https://www.pulmccm.org/p/icu-physiology-in-1000-words-the

"Understanding the Non-Causal Relationship Between Right Atrial Pressure and Cardiac Output for Bedside Hemodynamic Management"

• Understanding the Relationship Between Right Atrial Pressure and Cardiac Output:

  • The association between right atrial pressure (P) and cardiac output (CO) does not imply causation. It is essential to distinguish cause and effect at the bedside for accurate diagnosis and therapy.
  • Distinguishing between cause and effect is important for diagnosis and therapy. Non-invasive ultrasound methods, such as assessing the size and collapsibility of the inferior vena cava, are used to deduce CO.

• Guyton’s Approach to Hemodynamics in the ICU:

  • Guyton's approach to hemodynamics in the intensive care unit (ICU) emphasizes the concept of venous return (VR) and its relationship with cardiac output (CO). Understanding the primary determinants of CO and VR is crucial for clinical management.
  • Using a geometrical model based on Guytonian physiology, it becomes clear that the pressure surrounding the right atrium (P) is not an independent determinant of blood flow, as it is not included in the equation for CO/VR.

• Clinical Implications and Further Reading:

  • The equations solving for CO/VR and P provide valuable insights for bedside hemodynamic management. Understanding these relationships can help clinicians make informed decisions in the care of critically ill patients.
  • For more in-depth information, readers are encouraged to continue to Part 2 of the article to further explore the clinical implications of these findings.

  Exploring the Relationship Between Right Atrial Pressure and Cardiac Output/Venous Return

  • Right Atrial Pressure and Cardiac Output Association:
    • In part 1, the relationship between right atrial pressure (P) and cardiac output/venous return (CO/VR) was explored, revealing that P and CO do not affect each other.
    • The mean systemic filling pressure (Pmsf), pericardial pressure (P), cardiac resistance (R), and resistance to venous return (R) govern the true dependent variable - the operating point (OP) of the circulatory system.
  • Inspiratory Holds and Blood Administration:
    • Two clinical thought experiments were presented to illustrate the relationship between P and CO.
    • During inspiratory holds, an inverse linear association between P and CO was observed, caused by increased pleural pressure impacting both P and CO.
    • In contrast, blood administration showed a direct linear association between P and CO, driven by increased elastic recoil pressure from peripheral vessels.
  • Operating Points:
    • Combining the two thought experiments onto a single graph revealed an infinite number of operating points within physiologically possible P and CO values.
    • Thus, a large sample size representing all possible pathophysiologies would show no association between P and CO.
  • Operating Point Guided Resuscitation (OPGR):
    • OPGR involves quantifying or qualifying both P and CO/stroke volume (SV) at each step of resuscitation, presenting a two-dimensional approach to resuscitation.
    • The method dichotomizes P into 'low' vs. 'high' and SV into 'normal/high' vs. 'low', creating a two-by-two factorial classification for various hemodynamic states.
  • Clinical Examples:
    • The application of the Diamond and Forrester two-by-two factorial classification in clinical examples demonstrated the significance of understanding operating points in assessing hemodynamic states.
    • The example of two hypotensive patients with different responses to a passive leg raise illustrated that mathematical preload responsiveness does not indicate similar cardiac functions.
  • Conclusion:
    • P and CO do not cause each other, nor do they mathematically associate, as revealed by the operating point and Diamond - Forrester classification.
  • Cardiac Output Determination:
    • Guyton AC's method of determining cardiac output by equating venous return curves with cardiac response curves was a significant contribution to physiology.
    • Kenny et al.'s dynamic interpretation of the Diamond-Forrester classification unifies fluid responsiveness and tolerance with physiology.