Monitoring Hemodynamic and Metabolic Alterations during Severe Hemorrhagic Shock in Rat Brains

Rationale and Objectives

Our long-term goals are to identify imaging biomarkers for hemorrhagic shock and to understand how the preservation of cerebral microcirculation works. We also seek to understand how the damage occurs to the cerebral hemodynamics and the mitochondrial metabolism during severe hemorrhagic shock.

Materials and Methods

We used a multimodal cerebral cortex optical imaging system to obtain 4-hour observations of cerebral hemodynamic and metabolic alterations in exposed rat cortexes during severe hemorrhagic shock. We monitored the mean arterial pressure, heart rate, cerebral blood flow (CBF), functional vascular density (FVD), vascular perfusion and diameter, blood oxygenation, and mitochondrial reduced nicotinamide adenine dinucleotide (NADH) signals.

Results

During the rapid bleeding and compensatory stage, cerebral parenchymal circulation was protected by inhibiting the perfusion of dural vessels. During the compensatory stage, although the brain parenchymal CBF and FVD decreased rapidly, the NADH signal did not show a significant increase. During the decompensatory stage, FVD and CBF maintained the same low level and the NADH signal remained unchanged. However, the NADH signal showed a significant increase after the rapid blood infusion. FVD and CBF rebounded to the baseline after the resuscitation and then declined again.

Conclusions

We present for the first time simultaneous imaging of cerebral hemodynamics and NADH signals in vivo during the process of hemorrhagic shock. This novel multimodal method demonstrated clearly that severe hemorrhagic shock imparts irreversible tissue damage that is not compensated by the autoregulatory mechanism. Hemodynamic and metabolic signatures including CBF, FVD, and NADH may be further developed to provide sensitive biomarkers for stage transitions in hemorrhagic shock.

Hemorrhagic shock is a condition caused by a rapid and significant loss of intravascular volume, which may lead to hemodynamic instability, decreased tissue perfusion, cellular hypoxia, organ damage, and death . Usually, the process of hemorrhagic shock is described in three stages (or in four stages if defining an initial stage before the compensatory stage) . In the first stage (i.e., the compensatory stage) the body undergoes microcirculatory ischemia. This is characterized by compensatory hypotension during which the minimum perfusion level of vital organs is maintained by autoregulatory mechanism. In the second stage (i.e., the decompensatory stage) the body exhibits microcirculatory congestion, characterized by decompensatory hypotension that indicates the failure of autoregulatory mechanism. The last stage is the refractory stage, characterized by refractory hypotension and microcirculation failure followed by irreversible cell damage and organ failure.

Whether the cerebral circulation is protected by the autoregulatory mechanism during the process of the shock has long been controversial . Previous studies have highlighted that cerebral blood flow (CBF) in rats is maintained when mean arterial pressure (MAP) is kept between 60 and 140 mmHg . Animals would have lower MAP if they underwent a greater blood loss. Wan et al. found that the microcirculation of the cerebral cortex could be maintained after losing 35% of blood volume (MAP could be reduced to 60 mmHg or less). However, they found that the buccal microcirculation supplied by the carotid artery showed damages that are consistent with the changes in peripheral circulation. Furthermore, the protection of cerebral circulation resulting from autoregulatory mechanism may be temporary during the process of shock . The autoregulatory mechanism will show signs of failure once the blood pressure (BP) exceeds the lower threshold for an extended period .

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Materials and methods

Animal Preparation

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Experimental Procedures

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Imaging Instrument

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Data Analyses

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log(Iλ0/Iλt)=μλHbRDλΔHbR+μλHbODλΔHbO. log

(

)

HbR

HbO

where λ is the light wavelength, Iλ0 I

λ and Iλt I

λ are the measured intensities of reflected light at the light wavelength λ for the baseline ( t = 0) and time t , respectively; μλHbR μ

HbR

λ and μλHbO μ

HbO

λ are absorption coefficients of oxyhemoglobin (HbO) and deoxyhemoglobin (HbR), respectively; D__λ is the differential pathlength factor, that is, the mean optical pathlength of the photons travelling in the tissue; ΔHbO, ΔHbR, and ΔHbT represent the concentration changes of oxyhemoglobin, deoxyhemoglobin, and total hemoglobin (HbT), respectively. D__λ in brain tissue was evaluated and quantified by the Monte Carlo simulation at 350 nm (∼365 nm), 480 nm (∼475 nm), 560 nm, and 570 nm using the approach of Kohl et al. and Andrew et al. . We used the simulated D__λ for 560 nm and 570 nm to compute ΔHbO, ΔHbR, and ΔHbT (ΔHbT = ΔHbO + ΔHbR) by Equation ( 1 ). The intrinsic signals (reflectance) at 350 nm and 480 nm were predicted from ΔHbO and ΔHbR using the approach of Yevgeniy et al. , and their reflected signals were then used to correct the NADH signals measured from the first CCD . CBF was obtained through online processing (customized C program) by the laser speckle temporal contrast analysis of 100 consecutive raw speckles images .

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Results

Alterations of Hemodynamics

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Alteration of Metabolism

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Discussion

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Acknowledgments

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Monitoring Hemodynamic and Metabolic Alterations during Severe Hemorrhagic Shock in Rat Brains

Rationale and Objectives

Materials and Methods

Results

Conclusions

Materials and methods

Animal Preparation

Experimental Procedures

Imaging Instrument

Data Analyses

Results

Alterations of Hemodynamics

Alteration of Metabolism

Discussion

Acknowledgments

References

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