On bioenergetic failure in septic shock : lactate kinetics and mitochondrial respiration

Abstract

Septic shock, a dysregulated immune response to an infection, is a major cause of mortality and morbidity in patients treated in intensive care units (ICUs). The cause of the organ dysfunction seen in sepsis is still not fully elucidated, but a disrupted intracellular metabolism, a bioenergetics failure, has been suggested as a possible mechanism. In this thesis we investigate two aspects of these metabolic alterations – mitochondrial dysfunction and lactate metabolism. Compromised mitochondrial function and damaged mitochondria are described in septic ICU patients and various animal septic models. In our study we investigated if plasma from septic patients was able to alter the respiratory function of mitochondria of rat skeletal muscle. We examined this through incubation of isolated mitochondria as well as incubation of permeabilized muscle cells, but could not detect any difference in respiration compared to mitochondria incubated in control plasma.

Elevated concentrations, and impaired clearance, of plasma lactate are known to correlate with increased mortality in patients with septic shock. We examined the development of plasma lactate in septic shock patients treated in our ICU to determine the optimal rate of hourly reduction to use for prognostication. All patients treated for septic shock with plasma lactate >2mmol/L and treatment with vasopressors during 2015 and 2016 (n= 104, mortality 34%) were included. Lactate values from all blood gas analyzes, until normalization (<1.5 mmol/L) or 24 hours, whichever came first, were included. Changes in lactate concentrations between each sampling point were used to determine the mean hourly lactate reduction in percent. Mean reduction <2.5%/h and admission lactate ≥4mmol/L provided the best cut-off values for prognostication of poor outcome. Of the 22 patients with admission lactate >4mmol/L and decrease <2.5%/h, 14 died within 60 hours of ICU admission.

To study lactate metabolism, production as well as consumption, we examined two different approaches using intravenous administration of 13C-labeled lactate, which can be distinguished from endogenous lactate through laboratory analysis. First we examined a continuous infusion of 13C-lactate in healthy subjects. Samples were drawn from arterial and femoral venous lines and a skeletal muscle biopsy was taken at tracer steady state. An infusion of adrenaline was used to increase plasma lactate (from 1 to 4 mmol/L) and sampling was repeated after three hours. Skeletal muscle was a net contributor of lactate and adrenaline increased maximal mitochondrial respiration by 30%. The muscle biopsies showed a large variability and will not be used in clinical studies on septic patients. We then investigated a less invasive protocol for studying whole body lactate metabolism. First healthy volunteers were administered a bolus dose of 13C-lactate and a total of 43 blood samples were drawn during 2 hours. 10 ICU patients were then investigated using the same protocol. This protocol rendered similar values for lactate rate of appearance as other methods in healthy volunteers. ICU patients with normal lactate concentrations showed similar lactate metabolism as healthy volunteers. Simulations showed that blood samples can be decreased from 43 to 14 with the same accuracy.

We conclude that the protocol using infusion and arterio-venous sampling yield more information, but is more invasive and may be suitable for smaller physiological studies while the bolus approach is simpler and may be useful in larger cohorts. We plan on using these approaches to examine lactate kinetics in patients with septic shock. Other patient groups that may be interesting to study in the future includes post cardiac arrest, traumatic brain injury or postoperatively to major surgery.