VET90 blood gas analyzer

Potential of hydrogen

The degree of acidity or alkalinity of any liquid (including blood) is a function of its hydrogen ion concentration [H⁺], and pH is simply a way of expressing hydrogen ion activity. The relationship between pH and hydrogen ion concentration is described thus:

pH = -log aH⁺
where aH⁺ is hydrogen ion activity.

Low pH is associated with acidosis and high pH with alkalosis [1].

1. CLSI. Blood gas and pH analysis and related measurements; Approved Guidelines. CLSI document CA46-A2, 29, 8. Clinical and Laboratory Standards Institute, 940 West Valley Road, Suite 1400, Wayne, Pennsylvania 19087-1898 USA, 2009.

Partial pressure of carbon dioxide

Carbon dioxide (CO₂) is an acidic gas; the amount of CO₂ in blood is largely controlled by the rate and depth of breathing or ventilation. pCO₂ is the partial pressure of CO₂ in blood. It is a measure of the pressure exerted by that small portion (~5 %) of total CO₂ that remains in the gaseous state, dissolved in the blood plasma. pCO₂ is the respiratory component of acid-base balance and reflects the adequacy of pulmonary ventilation. The severity of ventilator failure as well as the chronicity can be judged by the accompanying changes in acid-base status [1].

1. Higgins C. Parameters that reflect the carbon dioxide content of blood. www.acutecaretesting.org Oct 2008.

Partial pressure of oxygen

The amount of oxygen in blood is controlled by many variables, e.g. ventilation/perfusion. pO₂ is the partial pressure of oxygen in a gas phase in equilibrium with the blood. pO₂ only reflects a small fraction (1 – 2 %) of total oxygen in blood that is dissolved in blood plasma [1]. The remaining 98 – 99 % of oxygen present in blood is bound to the hemoglobin in the erythrocytes. pO₂ primarily reflects the oxygen uptake in the lungs. 

1. Wettstein R, Wilkins R. Interpretation of blood gases. In: Clinical assessment in respiratory care, 6th ed. St. Louis: Mosby, 2010.

Glucose

Glucose, the most abundant carbohydrate in human metabolism, serves as the major intracellular energy source (see lactate). Glucose is derived principally from dietary carbohydrate, but it is also produced – primarily in the liver and kidneys – via the anabolic process of gluconeogenesis, and from the breakdown of glycogen (glycogenolysis). This endogenously produced glucose helps keep blood glucose concentration within normal limits, when dietary-derived glucose is not available, e.g. between meals or during periods of starvation.

Lactate

Lactate, the anion that results from dissociation of lactic acid, is an intracellular metabolite of glucose. It is produced by skeletal muscle cells, red blood cells (erythrocytes), the brain, and other tissues during anaerobic energy production (glycolysis). Lactate is formed in the intracellular fluid from pyruvate; the reaction is catalyzed by the enzyme lactate dehydrogenase (LDH) [1].

1. Robergs RA, Ghiasvand F, Parker D. Biochemistry of exercise-induced metabolic acidosis. Am J Physiol Regul Integr Comp Physiol 2004; 287: R502-16.

Creatinine

Creatinine is an endogenous waste product of muscle metabolism, derived from creatine, a molecule of major importance for energy production within muscle cells. Creatinine is removed from the body in urine and its concentration in blood reflects glomerular filtration and thereby kidney function.

Urea

Urea (molecular formula CO(NH₂)₂) is the principal nitrogenous waste product of protein catabolism, which is eliminated from the body in urine. It is the most abundant organic component of urine. Urea is transported in blood from the liver to the kidneys, where it is filtered from the blood and excreted in the urine. Renal failure is associated with the reduced excretion of urea in urine, and a consequent rise in blood (plasma/serum) urea concentration.

Calsium

The calcium ion (Ca²⁺) is one of the most prevalent cations in the body, where approximately 1 % is present in the extracellular fluid of blood. Ca²⁺ plays a vital role for bone mineralization and many cellular processes, e.g. contractility of the heart and the skeletal musculature, neuromuscular transmission, hormone secretion and action in various enzymatic reactions such as, e.g. blood coagulation.

Chloride

Chloride (Cl^-) is the major anion in the extracellular fluid and one of the most important anions in blood. The main function of Cl^- is to maintain osmotic pressure, fluid balance, muscular activity, ionic neutrality in plasma, and help elucidate the cause of acid-base disturbances.

