Concurrent quantification of tissue metabolism and blood flow via 2H/31P NMR in vivo. III. Alterations of muscle blood flow and metabolism during sepsis

Assessment and treatment of perfusion abnormalities in the emergency patient

Many patients presented to the emergency veterinarian are suffering from global or local tissue hypoperfusion. Global or systemic hypoperfusion can occur secondary to a reduction in the effective circulating intravascular volume (hypovolemic shock) or reduced ability of the heart to pump blood around the body secondary to reduced cardiac function (cardiogenic shock),obstruction to blood flow (obstructive shock), or maldistribution of the circulating intravascular volume (distributive shock). Initial assessment involving physical examination supplemented by measurement of hemodynamic and metabolic parameters allows the clinician to recognize and treat patients with severe global hypoperfusion. Use of techniques like sublingual capnometry and measurement of central venous oxygen saturation may aid recognition and evaluation of early hypoperfusion. Treatment decisions are made based on an assessment of the severity of the hypoperfusion and its probable underlying cause. Early effective treatment of hypoperfusion is likely to lead to a better outcome for the patient.

Competitive and Noncompetitive Inhibition of Myocardial Cytochrome C Oxidase in Sepsis

Sepsis is the most common cause of death in intensive care units worldwide. The basic pathophysiologic defect in sepsis, causing functional abnormalities in many organ systems, remains elusive. One potential cause is disruption of oxidative phosphorylation in mitochondria. Here, we report that oxidation of cytochrome c by myocardial cytochrome c oxidase, the terminal oxidase in the electron transport chain, is competitively inhibited early in experimental sepsis (cecal ligation with single or double 23-gauge puncture) in mice. In severe sepsis (cecal ligation and double puncture, 75% mortality at 48 h), inhibition becomes noncompetitive by 48 h. The development of noncompetitive inhibition is associated with a decrease in heme a,a3 content, which is the key active site in the functional subunit (I) and catalyzes the reduction of molecular oxygen. In addition, there are persistently decreased steady-state levels of subunit I mRNA and protein after cecal ligation and double puncture. Both loss of heme and loss of subunit I could explain the observed irreversible inhibition of cytochrome c oxidase. Noncompetitive inhibition of cytochrome c oxidase may interrupt oxidative phosphorylation, leading to sepsis-associated cardiac depression. Importantly, this abnormality may underlie sepsis-associated dysfunction in other organ systems.

Sublingual capnography: a clinical validation study

OBJECTIVE

To compare sublingual PCO(2) (PslCO(2)) measurements with gastric intramucosal PCO(2) (PimCO(2)) as well as with the traditional indexes of tissue oxygenation in hemodynamically unstable ICU patients.

DESIGN

A prospective, validation study.

SETTING

The medical and coronary ICUs of a community teaching hospital.

PATIENTS

Consecutive patients with severe sepsis, septic shock, or cardiogenic shock requiring pulmonary artery catheterization for hemodynamic management.

INTERVENTIONS

During the first 24 h of ICU admission, the PslCO(2), PimCO(2), and blood lactate concentrations as well conventional hemodynamic and oxygenation parameters were recorded every 4 to 6 h. The PslCO(2)-PaCO(2) and PimCO(2)-PaCO(2) differences were used as indexes of tissue dysoxia. These variables were correlated with each other as well as with the traditional markers of tissue oxygenation.

RESULTS

Seventy-six data sets were obtained on 22 patients. Fifteen patients had severe sepsis/septic shock, and 7 patients did not have sepsis. A patient with ischemic bowel who had a large PimCO(2)-PslCO(2) difference (60.2 mm Hg) was excluded. The initial PslCO(2) and PimCO(2) measurements were 43.5 +/- 10.4 mm Hg and 42.8 +/- 10.9 mm Hg, respectively (correlation coefficient [r] of 0.86; p < 0.001). The mean PslCO(2) and PimCO(2) for the entire data set were 48.0 +/- 13.4 mm Hg and 46.1 +/- 12.3 mm Hg, respectively (r = 0.78; p < 0.001). Ten patients died. The initial PslCO(2)-PaCO(2) difference was 9.2 +/- 5.0 mm Hg in the survivors and 17.8 +/- 11.5 mm Hg in the nonsurvivors (p = 0.04). The initial PimCO(2)-PaCO(2) difference was 8.4 +/- 4.8 mm Hg in the survivors and 16.1 +/- 13.7 mm Hg in the nonsurvivors (p = 0.08, not significant). The initial PslCO(2)-PaCO(2) difference correlated with the initial mixed venous-arterial CO(2) gradient (r = 0.66; p = 0.001), but correlated poorly with the initial blood lactate concentration (r = 0.38), mixed venous PO(2) (r = 0.05), and systemic oxygen delivery (r = - 0.39).

CONCLUSION

In this study, sublingual capnometry yielded measurements that correlated well with those of gastric tonometry. PslCO(2) may serve as a technically simple and noninvasive clinical measurement of tissue dysoxia in critically ill and injured patients.

