, raised intrathoracic pressure can reduce LV transmural pressure (afterload) and sometimes improve LV ejection despite reduced preload.

A patient has a high CVP but low cardiac output. Use Frank–Starling concepts to give differential explanations.

Show you understand that filling pressure ≠ effective preload and that curve position/shape matters.

May be on flat portion of Starling curve (poor contractility/heart failure): extra preload raises CVP without increasing SV.
High CVP may not reflect high transmural preload: raised intrathoracic pressure (PEEP), tamponade, constriction.
RV failure/acute pulmonary hypertension: high CVP from RV dilation; low LV preload due to reduced forward flow and septal shift.
Tricuspid regurgitation can elevate CVP and reduce effective forward stroke volume.

How would a pressure–volume loop change with an isolated increase in preload? What stays the same?

Link Starling to PV loop geometry and ESPVR.

EDV increases (rightward shift of end-diastolic point) → loop becomes wider → SV increases.
If contractility and afterload unchanged, ESPVR slope and end-systolic point relationship remain essentially unchanged (though ESV may change slightly depending on afterload).

Why can two patients with the same LVEDP have different stroke volumes?

This tests compliance and contractility.

Different LV compliance: stiff ventricle (LVH/diastolic dysfunction) generates higher LVEDP for a given LVEDV, so true preload (fibre length) may be lower.
Different contractility: at the same preload, a more contractile ventricle generates higher SV (curve shifted up).
Different surrounding pressures: PEEP/pericardial pressure alters transmural LVEDP at the same measured LVEDP.

Give examples of clinical manoeuvres/tests that use Frank–Starling principles to assess fluid responsiveness, and state key limitations.

FRCA often expects dynamic tests rather than static pressures.

Passive leg raise (autotransfusion) with real-time SV/CO measurement; a rise in SV suggests preload responsiveness.
Stroke volume variation/pulse pressure variation during controlled ventilation (reflects cyclic preload changes) when prerequisites met.
- Limitations: requires sinus rhythm, controlled ventilation with adequate tidal volume, no significant RV failure, low spontaneous effort.
Small fluid challenge with SV monitoring; limitation: risk of fluid overload and confounding by changes in vascular tone/afterload.

Viva: ‘Talk me through the factors determining stroke volume and where Frank–Starling fits in.’

Structure: preload, afterload, contractility, heart rate/diastolic time; then place Starling as preload–SV link.

Stroke volume determined by preload, afterload, and contractility (and synchrony/valve competence).
Frank–Starling describes how increasing preload increases SV at constant contractility/afterload (movement along a curve).
Afterload increases tend to reduce SV (increase ESV); contractility increases raise SV (shift curve up).
Heart rate affects SV via filling time and force–frequency effects; extreme tachycardia reduces preload and SV.

Written/SAQ theme: ‘Explain why CVP is a poor guide to fluid therapy using the Frank–Starling mechanism.’

Marking points: CVP ≠ preload, compliance, transmural pressure, curve position, RV issues.

CVP is a pressure, not fibre length; relationship to EDV depends on RV compliance and pericardial constraint.
CVP is influenced by intrathoracic pressure (PEEP), venous tone, and tricuspid valve disease; therefore it does not reliably indicate where the patient sits on the Starling curve.
Even if CVP reflects preload, the response depends on curve slope: on the flat portion, fluids raise CVP without raising SV.
RV failure/pulmonary hypertension can cause high CVP with low LV preload and low CO; fluid may worsen RV dilation and output.

Viva: ‘How do inotropes and fluids differ in their effect on cardiac output on a Starling diagram?’

Key distinction: move along vs shift curve.

Fluids increase venous return/preload → move along the same Starling curve to a higher SV (if on steep portion).
Inotropes increase contractility → shift the Starling curve up/left, increasing SV at any given preload.
In heart failure (flat curve), inotropes may increase SV more effectively than fluids, which mainly increase filling pressures.