Editing Blood gas interpretation

== Measures: ==

* pH: normally 7.35-7.45
* PaO2: arterial partial pressure of oxygen - a reflection of the amount of oxygen dissolved in the blood. Normal 10-14kPa.
* PaCO2: Normal 4.5-6kPa - reflects the absolute ventilatory state of a patient and possible respiratory compensatory mechanisms
* HCO3-: normal 24-28mmol/L. Bicarbonate is the main plasma buffer. A low value suggests consumption, often due to increased acid load e.g. lactic acid. A high value suggests retention of base to compensate for hypoventilation causing an acidaemia.
* Base deficit/base excess: normal +2 to -2. This describes whether the body's buffers are being consumed (deficit) or retained (excess).
** A measure of the amount of fixed base added to an aliquot (1L) of blood to bring pH to 7.4
** Essentially goes lower in anaerobic metabolism and acidosis
** Might actually be a better predictor of poor outcomes in trauma patients than lactate - in one study a moderate BD on admission meant a mortality of 15%, while severe BD meant 35% mortality
** Normal 2 to -2
** Mild -3 to -5
** Moderate -6 to -9
** Severe <-9
* Serum lactate - normal <1.2mmol/L. A reflection of the extent of anaerobic metabolism occurring, and secondarily a measure of the liver's ability to metabolise lactate and regenerate bicarbonate ions.
* Anion gap - normal range 10-15mmol/L.


Relative hypoxaemia is a more useful measure than absolute PaO2, which is essentially comparing it to FiO2

* As FiO2 increases towards 1, PaO2 should also increase
* Use PaO2/FiO2 (should be >40kPa, or patient is hypoxic)

== Metabolic acidosis ==

* Impaired tissue perfusion - deal with cause
* Renal failure - deal with cause, bicarbonate, RRT
* Hepatic failure - deal with cause
** Or KULT (ketones urea lactate toxins)
* NAGMA - usually hyperchloraemic acidaemia - most often seen following vigorous resus with 0.9% saline, also bladder surgery and ileal conduit formation.
*

== Resp acidosis ==

* Inadequate CO2 elimination by lungs relative to rate of CO2 produced by cellular metabolism
** Head or spinal injury - ventilation
** Drug overdose - ventilate, antidote
** Chest wall injury - ventilate
** Myopathy/peripheral neuropathy - ventilate
** Pulmonary disease - treat disease, resp support +/- ventilate
** Massive PE - re-establish perfusion of ventilated lung

== Metabolic alkalosis ==

* Typically occurs when there is both an increase in alkali and impaired excretion of bicarb
* In surgical patients, frequently as a result of H+ loss (vomiting or NGT drainage, especially in gastric outlet obstruction), or from excess bicarb regeneration as a result of diuretic therapy
* Can also occur with mineralocorticoid excess

== Resp alkalosis ==

* Occurs when alveolar ventilation exceeds the rate necessary to eliminate CO2 produced by cellular metabolism




[[Category:Critical care]]
[[Category:Intern education]]