Transmural Cardiac Pressures and Ventricular Loading Conditions

A pressure inside a cardiac chamber does not reveal how strongly that chamber is distended. The pressure outside its wall also matters, and the relation between pressure and myocardial stretch changes with relaxation, geometry, and constraint. Transmural reasoning helps explain these differences without turning an estimated pressure into a direct measurement of preload or a treatment target.

41.1 Preload Describes Myocardial Stretch Before Contraction

Preload refers to the mechanical state of the myocardium at the end of filling, before the next contraction. End-diastolic fiber or sarcomere stretch is central to the concept. At a given contractile state and afterload, increasing initial stretch within the physiological operating range can increase subsequent force and stroke volume.

At the bedside, fiber stretch is rarely measured directly. End-diastolic chamber volume, end-diastolic pressure, and surrounding pressure provide related information. They are substitutes with assumptions, rather than interchangeable definitions of one measured quantity.

Volume is generally closer to chamber distension than an unqualified pressure, but even volume does not specify individual fiber length. The same cavity volume can occur in hearts with different dimensions, wall thicknesses, shapes, or myocardial architecture. A dilated spherical ventricle and a smaller thick-walled ventricle do not distribute wall stress identically.

Pressure adds another layer. A measured end-diastolic pressure reflects both the force opposing distension within the wall and the pressure applied from outside. A higher pressure may indicate greater filling, a stiffer chamber, incomplete relaxation, greater external constraint, or a combination.

End diastole must also be defined consistently. An atrial contraction can raise late-diastolic pressure substantially. A mean atrial pressure averages a different part of the cardiac cycle. Comparing one with an instantaneous ventricular end-diastolic value without considering timing can create an apparent disagreement that is partly built into the measurements.

The Frank–Starling response is a relationship between changing initial myocardial state and changing output under specified conditions. One static pressure cannot locate the patient precisely on that relationship. Contractility, afterload, heart rate, valve function, and ventricular interaction can all change the output obtained from the same filling state.

Use the word preload precisely enough to preserve these distinctions. State whether the observation is a pressure, a volume, an estimated distending pressure, or a demonstrated flow response. That makes the next inference visible and allows its assumptions to be assessed.

41.2 Transmural Pressure Requires an Internal and a Local External Pressure

The pressure distending a wall is the pressure inside it minus the pressure immediately outside it. Let Pint be intracavitary pressure, Pext the local external pressure, and Ptm transmural pressure. All three must use the same pressure units and reference convention.

For a cardiac chamber, the relevant external pressure is the pressure acting on its surface. Pleural pressure influences this environment, but pericardial constraint and contact with adjacent structures can add local effects. The external pressure for a ventricular free wall is also not necessarily the pressure on the other side of the interventricular septum.

Consider illustrative end-diastolic measurements expressed in mm Hg. Initially, intracavitary pressure is 18 and local external pressure is 5. Transmural pressure is 18 − 5 = 13 mm Hg. Later, internal pressure is 23 while external pressure is 12. Transmural pressure is 23 − 12 = 11 mm Hg.

The displayed intracavitary value has risen by 5 while the distending difference has fallen by 2. A higher catheter reading can therefore coexist with less pressure distending the chamber. The arithmetic does not establish whether end-diastolic volume or stroke volume changed; those require the pressure–volume relationship and functional response.

The example assumes reliable paired measurements at comparable cardiac and respiratory moments. Zeroing, transducer height, damping, catheter position, and waveform interpretation affect the internal measurement. External-pressure estimates introduce their own uncertainty. Subtracting two uncertain values produces an estimate with uncertainty from both.

A common atmospheric reference is useful because it allows subtraction of compatible measurements. A shared offset cancels, but two independently misleveled or differently referenced signals may not share the same offset. Recording pressures in the same patient does not automatically make them comparable.

Before calculating, identify the wall, its two pressure environments, the sampling time, and the units. This is more informative than calling any pressure with a correction applied to it a true filling pressure.

41.3 Relaxation and Passive Stiffness Change the Pressure–Volume Relationship

Ventricular filling occurs while active myocardial relaxation and passive tissue properties interact. Relaxation removes residual contractile tension after systole. Passive stiffness describes how much additional pressure is needed to distend the chamber once its mechanical state is specified. These are different processes.