Cytopathic hypoxia. Mitochondrial dysfunction as mechanism contributing to organ dysfunction in sepsis

Several lines of evidence support the notion that cellular energetics are deranged in sepsis, not on the basis of inadequate tissue perfusion, but rather on the basis of impaired mitochondrial respiration and/or coupling; that is, organ dysfunction in sepsis may occur on the basis of cytopathic hypoxia. If this concept is correct, then the therapeutic implications are enormous. Efforts to improve outcome in patients with sepsis by monitoring and manipulating cardiac output, systemic Do2, and regional blood flow are doomed to failure. Instead, the focus should be on developing pharmacologic strategies to restore normal mitochondrial function and cellular energetics.

Escherichia coli endotoxin reduces cytochrome aa3 redox status in pig skeletal muscle

OBJECTIVE

To assess the effect of endotoxin on cytochrome aa3 (Caa3) redox status in a controlled blood flow preparation of pig isolated hindlimb, at a constant oxygen delivery (Do2limb) (constant flow period) and during progressive ischemia (decreasing flow period).

DESIGN

Randomized, controlled experimental study.

SETTING

University hospital experimental laboratory.

SUBJECTS

Ten piglets.

INTERVENTIONS

Hindlimb blood flow was restricted to the femoral vessels. The arterial femoral blood flow coming from the carotid artery was controlled by a roller occlusive pump. The femoral venous blood flow was returned to the jugular vein. During the first 100 mins, the hindlimb blood flow was maintained at a normal level and then decreased stepwise. Animals were randomized to receive 150 microg/kg endotoxin lipopolysaccharide (LPS; n = 5) or saline (control; n = 5).

MEASUREMENTS AND MAIN RESULTS

Hindlimb muscle Caa3 redox status was monitored by near-infrared spectroscopy. Hindlimb Do2limb and oxygen consumption (Vo2limb) were calculated. In the LPS group, a rapid reduction of Caa3 redox status was observed after LPS administration, whereas the hindlimb blood flow remained normal with no change in Do2limb and Vo2limb. A progressive simultaneous decrease in Do2limb and Vo2limb was observed during the decreasing flow period with no further reduction in Caa3 redox status. In the control group, no change was observed in Caa3, Do2limb, or Vo2limb during the constant flow period. During the decreasing flow period, Caa3 redox status was reduced as Do2limb and Vo2limb decreased.

CONCLUSION

Our results suggest that endotoxin may induce a reduction of Caa3 redox status independently of Do2 and Vo2.

Metabolic derangements in sepsis and septic shock

This article examines the spectrum of metabolic alterations in sepsis and septic shock. The clinical manifestations, neuroendocrine control, and bioenergetics of the "ebb" and "flow" phases of sepsis are reviewed. Characteristic alterations in carbohydrate, fat, and protein metabolism induced by sepsis are outlined. Finally, the implications of these metabolic alterations for the nutritional support of patients with sepsis are discussed.

The hemodynamic derangements in sepsis: implications for treatment strategies

The incidence of the sepsis syndrome has increased dramatically in the last few decades. During this time, we have gained new insights into the pathophysiologic mechanisms leading to organ dysfunction in this syndrome. Yet, despite this increased knowledge and the use of novel therapeutic approaches, the mortality associated with the sepsis syndrome has remained between 30% and 40%. Appropriate antibiotic selection and hemodynamic support remain the cornerstone of treatment of patients with sepsis. Recent studies have failed to demonstrate a global oxygen debt in patients with sepsis. Furthermore, therapy aimed at increasing systemic oxygen delivery has failed to consistently improve patient outcome. The primary aim of the initial phase of resuscitation is to restore an adequate tissue perfusion pressure. Aggressive volume resuscitation is considered the best initial therapy for the cardiovascular instability of sepsis. Vasoactive agents are required in patients who remain hemodynamically unstable or have evidence of tissue hypoxia after adequate volume resuscitation.

A new method for imaging perfusion and contrast extraction fraction: input functions derived from reference tissues

This study describes a new method for analysis of dynamic MR contrast data that greatly increases the time available for data acquisition. The capillary input function, CB(t), is estimated from the rate of contrast agent uptake in a reference tissue such as muscle, based on literature values for perfusion rate, extraction fraction, and extracellular volume. The rate constant for contrast uptake (the product of perfusion rate, F, and extraction fraction, E; F x E) is then determined in each image pixel using CB(t), extracellular volume (relative to the reference tissue) measured from MR and the tissue concentration of contrast media as a function of time calculated from the MR data. The "reference tissue method" was tested using rats with mammary (n = 10) or prostate (n = 15) tumors implanted in the hindlimb. Dynamic MR images at 4.7 T were acquired before and after Gd-DTPA intravenous bolus injections to determine F x E(Gd-DTPA). Acquisition parameters were optimized for detection of the first pass of the contrast agent bolus, so that "first-pass analysis" could be used as the "gold standard" for determination of F x E. The accuracy of values of F x E determined using the reference tissue method was determined based on comparison with first-pass analysis. In some cases, deuterated water (D2O) was injected i.v. immediately after Gd-DTPA measurements, and the reference tissue method was used to calculate F, based on the rate of uptake of D2O. Comparison of rate constants for Gd-DTPA uptake and D2O uptake allowed calculation of E(Gd-DTPA). Values for F x E(Gd-DTPA), F, and E(Gd-DTPA) were determined for selected regions and on a pixel-by-pixel basis. Values for F x E and E(Gd-DTPA) measured using the reference tissue method correlated well (P = .90 with a standard error of +/- .016, n = 15) with values determined based on first-pass contrast media uptake. The reference tissue method has important advantages: (a) A large volume of reference tissue can be used to determine the contrast agent input function with high precision. (b) Data obtained for 20 minutes after injection are used to calculate F or F x E. The greatly increased acquisition time can be used to increase the spatial resolution, field of view or SNR of measurements. The reference tissue method is most useful when the volume of tissue that must be imaged and/or the spatial resolution required precludes use of traditional first-pass methods.

