Group B streptococcal sepsis impairs cerebral vascular reactivity to acute hypercarbia in piglets

Effects of hypercapnia in sepsis: A scoping review of clinical and pre-clinical data

BACKGROUND

Perform a scoping review of (1) pre-clinical studies testing the physiological effects of higher PaCO2 levels in the setting of sepsis models and (2) clinical investigations testing the effects of hypercapnia on clinical outcomes in mechanically ventilated patients with sepsis.

METHODS

We performed a search of CENTRAL, PUBMED, CINAHL, and EMBASE. Study inclusion criteria for pre-clinical studies were: (1) bacterial sepsis model (2) measurement of PaCO2 , and (3) comparison of outcome measure between different PaCO2 levels. Inclusion criteria for clinical studies were: (1) diagnosis of sepsis, (2) receiving invasive mechanical ventilation, (3) measurement of PaCO2 , and (4) comparison of outcomes between different PaCO2 levels. We performed a qualitative analysis to collate and summarize the physiological and clinical effects of hypercapnia according to the recommended methodology from the Cochrane Handbook.

RESULTS

Fifteen pre-clinical and nine clinical studies were included. Among pre-clinical studies, the individual studies found higher PaCO2 augments tissue blood flow and oxygenation, and attenuates inflammation and lung injury; however, all pre-clinical studies were found to have some degree of risk of bias. Six of the nine clinical studies were deemed to be good quality. Among clinical studies hypercapnia was associated with increased cerebral perfusion and oxygenation; however, there were conflicting results testing the association between hypercapnia and mortality.

CONCLUSION

While individual pre-clinical studies identified potential mechanisms by which changes in PaCO2 levels could affect pathophysiology in sepsis, there is a paucity of clinical data as to the optimal PaCO2 range, demonstrating a need for future research.

REGISTRATION

PROSPERO number CRD42018086703.

Impaired cerebrovascular reactivity in sepsis-associated encephalopathy studied by acetazolamide test

INTRODUCTION

The pathophysiology of sepsis-associated encephalopathy (SAE) is not entirely clear. One of the possible underlying mechanisms is the alteration of the cerebral microvascular function induced by the systemic inflammation. The aim of the present work was to test whether cerebral vasomotor-reactivity is impaired in patients with SAE.

METHODS

Patients fulfilling the criteria of clinical sepsis and showing disturbance of consciousness of any severity were included (n = 14). Non-septic persons without previous diseases affecting cerebral vasoreactivity served as controls (n = 20). Transcranial Doppler blood flow velocities were measured at rest and at 5, 10, 15 and 20 minutes after intravenous administration of 15 mg/kgBW acetazolamide. The time course of the acetazolamide effect on cerebral blood flow velocity (cerebrovascular reactivity, CVR) and the maximal vasodilatory effect of acetazolemide (cerebrovascular reserve capacity, CRC) were compared among the groups.

RESULTS

Absolute blood flow velocities after administration of the vasodilator drug were higher among control subjects than in SAE. Assessment of the time-course of the vasomotor reaction showed that patients with SAE reacted slower to the vasodilatory stimulus than control persons. When assessing the maximal vasodilatory ability of the cerebral arterioles to acetazolamide during vasomotor testing, we found that patients with SAE reacted to a lesser extent to the drug than did control subjects (CRC controls:46.2 +/- 15.9%, CRC SAE: 31,5 +/- 15.8%, P < 0.01).

CONCLUSIONS

We conclude that cerebrovascular reactivity is impaired in patients with SAE. The clinical significance of this pathophysiological finding has to be assessed in further studies.

Cerebral haemodynamics and carbon dioxide reactivity during sepsis syndrome

Background

Most patients with sepsis develop potentially irreversible cerebral dysfunctions. It is yet not clear whether cerebral haemodynamics are altered in these sepsis patients at all, and to what extent. We hypothesized that cerebral haemodynamics and carbon dioxide reactivity would be impaired in patients with sepsis syndrome and pathological electroencephalogram patterns.

