Sekhon, Ainslie and Griesdale discuss the effect of Carbon Dioxide levels on outcome in secondary brain injury after Hypoxic Ischaemic Brain Injury (HIBI) following cardiac arrest.
It is useful to talk about Carbon Dioxide during cardiac arrest in the first place. The respiratory cycle involves inhalation (Oxygen is approximately 21% of the atmosphere by volume) and exhalation. Exhaled Carbon Dioxide is measured with capnometry and the typical values for Carbon Dioxide in exhaled air are between 4 and 5.5% (e.g (Han et al, 1997)).
Cardiac arrest is effectively cardio-respiratory arrest and the absence of exhalation of Carbon Dioxide would be expected to lead to the retention of Carbon Dioxide. There are many causes of a cardiac-arrest and these different causes can differentially affect the Carbon Dioxide levels in the period leading up to the cardiac arrest. Nevertheless in one study, arterial Carbon Dioxide levels were checked during out of hospital cardiac arrest and Hypercapnia was identified in 94% of cases and Hypocapnia in 6% of cases (Spinelboeck et al, 2016). Therefore in terms of modelling it can be assumed that during the period of Hypoxic injury there is associated Hypercapnia.
In secondary injury after Hypoxic Ischaemic Brain Injury (HIBI), Sekhon and colleagues note that there is a complex relationship between subsequent Carbon Dioxide levels and clinical outcomes (Sekhon et al, 2017). They note the physiological effects of Carbon Dioxide on vascular smooth muscle tone. This in turn affects resistance and Cerebral Blood Flow (CBF). Hypercapnia leads to Cerebral Vasodilatation and Hyperemia which the authors note can lead to reduced CBF.
Following HIBI the authors note that there has been a lot of discussion about the optimal Carbon Dioxide levels in terms of clinical outcomes. They identify that multiple lines of evidence show that Normocapnia is associated with good neurologic outcomes compared to Hypocapnia and Hypercapnia. They also suggest future studies to examine physiological values which in turn would be helpful for modelling.
Interestingly one study found a correlation between Near Death Experiences and Carbon Dioxide levels in survivors of out-of-hospital cardiac arrest (Klemenc-Ketis, Kersnik and Grmec, 2010).
The Context of Hypoxic Ischaemic Brain Injury
Sekhon, Ainslie and Griesdale have written an open access article on hypoxic ischaemic brain injury titled “Clinical Pathophysiology of Hypoxic Ischemic Brain Injury after Cardiac Arrest:A “two-hit” Model“. This paper can be used as a starting point for discussion of the events that lead to brain injury following hypoxia. This in turn is relevant to the question of energy usage in the Brain Hypometabolism Hypothesis.
Sekhon, Ainslie and Griesdale posit a simple two stage model of brain injury following cardiac arrest in which injury results from
- Primary cerebral hypoxia
- Secondary mechanisms after return of cerebral perfusion
In Sekhon, Ainslie and Griesdale’s model they discuss primary and secondary brain injury following a cardiac arrest.
Primary Brain Injury after Hypoxia
Looking more closely at the primary brain injury they state that with a reduction in cerebral oxygen ATP production decreases and there is a switch to anaerobic respiration. This in turn leads to a reduction in ATP dependent ion channel action. There are three main effects
- Accumulation of Na+ ions
- Accumulation of lactate with acidosis
- An influx of Calcium ions into the cells
Secondary Brain Injury after Hypoxia
Sekhon, Ainslie and Griesdale identify 7 factors associated with secondary brain injury after hypoxia in their two stage model. These 7 factors are
- Microvascular dysfunction
- Cerebral oedema
- Impaired autoregulation
- Carbon Dioxide
What is Metabolism?
Metabolism can be defined as the chemical processes that occur in living organisms. There are three types of metabolic processes
(a) Generation of energy
(b) Generation of basic chemicals including fatty acids, amino acids and sugars
(c) Elimination of Nitrogen waste products
Brain Hypometabolism Hypothesis
The Brain Hypometabolism Hypothesis focuses on energy metabolism. More specifically the hypothesis states that
‘Energy hypometabolism in the brain leads to neuropathology‘
Glycolysis is one of the key pathways for energy metabolism in the human body. In this metabolic pathway the molecule Glucose is converted into Pyruvate. This pathway generates energy in the form of ATP. This pathway however does not use oxygen although the products generated are metabolised using oxygen. This is relevant to the bigger picture of energy metabolism in the brain.
Acetyl CoA Space Filling Molecule by Benjah-bmm27 (Public Domain)
Acetyl Coenzyme A is an important molecule for many pathways involved in energy metabolism. Acetyl Coenzyme A is derived from
(a) Glucose via the Glycolysis pathway
(b) Amino acids via Acetoacetyl-CoA, Pyruvate and directly through multiple pathways
(c) Fatty acids via Beta-oxidation
Vitamin B5 is required for the synthesis of Acetyl CoA.
The Citric Acid Cycle
The Citric Acid Cycle is one of the main energy metabolism pathways in humans. Acetyl Co-A which is generated from other pathways is utilised in the Citric Acid Cycle. The Citric Acid Cycle has a number of properties
- Generation of energy in the form of ATP
- Generating NADH which is utilised in oxidative phosphorylation
- Citric Acid is regenerated
- Carbon Dioxide is produced
The Citric Acid Cycle takes place in the Mitochondria.
The Citric Acid Cycle is important for the discussion of the Brain Hypometabolism Hypothesis where we have already discussed the metabolism of Glucose.
Han JN, Stegen K, Simkens K, Cauberghs M, Schepers R, Van den Bergh O, Clément J, Van de Woestijne KP. Unsteadiness of breathing in patients with hyperventilation syndrome and anxiety disorders. Eur Respir J. 1997 Jan;10(1):167-76.
Mypinder S. Sekhon, Philip N. Ainslie and Donald E. Griesdale
Clinical pathophysiology of hypoxic ischemic brain injury after cardiac arrest: a “two-hit” model. Critical Care. 2017. 21:90. DOI: 10.1186/s13054-017-1670-9
Spindelboeck W, Gemes G, Strasser C, Toescher K, Kores B, Metnitz P, Haas J, Prause G. Arterial blood gases during and their dynamic changes after cardiopulmonary resuscitation: A prospective clinical study.Resuscitation. 2016 Sep;106:24-9. doi: 10.1016/j.resuscitation.2016.06.013. Epub 2016 Jun 18.
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