Heterogeneity and Variability in Pressure Autoregulation of Organ Blood Flow: Lessons Learned Over 100+ Years

Essential Role of Hemoglobin βCys93 in Cardiovascular Physiology

The supply of oxygen to tissues is controlled by microcirculatory blood flow. One of the more surprising discoveries in cardiovascular physiology is the critical dependence of microcirculatory blood flow on a single conserved cysteine within the β-subunit (βCys93) of hemoglobin (Hb). βCys93 is the primary site of Hb S-nitrosylation [i.e., S-nitrosothiol (SNO) formation to produce S-nitrosohemoglobin (SNO-Hb)]. Notably, S-nitrosylation of βCys93 by NO is favored in the oxygenated conformation of Hb, and deoxygenated Hb releases SNO from βCys93. Since SNOs are vasodilatory, this mechanism provides a physiological basis for how tissue hypoxia increases microcirculatory blood flow (hypoxic autoregulation of blood flow). Mice expressing βCys93A mutant Hb (C93A) have been applied to understand the role of βCys93, and RBCs more generally, in cardiovascular physiology. Notably, C93A mice are unable to effect hypoxic autoregulation of blood flow and exhibit widespread tissue hypoxia. Moreover, reactive hyperemia (augmentation of blood flow following transient ischemia) is markedly impaired. C93A mice display multiple compensations to preserve RBC vasodilation and overcome tissue hypoxia, including shifting SNOs to other thiols on adult and fetal Hbs and elsewhere in RBCs, and growing new blood vessels. However, compensatory vasodilation in C93A mice is uncoupled from hypoxic control, both peripherally (e.g., predisposing to ischemic injury) and centrally (e.g., impairing hypoxic drive to breathe). Altogether, physiological studies utilizing C93A mice are confirming the allosterically controlled role of SNO-Hb in microvascular blood flow, uncovering essential roles for RBC-mediated vasodilation in cardiovascular physiology and revealing new roles for RBCs in cardiovascular disease.

Perioperative Quality Initiative (POQI) consensus statement on fundamental concepts in perioperative fluid management: fluid responsiveness and venous capacitance

Background

Optimal fluid therapy in the perioperative and critical care settings depends on understanding the underlying cardiovascular physiology and individualizing assessment of the dynamic patient state.

Methods

The Perioperative Quality Initiative (POQI-5) consensus conference brought together an international team of multidisciplinary experts to survey and evaluate the literature on the physiology of volume responsiveness and perioperative fluid management. The group used a modified Delphi method to develop consensus statements applicable to the physiologically based management of intravenous fluid therapy in the perioperative setting.

Discussion

We discussed the clinical and physiological evidence underlying fluid responsiveness and venous capacitance as relevant factors in fluid management and developed consensus statements with clinical implications for a broad group of clinicians involved in intravenous fluid therapy. Two key concepts emerged as follows: (1) The ultimate goal of fluid therapy and hemodynamic management is to support the conditions that enable normal cellular metabolic function in order to produce optimal patient outcomes, and (2) optimal fluid and hemodynamic management is dependent on an understanding of the relationship between pressure, volume, and flow in a dynamic system which is distensible with variable elastance and capacitance properties.

Muscular Tissue Oxygen Saturation and Posthysterectomy Nausea and Vomiting

Background

Suboptimal tissue perfusion and oxygenation during surgery may be responsible for postoperative nausea and vomiting in some patients. This trial tested the hypothesis that muscular tissue oxygen saturation–guided intraoperative care reduces postoperative nausea and vomiting.

Methods

This multicenter, pragmatic, patient- and assessor-blinded randomized controlled (1:1 ratio) trial was conducted from September 2018 to June 2019 at six teaching hospitals in four different cities in China. Nonsmoking women, 18 to 65 yr old, and having elective laparoscopic surgery involving hysterectomy (n = 800) were randomly assigned to receive either intraoperative muscular tissue oxygen saturation–guided care or usual care. The goal was to maintain muscular tissue oxygen saturation, measured at flank and on forearm, greater than baseline or 70%, whichever was higher. The primary outcome was 24-h postoperative nausea and vomiting. Secondary outcomes included nausea severity, quality of recovery, and 30-day morbidity and mortality.

Results

Of the 800 randomized patients (median age, 50 yr [range, 27 to 65]), 799 were assessed for the primary outcome. The below-goal muscular tissue oxygen saturation area under the curve was significantly smaller in patients receiving muscular tissue oxygen saturation–guided care (n = 400) than in those receiving usual care (n = 399; flank, 50 vs. 140% · min, P < 0.001; forearm, 53 vs. 245% · min, P < 0.001). The incidences of 24-h postoperative nausea and vomiting were 32% (127 of 400) in the muscular tissue oxygen saturation–guided care group and 36% (142 of 399) in the usual care group, which were not significantly different (risk ratio, 0.89; 95% CI, 0.73 to 1.08; P = 0.251). There were no significant between-group differences for secondary outcomes. No harm was observed throughout the study.

Conclusions

In a relatively young and healthy female patient population, personalized, goal-directed, muscular tissue oxygen saturation–guided intraoperative care is effective in treating decreased muscular tissue oxygen saturation but does not reduce the incidence of 24-h posthysterectomy nausea and vomiting.

