Physiology Note - Mean Systemic Filling Pressure

 

DR. MANOJ RAJU PRABANDHANKAM

MBBS, MD, DM, IDCCM, IFCCM

CONSULTANT CRITICAL CARE MEDICINE

ASTER NARAYANADRI HOSPITAL, TIRUPATI.

                                                            PHYSIOLOGY NOTE - A HICS INITIATIVE OCTOBER 16 2025

MEAN SYSTEMIC FILLING PRESSURE

Introduction:

Ø  Mean systemic filling pressure (Pmsf) describes the pressure within the systemic circulation when there is no blood flow, that is, when the heart stops and pressures between arterial and venous systems equilibrate.

Ø  It represents the upstream pressure driving venous return to the heart and provides important insights into the interaction between blood volume, venous tone, and cardiac function.

Ø  Although not routinely measured at the bedside, understanding Pmsf is critical for interpreting fluid responsiveness, vascular capacity, and hemodynamic stability in critically ill patients.

 

 

 

 

 

Definition and Concept behind it: GUYTONS principle

MSFP is defined as the equilibrated pressure in the systemic circulation after cessation of cardiac output. It reflects the “average” filling of the systemic vasculature relative to its capacitance. It is usually around 7mmHg.

The value is not only the measure of total blood volume alone but also function of both stressed and unstressed volume.

Determinants

•        Tone of the smooth muscle in the systemic circulation

•        Play a role in the elastic recoil pressure which produces the MSFP

•        Volume of fluid in the systemic circulation

•        Pouring fluid into a vessel places a pressure on the vessel walls, and in the circulatory system. The pressure these soft squishy walls produce is mainly dependent on the tone of the smooth muscle.

•        Separated into the "unstressed" and "stressed" volumes.

•        Unstressed volume: The amount of blood filling the vasculature without exerting pressure on the walls.

•        Stressed volume: The additional volume that stretches vessel walls and creates an effective pressure gradient for venous return. The greater the stressed volume, the greater Pmsf, and in turn, the greater the venous return

•        Explained very well by Guyton’s bath tub model

•        Imagine a bathtub three quarters to capacity with the stopper located halfway up the side of the tub (rather than at the base). The stopper is unplugged and a drainpipe is inserted which leads to a bucket. The flow of water from the bathtub (vasculature) to Unstressed volume the bucket (the right atrium), is determined by the relative pressure between the tub (the Pmsf), the bucket (the right atrial pressure) as well as the length and diameter of the drainpipe (venous resistance). In this model, the tub will drain to the level of the drainpipe but no farther. The remainder of the water in the system is considered the unstressed volume. In order to get more water to flow from the tub to the bucket, either more water can be added to the system, increasing both the total volume and stressed volume, or one can compress the walls of the tub. This will decrease the system’s compliance, shifting a portion of the unstressed volume into stressed volume, which will allow more water to drain from the bed without adding additional volume to the system.

(A) Stressed volume responsible for Pmsf

(B) Illustrating how a fluid bolus augments venous return

(C) Illustrating how a vasopressors augment venous return

Role in Venous Return and Circulatory Physiology:

The concept of MSFP is central to Guyton’s model of circulation. According to this framework:

Cardiac output is primarily governed by venous inflow. Venous return (VR) is driven by the pressure gradient between Pmsf and right atrial pressure (RAP). (pressure difference between the veins and venules on the one hand and the right atrium on the other hand, with the heart emptying the right atrium and keeping the right atrial pressure (RAP) low)

The relationship is expressed as:

•        Venous return = (Pmsf–RAP)/RVr. 

•        Its three determinants are

•        Mean systemic filling pressure (Pmsf) forward pressure that drives flow towards the right atrium, results from the elastic recoil potential stored in the walls of the veins.

