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Normal values for humans are in the range 35–45 mmHg. Values less than this may indicate hyperventilation and (if blood pH is greater than 7.45) respiratory alkalosis. Values greater than 45 mmHg may indicate hypoventilation, and (if blood pH is less than 7.35) respiratory acidosis. [2] [3]
Arterial blood oxygen tension (normal) P a O 2 – Partial pressure of oxygen at sea level (160 mmHg (21.3 kPa) in the atmosphere, 21% of the standard atmospheric pressure of 760 mmHg (101 kPa)) in arterial blood is between 75 and 100 mmHg (10.0 and 13.3 kPa).
An ABG test measures the blood gas tension values of the arterial partial pressure of oxygen (PaO2), and the arterial partial pressure of carbon dioxide (PaCO2), and the blood's pH. In addition, the arterial oxygen saturation (SaO2) can be determined. Such information is vital when caring for patients with critical illnesses or respiratory disease.
Hypoventilation exists when the ratio of carbon dioxide production to alveolar ventilation increases above normal values – greater than 45mmHg. If pH is also less than 7.35 this is respiratory acidosis.
Diagnoses can be done by doing an ABG (Arterial Blood Gas) laboratory study, with a pH <7.35 and a PaCO2 >45 mmHg in an acute setting. Patients with COPD and other Chronic respiratory diseases will sometimes display higher level of PaCO2 with HCO3- >30 and normal pH. [citation needed] [2]
Respiratory failure results from inadequate gas exchange by the respiratory system, meaning that the arterial oxygen, carbon dioxide, or both cannot be kept at normal levels. A drop in the oxygen carried in the blood is known as hypoxemia ; a rise in arterial carbon dioxide levels is called hypercapnia .
Base excess is defined as the amount of strong acid that must be added to each liter of fully oxygenated blood to return the pH to 7.40 at a temperature of 37°C and a pCO 2 of 40 mmHg (5.3 kPa). [2]
In fact, for each 1 mmHg increase or decrease in PaCO2, between the range of 20–60 mmHg, there is a corresponding CBF change in the same direction of approximately 1–2 ml/100g/min, or 2–5% of the CBF value. [17] This is why small alterations in respiration pattern can cause significant changes in global CBF, specially through PaCO2 ...