Potassium

Potassium (K⁺) is the major cation in the intracellular fluid, where it has a 25 - 37-fold higher concentration (∼150 mmol/L in tissue cells, ∼105 mmol/L in erythrocytes) than in the extracellular fluid (∼4 mmol/L) [1, 2]. K⁺ has several vital functions in the body, e.g. regulation of neuromuscular excitability, regulation of heart rhythm, regulation of intracellular and extracellular volume and acid-base status.

1. Burtis CA, Ashwood ER, Bruns DE. Tietz textbook of clinical chemistry and molecular diagnostics. 5th ed. St. Louis: Saunders Elsevier, 2012. Engquist A. Fluids/Electrolytes/Nutrition. 1st ed. Copenhagen: Munksgaard, 1985.
2. Engquist A. Fluids/Electrolytes/Nutrition. 1st ed. Copenhagen: Munksgaard, 1985.

Sodium

Sodium (Na⁺) is the dominant cation in the extracellular fluid, where it has a 14-fold higher concentration (∼140 mmol/L) than in the intracellular fluid (∼10 mmol/L). Na⁺ is a major contributor of the osmolality of the extracellular fluid and its main function is largely in controlling and regulating water balance, and maintaining blood pressure. Na⁺ is also important for transmitting nerve impulses and activating muscle concretion.

Carboxyhemoglobin

FMetHb is the fraction of total hemoglobin (ctHb) that is present as methemoglobin (MetHb). By convention the fraction is expressed as a percentage (%).

In the range of 0 – 60 % COHb in arterial (COHb(a)) and venous blood (COHb(v)) is similar, i.e. either venous or arterial blood may be analyzed [1]. In most medical texts FCOHb(a) is referred to as simply COHb.

1. Lopez DM, Weingarten-Arams JS, Singer LP, Conway EE Jr. Relationship between arterial, mixed venous and internal jugular carboxyhemoglobin concentrations at low, medium and high concentrations in a piglet model of carbon monoxide toxicity. Crit Care Med 2000; 28: 1998-2001.

Bilirubin

Bilirubin is the yellow breakdown product of the degradation of the heme group of hemoglobin. It is transported in blood from its site of production – the reticuloendothelial system – to the liver, where it is biotransformed before excretion in bile. Jaundice, the pathological yellow discoloration of skin, is due to abnormal accumulation of bilirubin in the tissues, and is always associated with elevated blood concentration of bilirubin (hyperbilirubinemia).

Total hemoglobin

The concentration of total hemoglobin (ctHb) in blood includes oxyhemoglobin (cO₂Hb), deoxyhemoglobin (cHHb), as well as the dysfunctional hemoglobin species that are incapable of binding oxygen:

carboxyhemoglobin (cCOHb) (see COHb), methemoglobin (cMetHb) (see MetHb) and sulfhemoglobin (cSulfHb).

Thus:

ctHb = cO₂Hb + cHHb + cCOHb + cMetHb + cSulfHb

The rare sulfHb is not included in the reported c tHb in most oximeters.

Fraction of deoxyhemoglobin

FHHb in total hemoglobin in blood.

Methemoglobin

FMetHb is the fraction of total hemoglobin (ctHb) that is present as methemoglobin (MetHb). By convention the fraction is expressed as a percentage (%) [1].

In most medical text boxes MetHb(a) is referred to as simply methemoglobin (MetHb).

1. CLSI. Blood gas and pH analysis and related measurements; Approved Guidelines. CLSI document CA46-A2, 29, 8. Clinical and Laboratory Standards Institute, 940 West Valley Road, Suite 1400, Wayne, Pennsylvania 19087-1898 USA, 2009.

Oxygen saturation

Oxygen saturation (sO₂) is the ratio of oxyhemoglobin concentration to concentration of functional hemoglobin (i.e. oxyhemoglobin (O₂Hb) and deoxyhemoglobin (HHb) capable of carrying oxygen [1].

The sO₂ reflects utilization of the currently available oxygen transport capacity.

In arterial blood 98 – 99 % of oxygen is transported in erythrocytes bound to hemoglobin. The remaining 1–2 % of the oxygen transported in blood is dissolved in the blood plasma – this is the portion reported as partial pressure of oxygen (pO₂) [2].

1. CLSI. Blood gas and pH analysis and related measurements; Approved Guidelines. CLSI document CA46-A2, 29, 8. Clinical and Laboratory Standards Institute, 940 West Valley Road, Suite 1400, Wayne, Pennsylvania 19087-1898 USA, 2009.

2. Higgins C. Parameters that reflect the carbon dioxide content of blood. www.acutecaretesting.org Oct 2008.

Fraction of oxyhemoglobin

FO₂Hb in total hemoglobin in blood.