Editor’s Perspective

What We Already Know about This Topic

What This Article Tells Us That Is New

[Perioperative blood pressure management : What is the optimal pressure?]

BACKGROUND

Measurement of blood pressure is part of standard monitoring procedures in anesthesia, in addition to the other vital parameters of heart frequency and peripheral oxygen saturation. In recent years the relevance of the duration and extent of perioperative episodes of hypotension for the occurrence of postoperative complications or even increased mortality have become the focus of scientific investigations.

OBJECTIVE

The aim of this review is to briefly recapitulate the physiological aspects of blood pressure and to describe the pathophysiology and risk factors of perioperative hypotension. It describes which potential organ damage can be caused by hypotension and discusses which perioperative blood pressure values are acceptable without harming the patient.

METHODS

Review and analysis of the currently available literature.

RESULTS

Perioperative hypotension is defined by either absolute systolic arterial pressure (SAP) or mean arterial pressure (MAP) thresholds and by relative blood pressure declines from an individual preoperative baseline value. For the definition of absolute and relative thresholds it needs to be considered that the ultimate target is an adequate perfusion pressure (and not the MAP) and that the preinduction blood pressure is a poor reflection of the patients' normal blood pressure profile. Risk factors for an intraoperative drop in blood pressure are advanced age, higher American Society of Anesthesiologists (ASA) status, low blood pressure prior to induction of anesthesia, the premedication, e.g. angiotensin-converting enzyme (ACE) inhibitors, the anesthesia technique (combination of general and epidural anesthesia) and emergency surgery. The lowest tolerable intraoperative blood pressure should be defined according to the individual patient's preoperative blood pressure and risk profile. Individual thresholds should be determined for the severity and duration of intraoperative hypotension. Empirically, MAP values <65 mm Hg and relative pressure declines of >20-30% are often recommended as thresholds. Below critical blood pressure values the risk of postoperative organ damage (myocardium, kidneys and central nervous system) and mortality increases with longer duration of hypotension. Older people and high-risk patients (e.g. patients in vascular surgery) have a poorer and shorter tolerance of low blood pressure. Postoperative organ complications can be minimized by maintenance of an adequate intraoperative blood pressure CONCLUSION: Anesthesiologists should avoid extensive and prolonged hypotension by timely interventions in order to improve the postoperative outcome of patients.

Association of Heart Rate Variability With Cognitive Performance: The Multi-Ethnic Study of Atherosclerosis

Background Heart rate variability (HRV) is associated with vascular risk factors for dementia, but whether HRV is associated with specific domains of cognitive performance is unclear. Methods and Results In the Multi-Ethnic Study of Atherosclerosis (N=3018; mean age 59.3±9.2 years), we assessed the relationship of 10-second HRV to scores on tests of global cognitive performance (Cognitive Abilities Screening Instrument), processing speed (Digit Symbol Coding), and working memory (Digit Span). HRV was computed as the SD of normal-normal intervals (SDNN) and root mean square of successive differences (RMSSD) at Exam 1 (2000-2002) and Exam 5 (2010-2012). Cognitive tests were administered at Exam 5. We report regression coefficients (β [95% CI]) representing cognitive test score change per 2-fold increase in HRV. After adjustment for age, race/ethnicity, sex, education, apolipoprotein E genotype, and cardiovascular risk factors and incident disease, higher Exam 1 (β=0.37 [0.06, 0.67]) and Exam 5 (β=0.31 [0.04, 0.59]) SDNN were associated with better Cognitive Abilities Screening Instrument performance. Higher Exam 1 (β=0.80 [0.17, 1.43]) and Exam 5 (β=0.63 [0.06, 1.20]) SDNN, and Exam 5 RMSSD (β=0.54 [0.01, 1.08]) were associated with better Digit Symbol Coding performance. Finally, higher Exam 5 SDNN was associated with better Digit Span performance (β=0.17 [0.01, 0.33]). Associations were attenuated after adjustment for resting heart rate. Conclusions Higher HRV is generally associated with better cognitive performance in this multi-ethnic cohort of aging adults, and further study of the relationship of autonomic function to cognition is warranted.

Neurological complications after cardiac surgery: anesthetic considerations based on outcome evidence

PURPOSE OF REVIEW

Neurological complications after cardiac surgery remain prevalent. This review aims to discuss the modifiable and outcome-relevant risk factors based on an up-to-date literature review, with a focus on interventions that may improve outcomes.

RECENT FINDINGS

There is a close relationship between intraoperative blood pressure and postoperative neurological outcomes in cardiac surgical patients based on cohort studies and randomized controlled trials. Adopting an optimal and personalized blood pressure target is essential; however, the outstanding issue is the determination of this target. Maintaining cerebral tissue oxygen saturation at least 90% patient's baseline during cardiac surgery may be beneficial; however, the outstanding issues are effective intervention protocols and quality outcome evidence. Maintaining hemoglobin at least 7.5 g/dl may be adequate for cardiac surgical patients; however, this evidence is based on the pooled results of thousands of patients. We still need to know the optimal hemoglobin level for an individual patient, which is of particular relevance during the decision-making of transfusion or not.

SUMMARY

The available evidence highlights the importance of maintaining optimal and individualized blood pressure, cerebral tissue oxygen saturation and hemoglobin level in improving neurological outcomes after cardiac surgery. However, outstanding issues remain and need to be addressed via outcome-oriented further research.