If relaxation is incomplete, pressure can remain high during filling even without a large end-diastolic volume. Tachycardia shortens the available filling interval, while ischemia and altered calcium handling can impair relaxation. A late-diastolic pressure is therefore not always a pure measurement of passive elasticity.

Weiss, Frederiksen, and Weisfeldt studied the decline in pressure in isolated canine ventricular preparations. Their experiments characterized the time course of relaxation and its dependence on the preceding contraction and interventions. This primary work helps separate active pressure decline from passive filling behavior; it does not supply one relaxation constant for every clinical state. 1

For a small change around a specified operating point, chamber compliance C can be expressed as a volume change divided by the corresponding transmural pressure change. If volume is in mL and pressure in mm Hg, compliance is in mL/mm Hg.

For illustration, a 20-mL increase accompanied by a 2-mm Hg transmural rise gives 20/2 = 10 mL/mm Hg. The same volume increase accompanied by an 8-mm Hg rise gives 20/8 = 2.5 mL/mm Hg. The second chamber requires a larger pressure increase to accept that additional volume.

These are local incremental examples, not whole-heart constants. Ventricular pressure–volume curves are nonlinear; compliance generally changes as filling increases. The same ventricle can therefore behave differently at two operating volumes. Changes in external constraint or incomplete relaxation can also alter the observed curve without representing a new intrinsic material property of the myocardium.

A stiff ventricle may develop pulmonary venous pressure elevation after a modest volume increment. A compliant dilated ventricle may contain a large volume at a lower pressure. Correcting external pressure cannot erase these differences or turn one transmural value into a universal volume estimate.

41.4 The Pericardium and the Other Ventricle Contribute to Constraint

The heart does not fill as an isolated elastic chamber. The pericardium surrounds the combined cardiac volume, and the ventricles share a septum and myocardial connections. An increase in right ventricular or atrial volume can change the space available to the left ventricle.

As total cardiac volume increases, the pericardium can oppose further expansion more strongly. Intracavitary diastolic pressure then rises partly because the pressure outside the myocardium has risen. Interpreting the entire rise as greater myocardial distension attributes an external contribution to the chamber itself.

Ventricular interaction adds a directional effect. Right ventricular dilation or pressure loading can shift the septum and alter left ventricular shape. Maintaining a given left ventricular volume may require more pressure, or volume may decrease despite a similar measured filling pressure. The external environment includes this shared geometry as well as pressure on the free wall.

Tyberg and colleagues measured pericardial surface pressure with a flat balloon during volume loading in nine patients immediately after thoracotomy for elective cardiac surgery. Right atrial and pericardial pressures tracked closely in that preparation. Their observations demonstrated meaningful normal pericardial constraint, but the selected operative setting does not establish equality of the pressures in all patients. 2

The method used to measure external pressure also matters. Smiseth and colleagues examined patients undergoing drainage of chronic pericardial effusions and a complementary canine preparation. In the animal experiments, a flat balloon continued to detect surface constraint after a fluid-pressure catheter reading fell as fluid was removed. Pressure in a small fluid space need not represent every contact force applied to the cardiac surface. 3

Pericardial effusion, postoperative changes, cardiac enlargement, and regional compression can therefore alter both the physiology and the validity of a proxy. One chamber's pressure cannot be assumed to describe the external pressure everywhere else. Chamber size, septal behavior, and the clinical setting help distinguish additional myocardial filling from additional external constraint.

41.5 Esophageal and Right Atrial Pressures Are Conditional Proxies

Esophageal pressure estimates pressure within the surrounding thoracic environment. Its usefulness depends on balloon position, filling, calibration, body position, and the physiological question. A validated respiratory swing is different from a proven absolute pressure at a particular cardiac surface.

Chiumello and colleagues compared esophageal measurements during controlled and assisted ventilation. Balloon position and paralysis changed absolute end-expiratory values more than the relationship between pressure changes during occlusion. Their findings support separating validation of changes from interpretation of absolute levels. The studied patients and techniques do not validate an exact pericardial subtraction in every patient with severe lung disease. 4

Unit conversion must precede subtraction. One cm H₂O is approximately 0.736 mm Hg. Suppose an illustrative intracavitary end-diastolic pressure is 20 mm Hg and an external-pressure estimate is 8 cm H₂O. Converting the external value gives 8 × 0.736 = 5.888 mm Hg, approximately 5.9. The estimated transmural difference is 20 − 5.9 = 14.1 mm Hg.

