Effects of endotoxin infusion on mean systemic filling pressure and flow resistance to venous return

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. At steady state, venous return equals cardiac output, as the venous and arterial systems operate in series. However, unlike the arterial one, the venous network is a capacitive system with a high compliance. It includes a part of unstressed blood, which is a reservoir that can be recruited via sympathetic endogenous or exogenous stimulation. Guyton’s model describes the three determinants of venous return: the mean systemic filling pressure, the right atrial pressure and the resistance to venous return. Recently, new methods have been developed to explore such determinants at the bedside. In this narrative review, after a reminder about Guyton’s model and current methods used to investigate it, we emphasize how Guyton’s physiology helps understand the effects on cardiac output of common treatments used in critically ill patients.

Cardiovascular Responses During Sepsis

Sepsis is the life-threatening organ dysfunction arising from a dysregulated host response to infection. Although the specific mechanisms leading to organ dysfunction are still debated, impaired tissue oxygenation appears to play a major role, and concomitant hemodynamic alterations are invariably present. The hemodynamic phenotype of affected individuals is highly variable for reasons that have been partially elucidated. Indeed, each patient's circulatory condition is shaped by the complex interplay between the medical history, the volemic status, the interval from disease onset, the pathogen, the site of infection, and the attempted resuscitation. Moreover, the same hemodynamic pattern can be generated by different combinations of various pathophysiological processes, so the presence of a given hemodynamic pattern cannot be directly related to a unique cluster of alterations. Research based on endotoxin administration to healthy volunteers and animal models compensate, to an extent, for the scarcity of clinical studies on the evolution of sepsis hemodynamics. Their results, however, cannot be directly extrapolated to the clinical setting, due to fundamental differences between the septic patient, the healthy volunteer, and the experimental model. Numerous microcirculatory derangements might exist in the septic host, even in the presence of a preserved macrocirculation. This dissociation between the macro- and the microcirculation might account for the limited success of therapeutic interventions targeting typical hemodynamic parameters, such as arterial and cardiac filling pressures, and cardiac output. Finally, physiological studies point to an early contribution of cardiac dysfunction to the septic phenotype, however, our defective diagnostic tools preclude its clinical recognition. © 2021 American Physiological Society. Compr Physiol 11:1605-1652, 2021.

Estimating mean circulatory filling pressure in clinical practice: a systematic review comparing three bedside methods in the critically ill

The bedside hemodynamic assessment of the critically ill remains challenging since blood volume, arterial–venous interaction and compliance are not measured directly. Mean circulatory filling pressure (P_mcf) is the blood pressure throughout the vascular system at zero flow. Animal studies have shown P_mcf provides information on vascular compliance, volume responsiveness and enables the calculation of stressed volume. It is now possible to measure P_mcf at the bedside. We performed a systematic review of the current P_mcf measurement techniques and compared their clinical applicability, precision, accuracy and limitations. A comprehensive search strategy was performed in PubMed, Embase and the Cochrane databases. Studies measuring P_mcf in heart-beating patients at the bedside were included. Data were extracted from the articles into predefined forms. Quality assessment was based on the Newcastle–Ottawa Scale for cohort studies. A total of 17 prospective cohort studies were included. Three techniques were described: P_mcf hold, based on inspiratory hold-derived venous return curves, P_mcf arm, based on arterial and venous pressure equilibration in the arm as a model for the entire circulation, and P_mcf analogue, based on a Guytonian mathematical model of the circulation. The included studies show P_mcf to accurately follow intravascular fluid administration and vascular compliance following drug-induced hemodynamic changes. Bedside P_mcf measures allow for more direct assessment of circulating blood volume, venous return and compliance. However, studies are needed to determine normative P_mcf values and their expected changes to therapies if they are to be used to guide clinical practice.

The effect of propofol on haemodynamics: cardiac output, venous return, mean systemic filling pressure, and vascular resistances

BACKGROUND

Although arterial hypotension occurs frequently with propofol use in humans, its effects on intravascular volume and vascular capacitance are uncertain. We hypothesized that propofol decreases vascular capacitance and therefore decreases stressed volume.