Quantification of phosphorus metabolites from chemical shift imaging spectra with corrections for point spread effects and B1 inhomogeneity

A method is described for quantifying phosphorus metabolites in tissue using spectra localized with surface coils and chemical shift imaging (CSI) and assuming that metabolites are uniformly distributed within a well-defined volume. An analytical expression is developed that yields a single numerical correction factor that takes into account the excitation and receiver profiles of the coil, T1 saturation, and point spread effects associated with Fourier transformation of CSI data. An external phosphorus standard is used to calibrate instrument gain and the B1 profile of the coil. For spherical samples, point spread effects can modulate the signal intensities of three-dimensional CSI spectra from -32% to +54%, depending on the voxel size. Measurements of phantoms of known concentrations showed systematic variations of +/- 10% and random errors of +/- 5%. We have used this method to measure the concentration of phosphocreatine in the thigh muscle of normal volunteers.

In vivo imaging of extraction fraction of low molecular weight MR contrast agents and perfusion rate in rodent tumors

Tissue uptake of a fully extractable MR detectable tracer, deuterated water (D2O), was compared with that of a less extractable contrast agent, Gadolinium-DTPA-dimeglumine (Gd-DTPA), in rodent tumor and muscle tissue. This dual tracer method allowed calculation of relative (to muscle) tissue perfusion and extraction fraction of Gd-DTPA in each image pixel in vivo. Solutions of Gd-DTPA and D2O were injected intravenously into Fisher female rats (n = 9) with R3230 mammary adenocarcinomas implanted in the hind limb. Perfusion rate was approximately two times greater (P < 0.005 by paired t test) in tumor than in muscle. Gd-DTPA extraction fraction at the interface between tumor and muscle was 2.0 times the extraction fraction in normal muscle (P < 0.005 by paired t test). Extraction fraction at the tumor center was 1.6 times the extraction fraction in muscle (P < 0.01 by paired t test). High extraction fraction of Gd-DTPA correlated with high capillary permeability determined from Evans Blue staining. Low molecular weight Gd-DTPA derivatives are widely used in clinical practice, and their extraction fractions are crucial determinants of image contrast during the first few passes of the contrast agent bolus. Therefore spatially resolved measurements of contrast agent extraction fractions obtained in vivo have significant clinical utility. The data demonstrate that extraction of low molecular weight tracers is sensitive to increased permeability in tumor vasculature and that this increased permeability can be imaged.

Cytopathic hypoxia in sepsis

Diminished availability of oxygen at the cellular level might account for organ dysfunction in sepsis. Although the classical forms of tissue hypoxia due to hypoxemia, anemia, or inadequate perfusion all might be important under some conditions, it seems increasingly likely that a fourth mechanism, namely cytopathic hypoxia, might play a role as well. The term cytopathic hypoxia is used to denote diminished production of adenosine triphosphate (ATP) despite normal (or even supranormal) PO2 values in the vicinity of mitochondria within cells. At least in theory, cytopathic hypoxia could be a consequence of several different (but mutually compatible) pathogenic mechanisms, including diminished delivery of a key substrate (e.g., pyruvate) into the mitochondrial tricarboxylic acid (TCA) cycle, inhibition of key mitochondrial enzymes involved in either the TCA cycle or the electron transport chain, activation of the enzyme, poly-(ADP)-ribosylpolymerase (PARP), or collapse of the protonic gradient across the inner mitochondrial membrane leading to uncoupling of oxidation (of NADH and FADH) from phosphorylation of ADP to form ATP. Tantalizing, but limited, data support the view that cytopathic hypoxia occurs in both animals and patients with sepsis or endotoxemia.

Alterations in carbohydrate metabolism during stress: a review of the literature

Patients with sepsis, burn, or trauma commonly enter a hypermetabolic stress state that is associated with a number of alterations in carbohydrate metabolism. These alterations include enhanced peripheral glucose uptake and utilization, hyperlactatemia, increased glucose production, depressed glycogenesis, glucose intolerance, and insulin resistance. The hypermetabolic state is induced by the area of infection or injury as well as by organs involved in the immunologic response to stress; it generates a glycemic milieu that is directed toward satisfying an obligatory requirement for glucose as an energy substrate. This article reviews experimental and clinical data that indicate potential mechanisms for these alterations and emphasizes aspects that have relevance for the clinician.