Methods

After approval of the institutional ethics committee, 10 mechanically ventilated patients with sepsis syndrome and pathological electroencephalogram patterns underwent measurements of cerebral blood flow and jugular venous oxygen saturation before and after reduction of the arterial carbon dioxide partial pressure by 0.93 ± 0.7 kPa iu by ypervent ilation. The cerebral capillary closing pressure was determined from transcranial Doppler measurements of the arterial blood flow of the middle cerebral artery and the arterial pressure curve. A t test for matched pairs was used for statistical analysis (P < 0.05).

Results

During stable mean arterial pressure and cardiac index, reduction of the arterial carbon dioxide partial pressure led to a significant increase of the capillary closing pressure from 25 ± 11 mmHg to 39 ± 15 mmHg (P < 0.001), with a consecutive decrease of blood flow velocity in the middle cerebral artery of 21.8 ± 4.8%/kPa (P < 0.001), of cerebral blood flow from 64 ± 29 ml/100 g/min to 39 ± 15 ml/100 g/min (P < 0.001) and of jugular venous oxygen saturation from 75 ± 8% to 67 ± 14% (P < 0.01).

Conclusion

In contrast to other experimental and clinical data, we observed no pathological findings in the investigated parameters of cerebral perfusion and oxygenation.

Comparative effects of propofol vs dexmedetomidine on cerebrovascular carbon dioxide reactivity in patients with septic shock

BACKGROUND

The use of sedative drugs is reportedly related to altered cerebrovascular CO2 reactivity. The present study examined the comparative effects of propofol vs dexmedetomidine on cerebrovascular CO2 reactivity in patients with septic shock.

METHODS

A total of 20 patients with septic shock who required mechanical ventilation were included in this study. Sedation during mechanical ventilation was maintained using either propofol or dexmedetomidine. A 2.5 MHz pulsed transcranial Doppler probe was attached to the head of the patient at the right temporal window for continuous measurement of mean blood flow velocity in the middle cerebral artery (V(mca)). After establishing baseline values of V(mca) and cardiovascular haemodynamics, end-tidal CO2 was increased by decreasing ventilatory frequency by 5-8 bpm.

RESULTS

The absolute and relative CO2 reactivity values in patients with septic shock were lower for both propofol and dexmedetomidine than those for control groups, with significant differences between these values in the two septic shock groups (absolute CO2 reactivity in septic shock under propofol: 2.6 (sd 0.3) cm s(-1) mm Hg(-1); absolute CO2 reactivity in septic shock under dexmedetomidine: 2.0 (0.3) cm s(-1) mm Hg(-1); P<0.01).

CONCLUSIONS

This study showed that cerebrovascular CO2 reactivity was lower under dexmedetomidine sedation than under propofol sedation during almost identical sedation in patients with septic shock.

Bench-to-bedside review: critical illness-associated cognitive dysfunction--mechanisms, markers, and emerging therapeutics

Cognitive dysfunction is common in critically ill patients, not only during the acute illness but also long after its resolution. A large number of pathophysiologic mechanisms are thought to underlie critical illness-associated cognitive dysfunction, including neuro-transmitter abnormalities and occult diffuse brain injury. Markers that could be used to evaluate the influence of specific mechanisms in individual patients include serum anticholinergic activity, certain brain proteins, and tissue sodium concentration determination via high-resolution three-dimensional magnetic resonance imaging. Although recent therapeutic advances in this area are exciting, they are still too immature to influence patient care. Additional research is needed if we are to understand better the relative contributions of specific mechanisms to the development of critical illness-associated cognitive dysfunction and to determine whether these mechanisms might be amenable to treatment or prevention.

Potential mechanisms and markers of critical illness-associated cognitive dysfunction

PURPOSE OF REVIEW

To review the current understanding of the potential mechanisms of critical illness-associated cognitive dysfunction and to provide insight into markers that could be used to evaluate the influence of specific mechanisms in individual patients.