METHODS

Cardiac output (CO) was measured using Modelflow(®) in 17 adult subjects after upper abdominal surgery. Mean systemic filling pressure (MSFP) and vascular resistances were calculated using venous return curves constructed by measuring steady-state arterial and venous pressures and CO during inspiratory hold manoeuvres at increasing plateau pressures. Measurements were performed at three incremental levels of targeted blood propofol concentrations.

RESULTS

Mean blood propofol concentrations for the three targeted levels were 3.0, 4.5, and 6.5 µg ml(-1). Mean arterial pressure, central venous pressure, MSFP, venous return pressure, Rv, systemic arterial resistance, and resistance of the systemic circulation decreased, stroke volume variation increased, and CO was not significantly different as propofol concentration increased.

CONCLUSIONS

An increase in propofol concentration within the therapeutic range causes a decrease in vascular stressed volume without a change in CO. The absence of an effect of propofol on CO can be explained by the balance between the decrease in effective, or stressed, volume (as determined by MSFP), the decrease in resistance for venous return, and slightly improved heart function.

CLINICAL TRIAL REGISTRATION

Netherlands Trial Register: NTR2486.

Can (and should) the venous tone be monitored at the bedside?

PURPOSE OF REVIEW

Most of our blood volume is contained in the venous compartment. The so-called 'compliant veins' are an adjustable blood reservoir, which is playing a paramount role in maintaining haemodynamic stability. The purpose of this study is to review what is known about this blood reservoir and how we can use this information to assess the cardiovascular state of critically ill patients.

RECENT FINDINGS

The mean systemic filling pressure (Pmsf) is the pivot pressure of the circulation, and a quantitative index of intravascular volume. The Pmsf can be measured at the bedside by three methods described in critically ill patients. The Pmsf can be modified by the fluid therapy and vasoactive medications.

SUMMARY

The Pmsf along with other haemodynamic variables can provide valuable information to correctly understand the cardiovascular status of critically ill patients and better manage the fluid therapy and cardiovascular support. Future studies using the Pmsf will show its usefulness for fluid administration.

Role of the venous return in critical illness and shock: part II-shock and mechanical ventilation

OBJECTIVE

To provide a conceptual and clinical review of the physiology of the venous system as it is related to cardiac function in health and disease.

DATA

An integration of venous and cardiac physiology under normal conditions, critical illness, and resuscitation.

SUMMARY

The usual clinical teaching of cardiac physiology focuses on left ventricular pathophysiology and pathology. Due to the wide array of shock states dealt with by intensivists, an integrated approach that takes into account the function of the venous system and its interaction with the right heart may be more useful. In part II of this two-part review, we describe the physiology of venous return and its interaction with the right heart function as it relates to mechanical ventilation and various shock states including hypovolemic, cardiogenic, obstructive, and septic shock. In particular, we demonstrate how these shock states perturb venous return/right heart interactions. We also show how compensatory mechanisms and therapeutic interventions can tend to return venous return and cardiac output to appropriate values.

CONCLUSION

An improved understanding of the role of the venous system in pathophysiologic conditions will allow intensivists to better appreciate the complex circulatory physiology of shock and related therapies. This should enable improved hemodynamic management of this disorder.

The role of venous return in critical illness and shock-part I: physiology

OBJECTIVE

To provide a conceptual and clinical review of the physiology of the venous system as it is relates to cardiac function in health and disease.

DATA

An integration of venous and cardiac physiology under normal conditions, critical illness, and resuscitation.

SUMMARY

The usual teaching of cardiac physiology focuses on left ventricular function. As a result of the wide array of shock states with which intensivists contend, an approach that takes into account the function of the venous system and its interaction with the right and left heart may be more useful. This two-part review focuses on the function of the venous system and right heart under normal and stressed conditions. The first part describes the basic physiology of the venous system, and part two focuses on the role of the venous system in different pathophysiologic states, particularly shock.

CONCLUSION

An improved understanding of the role of the venous system in health and disease will allow intensivists to better appreciate the complex circulatory physiology of shock and may allow for better hemodynamic management of this disorder.