The result has a clear meaning only under the stated assumption that this external estimate represents the pressure acting on the chamber at that moment. Conversion solves a unit problem; it does not validate the physiological proxy. Airway PEEP of 8 cm H₂O could not simply be substituted for that external measurement.

Right atrial pressure has also been proposed as an estimate of pericardial pressure. Its relationship depends on the pressure required to distend the atrium and right ventricle, the degree of common constraint, and regional conditions. A shared external-pressure contribution does not mean that the right atrial wall has zero transmural pressure.

Pinsky, Vincent, and De Smet directly compared atrial and pericardial pressures during PEEP changes in fifteen postoperative cardiac surgery patients. Right atrial pressure exceeded pericardial pressure and changed differently as PEEP increased. This provides a concrete limit to treating right atrial pressure as an exact external-pressure measurement. 5

Neither esophageal nor right atrial pressure should be discarded because it is imperfect. Each can add useful information when its measurement quality and physiological interpretation are stated. Label the result as an estimate, retain the original measurements, and assess whether its direction agrees with chamber geometry and flow.

41.6 Venous Return and Myocardial Distension Use Different Pressure Differences

The downstream pressure opposing systemic venous drainage is the intraluminal pressure encountered by returning blood. While the venous pathway remains open, right atrial pressure is an important part of that downstream condition. The distending pressure of the atrial wall answers a different question.

Subtracting local external pressure from right atrial pressure may help interpret atrial distension. It does not allow the resulting value to replace intracavitary right atrial pressure in a systemic return gradient whose upstream value retains an atmospheric reference.

Consider illustrative pressures in mm Hg. Let mean systemic filling pressure be 16, measured right atrial pressure 10, and local external pressure 6. The pressure distending the atrium is 10 − 6 = 4. The simple upstream-to-atrial pressure difference is 16 − 10 = 6.

Those two differences describe different physical relationships. Using 16 − 4 would mix the upstream intravascular reference with a locally corrected downstream value. It would not describe the actual pressure drop between those vascular locations. A consistent change of reference must be applied to all relevant pressures, rather than to one endpoint only.

Respiration can change both relationships simultaneously. Increased surrounding pressure may raise measured right atrial pressure while reducing its transmural pressure. The systemic return gradient may narrow even as the chamber becomes less distended. Conversely, negative inspiratory pressure may lower measured atrial pressure while preserving or increasing its transmural value.

Upstream pressure and vascular resistance can change as well. Fessler and colleagues demonstrated mechanical and reflex increases in mean systemic pressure during PEEP in dogs. The change was not simply explained by abdominal pressure, and preservation of a gradient did not remove all possible effects on return. The experiment reinforces the need to consider both vascular endpoints and the pathway between them. 6

Collapsible segments add a further limitation. When a vessel narrows enough to limit flow, its surrounding and critical closing pressures can become relevant downstream conditions. Transmural cardiac pressure remains useful for chamber loading, but it does not replace a complete model of vascular flow.

41.7 Wedge, Left Atrial, and Left Ventricular Pressures Are Related but Distinct

Pulmonary artery wedge pressure, also called pulmonary artery occlusion pressure, is measured after a pulmonary arterial branch is occluded. Under suitable conditions, a fluid pressure column connects the occluded region to the pulmonary venous and left atrial circulation. The measurement can then estimate left atrial pressure.

The pathway must remain suitable for pressure transmission. Catheter position, complete occlusion, regional alveolar pressure, vascular obstruction, and lung disease can affect it. If surrounding pressure limits the vascular connection, the recorded value may not represent left atrial pressure adequately. A technically recognizable wedge waveform is necessary evidence, but not a guarantee of every downstream assumption.

Left atrial pressure itself differs from left ventricular end-diastolic pressure. With an open mitral valve and little flow-related gradient, their instantaneous pressures can approach one another during diastole. Mitral stenosis creates a gradient. Regurgitant v waves, atrial stiffness, rhythm, and the timing used for averaging can make mean atrial pressure differ from a ventricular end-diastolic value.