RECENT FINDINGS

Cognitive dysfunction is common in critically ill patients, not only during the acute illness but also long after its resolution. Several pathophysiologic mechanisms are thought to underlie critical illness-associated cognitive dysfunction, including neurotransmitter abnormalities and occult diffuse brain injury. Markers that could be used to evaluate the influence of specific mechanisms in individual patients include serum anticholinergic activity, certain brain proteins, and tissue sodium concentration determination by way of high-resolution three-dimensional magnetic resonance imaging.

SUMMARY

Although recent advances in this area are exciting, they are still too immature to influence patient care. Additional research is needed to provide a better understanding of the relative contribution of specific mechanisms to the development of critical illness-associated cognitive dysfunction and to determine whether these mechanisms might be amenable to treatment or prevention.

Cerebrovascular reactivity to carbon dioxide in sepsis syndrome

Cerebral dysfunction in sepsis is common in critically ill adults. However, little is known of the effects of sepsis on cerebral haemodynamics. We studied 12 sedated and ventilated patients in whom sepsis had been established for > 24 h. Transcranial Doppler measurements of the middle cerebral artery flow velocity were made at normocapnia, then hypocapnia (-1 kPa) and hypercapnia (+1 kPa). From these data, cerebrovascular reactivity to carbon dioxide was calculated. Variables indicating disease severity, systemic cardiovascular status and outcome were also recorded. We found significant changes in cerebrovascular reactivity to carbon dioxide. Only three of 12 patients had a cerebrovascular reactivity to carbon dioxide in the normal range; seven patients had a reduced cerebrovascular reactivity to carbon dioxide, whereas in two patients it was raised. In this smaD sample, we could not find any trend of association between altered cerebrovascular reactivity to carbon dioxide and severity of illness, cardiovascular status or outcome. This study suggests that established sepsis profoundly affects the vascular tone and reactivity, not only of the systemic circulation, but also of the cerebral vasculature.

Hemodynamic homeostasis during acute hypoxia in septic and nonseptic piglets: differential role of prostaglandins and nitric oxide

We studied the hemodynamic responses of 29 anesthetized and mechanically ventilated piglets to acute hypoxia [reduction of Pao2 from 130 to 38 mm Hg induced by inhalation of 7% fraction of inspired oxygen (Fio2) for 7.5 min] before and during group B beta-hemolytic streptococci (GBS) sepsis. During hypoxia, nonseptic piglets maintained stable systemic blood pressure [105+/-9 (SD) to 97+/-14 mm Hg] and cardiac output (CO) (667+/-72 to 685+/-113 mL/min). However, during GBS/hypoxia, systemic blood pressure fell from 94+/-17 to 49+/-25 mm Hg, CO fell from 397+/-146 to 223+/-142 mL/min (both p < 0.001 versus pre-GBS), and cardiac arrest often ensued. We tested three hypotheses that might underlie GBS-induced intolerance to systemic hypoxia: 1) GBS-induced reduction of systemic CO/systemic oxygen delivery (QO2) below a critical QO2 beyond which the superimposition of hypoxia becomes intolerable; this mechanism is unlikely as nonseptic piglets with comparable reductions in CO/QO2 (induced by inflation of a left atrial balloon) tolerated hypoxia well; 2) GBS-induced inhibition of nitric oxide (NO) synthesis that is vital to tolerance of hypoxia; this mechanism is unlikely as infusion of the NO substrate L-arginine did not restore tolerance to hypoxia during GBS infusion (as it did after inhibition of NO synthesis during infusion of N-nitro-L-arginine in nonseptic piglets); and 3) GBS-induced production of pathologic prostaglandins that impaired the piglet's capacity to tolerate hypoxia; this mechanism finds support in the observation that inhibition of prostaglandins with the cyclooxygenase inhibitor indomethacin completely restored the ability of septic piglets to tolerate hypoxia. Further evaluation of GBS-induced intolerance to systemic hypoxia may provide insight into the incompletely understood mechanisms by which sepsis induces circulatory collapse in experimental animals and in humans.