Bedside assessment of mean systemic filling pressure

PURPOSE OF REVIEW

The physiology of the venous part of the human circulation seems to be a forgotten component of the circulation in critical care medicine. One of the main reasons, probably, is that measures of right atrial pressure (Pra) do not seem to be directly linked to blood flow. This perception is primarily due to an inability to measure the pressure gradient for venous return. The upstream pressure for venous return is mean systemic filling pressure (Pmsf) and it does not lend itself easily to be measured. Recent clinical studies now demonstrate the basic principles underpinning the measure of Pmsf at the bedside.

RECENT FINDINGS

Using routinely available minimally invasive monitoring of continuous cardiac output and Pra, one can accurately construct venous return curves by performing a series of end-inspiratory hold maneuvers, in ventilator-dependent patients. From these venous return curves, the clinician can now finally obtain at the bedside not only Pmsf but also the derived parameters: resistance to venous return, systemic compliance and stressed volume.

SUMMARY

Measurement of Pmsf is essential to describe the control of vascular capacitance. It is the key to distinguish between passive and active mechanisms of blood volume redistribution and partitioning total blood volume in stressed and unstressed volume.

Assessment of venous return curve and mean systemic filling pressure in postoperative cardiac surgery patients

OBJECTIVE

To measure the relationship between blood flow and central venous pressure (Pcv) and to estimate mean systemic filling pressure (Pmsf), circulatory compliance, and stressed volume in patients in the intensive care unit.

DESIGN

Intervention study.

SETTING

Intensive care unit of a university hospital.

PATIENTS

Twelve mechanically ventilated postoperative cardiac surgery patients.

INTERVENTIONS

Inspiratory holds were performed during normovolemia in supine position (baseline), relative hypovolemia by placing the patients in 30 degree head-up position (hypo), and relative hypervolemia by volume loading with 0.5 L colloid (hyper).

MEASUREMENTS AND MAIN RESULTS

We measured the relationship between blood flow and Pcv using 12-second inspiratory-hold maneuvers transiently increasing Pcv to three different steady-state levels and monitored the resultant blood flow via the pulse contour method during the last 3 seconds. The Pcv to blood flow relation was linear for all measurements with a slope unaltered by relative volume status. Pmsf decreased with hypo and increased with hyper (18.8 +/- 4.5 mm Hg, to 14.5 +/- 3.0 mm Hg, to 29.1 +/- 5.2 mm Hg [baseline, hypo, hyper, respectively, p < 0.05]). Baseline total circulatory compliance was 0.98 mL x mm Hg x kg and stressed volume was 1677 mL.

CONCLUSIONS

Pmsf can be determined in intensive care patients with an intact circulation with use of inspiratory pause procedures, making serial measures of circulatory compliance and circulatory stressed volume feasible.

Low glutamine concentrations induce phenotypical and functional differentiation of U937 myelomonocytic cells

L-Glutamine is the most abundant free amino acid of the human body and is essential for the culture of many cell types. Clinically, reduction of glutamine by administration of glutaminase or the use of glutamine analogs is a common therapy for patients with acute lymphocytic leukemia. In the current study, we investigated the influence of glutamine concentrations on the human myelomonocytic cell line U937. Decreasing the glutamine concentration evoked a reduction in DNA synthesis (R2 = 0.9885, P < 0.0001), increased cell volume (P < 0.01) and the cytoplasm/nuclear ratio, and enhanced the development of vacuoles but did not influence cell viability. Culturing cells in reduced concentrations of glutamine augmented the percentage of cells expressing CD64 (Fc receptor for IgG/FcgammaRI, P < 0.01), CD11b (complement receptor type 3/CR3, P < 0.001) and CD71 (transferrin receptor, P < 0.05). The percentage of U937 cells expressing CD23 (low affinity receptor for IgE/FcepsilonRII) was increased at low concentrations of glutamine at both the protein (P < 0.01) and mRNA levels. The percentage of U937 cells phagocytizing opsonized E. coli (P < 0.001) or latex particles (P < 0.001) was enhanced by lowering the glutamine concentration. In conclusion, reducing glutamine concentration causes differentiation of the cell line U937 along the monocytic pathway. These effects may indicate a mechanistic basis for prior published evidence that glutaminase and glutamine antagonists are effective anti-tumor agents.