Teboul and colleagues directly compared occlusion pressure and left ventricular end-diastolic pressure in twelve supine patients with severe ARDS across PEEP conditions. With the catheter tips in dependent positions relative to the left atrium, the pressures remained closely related even at the higher PEEP studied. This refutes a blanket rule that PEEP above one value necessarily invalidates wedge pressure, while retaining the study's anatomical and disease conditions. 7

In a larger clinical catheterization cohort, Hemnes and colleagues documented important disagreement between wedge and ventricular end-diastolic pressures. Rhythm, valve history, and atrial characteristics were associated with discordance. These clinical observations show why agreement cannot be assumed from population correlation alone. 8

Even perfect agreement between two intracavitary pressures would not establish a transmural pressure. Both may contain the same external-pressure contribution. Nor is wedge pressure a direct measurement of pulmonary microvascular filtration pressure, chamber volume, or fluid responsiveness. Specify which anatomical pressure is being estimated before using it to answer a loading question.

41.8 Respiratory Conditions Change the Meaning of an End-Expiratory Value

End expiration provides a reproducible respiratory landmark when the pattern is stable. It is not automatically a moment of zero external cardiac pressure or complete muscle relaxation. PEEP, dynamic hyperinflation, abdominal loading, posture, and active expiration can all influence the pressure around the heart at that point.

PEEP is an airway pressure, not a pericardial measurement. The fraction appearing in pleural or cardiac surroundings depends on lung and chest wall mechanics, regional inflation, and the intervention being considered. A relation describing tidal pressure changes does not necessarily provide the absolute external pressure at end expiration.

This is why subtracting one fixed fraction of PEEP from every filling pressure is unjustified. Even converting PEEP into mm Hg leaves the unknown transmission and baseline external-pressure contributions unresolved. The calculation can create false precision while mixing a measured internal pressure with an assumed external one.

Teboul and colleagues studied a transmission-based estimate in 107 ventilated patients, comparing it with a nadir pressure obtained after transient airway disconnection. Agreement depended on the presence of dynamic hyperinflation. This was a comparison of specific estimation methods, rather than universal validation against direct local pericardial pressure. It does not establish a general correction formula or a reason to disconnect a patient solely to obtain a number. 9

During spontaneous inspiration, internal and external pressures can fall by different amounts. During assisted breathing, positive airway pressure and negative muscle-generated pressure combine. Measurements need to be aligned to the actual effort and waveform; the ventilator mode name does not identify the external pressure.

Active expiration can similarly elevate or reshape the pressure signal. Magder and colleagues examined routine CVP tracings in a consecutive ICU series and identified active expiratory patterns in a substantial minority. The observation supports examining the waveform before treating its end-expiratory point as a relaxed baseline. It does not provide one correction applicable to every pattern. 10

Serial measurements are most informative when position, reference level, cardiac timing, respiratory state, and signal acquisition are comparable. If those conditions change, part of the apparent loading change may arise from the measurement environment itself.

41.9 Systolic Transmural Pressure Describes Part of Ejection Load

During systole, ventricular muscle generates pressure and shortens against an evolving load. The intracavitary pressure required to open the outflow valve depends on the pressure downstream, while the pressure across the ventricular wall also depends on the local external environment.

Suppose illustrative left ventricular systolic pressure is 120 mm Hg and surrounding pressure is −5. Transmural systolic pressure is 120 − (−5) = 125 mm Hg. If internal pressure remains 120 while surrounding pressure becomes +5, the transmural value becomes 120 − 5 = 115 mm Hg. The pressure difference across the wall has fallen by 10 even though the internal pressure has not changed.

The converse explains the challenge of a strong negative inspiratory effort. More negative external pressure can increase the pressure sustained across the wall despite a lower arterial pressure measured relative to atmosphere. Changes in external pressure can therefore alter ejection load without a matching change in the displayed systemic pressure.

Buda and colleagues examined pressure maneuvers in eight patients with implanted myocardial markers. Negative intrathoracic pressure reduced shortening and ejection fraction despite increased ventricular volume and lower arterial pressure. Their findings support a transmural pressure contribution to impaired ejection rather than diagnosing a primary fall in contractility from the arterial reading alone. 11

Using arterial systolic pressure as the intracavitary value requires assumptions. The aortic valve must be open without an important outflow gradient, and the pressures must refer to comparable times and sites. A peripheral systolic peak can differ from the central aortic waveform. In aortic stenosis, left ventricular systolic pressure can substantially exceed aortic pressure.

