https://ccforum.biomedcentral.com/articles/10.1186/s13054-022-04024-x?s=09

Understanding Venous Return and its Determinants in Critically Ill Patients: A Comprehensive Review

• Venous return and mean systemic filling pressure: physiology and clinical applications:
  • Venous return is the flow of blood from the systemic venous network towards the right heart.
  • Guyton’s model describes the determinants of venous return: mean systemic filling pressure, right atrial pressure, and resistance to venous return.
• Importance of venous return in cardiovascular system functioning:
  • Cardiac output is primarily governed by venous inflow.
  • The difference between mean systemic filling pressure and right atrial pressure drives venous return.
  • The venous system plays a crucial role in propelling flow towards the right atrium.
  • Venous compliance is 40 times greater than arterial compliance.
• Determinants of venous return:
  • Venous return is determined by the resistance to venous return and the pressure gradient between mean systemic filling pressure and right atrial pressure.
  • When right atrial pressure decreases, the pressure gradient increases and venous return increases.
  • At a certain point, venous return plateaus due to vein collapse.
  • Mean systemic filling pressure can be estimated from the x-intercept of the venous return curve.
  • Resistance to venous return is represented by the inverse of the curve slope.
• Role of venous system in circulation:
  • Venous system acts as a reservoir of unstressed blood, which can be recruited via sympathetic stimulation.
  • Approximately 70% of the total blood volume is in the venous system.
  • Venoconstriction moves unstressed blood volume to stressed blood volume.
• Clinical applications of understanding venous return:
  • Understanding venous return helps in the evaluation of common treatments used in critically ill patients.
  • Physiological studies of venous return in critically ill patients can improve clinical practice.
• Introduction:
  • RAP and venous return/cardiac output curves can be superimposed to study haemodynamic states.
  • Pmsf is determined by venules and small veins and is close to mean circulatory filling pressure.
• Factors Affecting Venous Return:
  • Pmsf varies depending on clinical circumstances, volume status, and vasomotor tone.
  • RAP is influenced by the efficiency of the cardiac pump and changes in pleural pressure.
  • RVr is influenced by vein diameter and sympathetic control.
• The Concept of Resistance to Venous Return:
  • Resistance to venous return is the sum of venous and arterial resistance.
  • Resistance near the right atrium has a major effect on venous return.
• Methods to Measure Venous Return Determinants:
  • Methods include cardiac arrest measurements, heart-lung interactions, and transient stop-flow arm method.
  • Mathematical estimation using Parkin and Leaning's method is also possible.
• Typical Patterns of Venous Return Determinants:
  • In hypovolaemic shock, Pmsf decreases and the point of equilibrium shifts to the bottom left.
  • In cardiogenic shock, the Frank-Starling curve slope decreases and the point of equilibrium shifts to the bottom right.
  • Dobutamine affects the three determinants of venous return.
• Effect of venous vasodilation on venous return:
  • Venous vasodilation decreases right ventricular afterload, increasing cardiac output during the hyperdynamic state of septic shock.
  • Mechanical ventilation with high positive end-expiratory pressure increases right ventricular afterload and decreases cardiac output.
• Physiological changes in venous return and central venous pressure:
  • Volume expansion increases stressed blood volume and pulmonary mean systemic filling pressure, leading to an increase in venous return and cardiac output in preload-responsive patients.
  • In preload-unresponsive patients, volume expansion increases both pulmonary mean systemic filling pressure and right atrial pressure to the same extent, resulting in unchanged venous return.
• Passive leg raising as a pseudo fluid challenge:
  • Passive leg raising increases stressed blood volume and pulmonary mean systemic filling pressure, mimicking the effects of volume expansion.
  • In patients with intra-abdominal hypertension, the efficacy of passive leg raising may be limited due to decreased splanchnic blood volume and flow limitation.
• Revisited effects of norepinephrine:
  • Norepinephrine increases cardiac preload by recruiting unstressed blood volume through venoconstriction.
  • The increase in cardiac preload leads to an increase in cardiac output in preload-responsive patients; however, it may increase right ventricular afterload and decrease venous return in preload-unresponsive patients.
  • Norepinephrine and fluid infusion may act synergistically to increase pulmonary mean systemic filling pressure.
• Norepinephrine as a means of restoring arterial pressure:
  • Norepinephrine is important in rapidly restoring arterial pressure during septic shock resuscitation.
  • It also potentiates the effects of fluid administration, which may lead to a reduction in the total amount of fluid administered.
• Haemodynamic effects of prone positioning:
  • Prone positioning reduces pulmonary arterial resistance and right ventricular afterload.
  • It also increases cardiac preload and cardiac output in patients with fluid responsiveness.
• Increase in Pmsf and RVr during prone positioning:
  • In patients with acute respiratory distress syndrome, Pmsf increased during prone positioning.
  • For most patients, the (Pmsf–RAP) gradient, venous return, and cardiac output increased during prone positioning in preload-responsive patients.