•        Right atrial pressure (RAP) the backward pressure of venous flow, impedes venous return and

•        Resistance to venous return (RVr)

1.      Venous Return (blue solid line, baseline MSFP \~7 mmHg)

a.      Negative slope: as RAP rises, venous return falls (backpressure).

b.      At RAP = MSFP (\~7 mmHg), venous return = 0 (because no driving gradient).

2.      Venous Return after ↑MSFP (cyan dashed line, \~9 mmHg)

a.      Parallel rightward shift of venous return curve.

b.      Happens after fluids or venoconstriction.

c.      Equilibrium CO is higher (3.3 L/min vs baseline 2.8).

 

3.      Cardiac Function Curves (red/orange/green)

a.      Red = normal cardiac function (baseline).

b.      Orange dashed = failure (weaker pump) → flatter slope, lower CO (2.0 L/min at equilibrium).

c.      Green dashed = inotropy (stronger pump) → steeper slope, higher CO (3.2 L/min).

4.Vertical Dashed Lines

Gray line at 7 mmHg = baseline MSFP.

Cyan line at 9 mmHg = MSFP after fluids.

Annotated points

1.      Baseline equilibrium: RAP ≈ 3 mmHg, CO ≈ 2.8 L/min.

2.      Failure: RAP higher (\~4 mmHg), CO lower (\~2.0).

3.      Inotropy: RAP lower (\~1.5 mmHg), CO higher (\~3.2).

4.      Fluids (↑MSFP): RAP slightly higher (\~3.5 mmHg), CO higher (\~3.3).

 

Clinical Interpretation of Guyton Diagram

CLINICAL SCENARIO	MSFP	CARDIAC FUNCTION CURVE	RAP	CARDIAC OUTPUT	EXAMPLE
Hypovolemia	↓ MSFP → venous return curve shifts leftward/down	Normal	Decreased	Decreased	Bleeding, Dehydration
Fluid Resuscitation	↑ MSFP → venous return curve shifts rightward/up	Normal	RAP ↑ slightly	CO ↑ modestly	Fluid bolus increases stressed volume; venoconstriction (e.g., norepinephrine) has similar effect.
Heart Failure (systolic dysfunction)	Normal MSFP	Cardiac function curve flattens (pump weak)	RAP ↑ (congestion)	CO ↓	Explains pulmonary edema & high CVP with low forward flow.
Inotropy (dobutamine, epinephrine)	Normal MSFP	Cardiac function curve steepens (stronger pump)	RAP ↓	CO ↑	Improves contractility → higher forward flow at lower filling pressure.
Sepsis (early, vasodilated phase)	↓ MSFP (vasodilation → unstressed volume ↑)	Variable: may be depressed	RAP ↓	CO often normal or ↑ (high-output state)	Explains why fluids + vasopressors are both needed
Tamponade / Tension Pneumothorax	Venous return curve displaced downward (impaired filling)	Normal or depressed	RAP markedly ↑	CO ↓	Restriction on heart filling → both curves intersect at low CO, high RAP.
PEEP / Intrathoracic Pressure ↑ (mechanical ventilation)	Venous return curve shifts downward (less gradient)	May depress cardiac function	RAP ↑	CO ↓	Explains why high PEEP reduces venous return & cardiac output.

 

 

This is why the Guyton model is powerful: it explains why fluids help hypovolemia but not in advanced heart failure, and why inotropes shift the curve differently than fluids.

References:

1)     Persichini, R., Lai, C., Teboul, JL. et al. Venous return and mean systemic filling pressure: physiology and clinical applications. Crit Care 26, 150 (2022). https://doi.org/10.1186/s13054-022-04024-x

2)     Rory Spiegel, Clin Exp Emerg Med 2016;3(1):52-54 http://dx.doi.org/10.15441/ceem.16.128

3)     Wijnberge, M., Sindhunata, D.P., Pinsky, M.R. et al. Estimating mean circulatory filling pressure in clinical practice: a systematic review comparing three bedside methods in the critically ill. Ann.Intensive Care 8, 73 (2018). https:/doi.org?10/1186/s13613-018-0418-2.