The right ventricle also responds to external pressure, but its downstream vascular bed lies within the thorax and changes with lung inflation. Its complete load cannot be inferred by applying a left ventricular arterial-pressure shortcut. For both ventricles, pressure, flow, geometry, and the dynamic behavior of the vascular bed determine the load encountered during ejection.

41.10 Geometry Connects Transmural Pressure to Myocardial Wall Stress

Wall stress is force per unit area within the myocardial wall. Pressure across a chamber generates that force, but the required stress also depends on chamber dimensions and wall thickness. A larger radius increases the force that the wall must sustain, while greater thickness distributes force across more tissue.

For an ideal thin-walled sphere, a simplified Laplace relationship uses circumferential stress σ, transmural pressure Ptm, internal radius r, and wall thickness h. Radius and thickness must use the same length units.

For an illustrative model, let Ptm be 100 mm Hg, radius 3 cm, and wall thickness 0.3 cm. The estimate is 100 × 3/(2 × 0.3) = 500 mm Hg in stress-equivalent units. At the same pressure and thickness, radius 4 cm gives approximately 667 mm Hg. The larger modeled chamber sustains greater wall stress despite identical pressure.

Stress and pressure share physical dimensions, but the calculated stress is not another catheter pressure. Using approximately 133.3 Pa per mm Hg, the first result is about 66.7 kPa. The example illustrates geometry, not a clinical threshold or an accurate individual ventricular stress calculation.

A real ventricle is thick walled, nonspherical, anisotropic, and dynamically changing. Curvature and thickness vary regionally, and the septum shares loads between chambers. A thin-wall equation cannot describe these details. More elaborate geometric models still depend on assumptions and measurement quality.

Grossman, Jones, and McLaurin measured pressure, dimensions, and wall stress in patients with different patterns of ventricular loading. Their observations linked wall thickness and chamber geometry to the stress associated with a given pressure. They do not establish that equal estimated stress means equal contractile reserve or clinical risk. 12

Full ventricular afterload also includes the arterial system's resistance, compliance, wave reflections, and the timing of pressure relative to flow. A transmural pressure correction or Laplace estimate captures part of this system. Neither replaces a complete assessment of ventricular–vascular interaction.

41.11 Use Loading Pressures to Explain Responses, Not to Prescribe Volume

Begin interpretation by naming the measurement. A mean CVP, a wedge pressure, an instantaneous ventricular end-diastolic pressure, and an estimated transmural pressure answer different questions. Record the respiratory and cardiac conditions under which each was obtained before comparing values across time.

Then connect pressure with chamber geometry and flow. A rising filling pressure with little volume change may indicate stiffness, incomplete relaxation, or increasing constraint. A small left ventricle can coexist with a dilated pressure-loaded right ventricle. A lower estimated transmural pressure does not establish that additional volume will increase useful systemic output.

Osman and colleagues reviewed volume challenges in septic patients and found that CVP and pulmonary artery occlusion pressure poorly predicted a subsequent flow response. Combining the pressures did not solve the prediction problem. Their retrospective study supports the distinction between a static filling observation and demonstrated responsiveness; it does not test every possible external-pressure estimate or prescribe one alternative treatment. 13

Fluid responsiveness is also different from fluid tolerance. Additional preload may increase stroke volume while raising pulmonary venous pressure or systemic venous congestion. Conversely, a high measured pressure may partly reflect surrounding pressure while still coexisting with an important upstream pressure against which an organ must drain. Calling that pressure externally transmitted does not make its vascular consequences disappear.

Consider a constructed patient whose wedge pressure rises after a ventilatory change, while output falls and the right ventricle enlarges. Increased external pressure, impaired right ventricular ejection, and ventricular interaction could all contribute. Subtracting an assumed PEEP fraction cannot distinguish them. Comparing effort, waveforms, chamber dimensions, septal motion, flow, and the response to a carefully monitored change provides a stronger explanation.

The objective of transmural reasoning is to identify what the wall and vascular pathway experience. Preserve the original observations alongside any estimate. A useful interpretation explains pressure, volume, and performance together, states where external pressure is uncertain, and remains open to revision when the patient's response differs from the proposed mechanism.

References

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