Abstract
Blood gas analysis is an important means to assess someone’s oxygenation, adequacy of ventilation, and also acid–base status. Blood gas analysis can be done with either arterial or venous blood, but it provides valuable information to make a diagnosis, to assess the severity of the condition, and also to assess the effectiveness of treatment. Anion gap is routinely measured which provides us with the information regarding “cations” and “anions” in the blood and also points toward the possible cause of acidosis. Winter’s formula is used to determine the compensatory mechanism of acid–base disorder, and delta (Δ) gap provides the information regarding mixed acid–base disorder.
Access provided by CONRICYT-eBooks. Download chapter PDF
Similar content being viewed by others
Keywords
A 36-year-old woman presents to the emergency room with severe abdominal pain, nausea, vomiting, anorexia, and somnolence.
ABG: pH 7.20, PCO2 35 mmHg, pO2 68 mmHg on room air
Laboratory values: Na 130 mEq/L, Cl 80 mEq/L, HCO3 10 mEq/L
-
1.
How do you diagnose a simple acid–base disorder?
-
2.
What blood gas abnormality does this patient have?
-
3.
How do you calculate anion gap and corrected anion gap?
-
4.
How do you diagnose a mixed acid–base disorder and does this patient have mixed acid–base disorder?
-
5.
What is Winter’s formula?
-
6.
Is there any compensation in this blood gas value?
-
7.
What are the possible causes of metabolic acidosis?
-
8.
What are the possible causes of respiratory acidosis?
Answers
-
1.
Initially the pH is used to determine acidosis or alkalosis, and then the value of PaCO2/HCO3 is used to determine if the derangement is metabolic or respiratory. If it is of respiratory origin, then we will have to determine whether the process is acute or chronic. If it is due to a metabolic component, then respiratory compensation should be calculated using the appropriate formula.
-
2.
Our patient has a pH less than 7.4, which signifies acidosis. The HCO3 is less than 24 mEq/L; therefore the primary abnormality in this patient is metabolic acidosis. This chart (Fig. 36.1) shows the steps to follow in order to diagnose an acid–base disorder [1].
-
3.
Anion gap (AG) = Na − (Cl + HCO3)
-
(a)
AG is the difference in the ‘routinely measured’ cations (Na) and ‘routinely measured’ anions (Cl and HCO3) in the blood and depends on serum phosphate and albumin concentrations [2]. Determination of AG is useful in determining the cause of acidosis [3, 4]. The normal value for serum AG is usually 8–12 mEq/L. In our patient, AG = 130 − (80 + 10) = 40 mEq/L. So, this patient has a high AG, most likely due to starvation or diabetic ketoacidosis.
-
(b)
In a normal healthy patient, negatively charged albumin is the single largest contributor to the AG [5]. Hypoalbuminemia causes a decrease in AG; hence AG is corrected to albumin level using the equation of Figge as follows: corrected AG = AG + [0.25 × (44 – Albumin)] [6].
-
If corrected AG >16, there is high AG acidosis.
-
If corrected AG <16, non-AG acidosis.
-
-
(a)
-
4.
Delta gap formula can be used to assess mixed acid–base disorder.
-
(a)
Δ gap = AG − 12 + HCO3 (12 is normal serum AG value)
-
If Δ gap <22 mEq/L, then concurrent non-gap metabolic acidosis exists.
-
If Δ gap >26 mEq/L, then concurrent metabolic alkalosis exists.
-
-
(b)
In our patient, Δ gap = 40 − 12 + 10 = 38 mEq/L. So, there is a concurrent metabolic alkalosis probably from vomiting in addition to high AG metabolic acidosis in this patient.
So, there is a concurrent metabolic alkalosis probably from vomiting in addition to high AG metabolic acidosis in this patient.
-
(a)
-
5.
Winter’s formula is used to determine whether there is an appropriate respiratory compensation during metabolic acidosis [1].
-
(a)
Winter’s formula: PCO2 = (1.5 × HCO3) + 8
-
If measured PCO2 > calculated PCO2, then concurrent respiratory acidosis is present.
-
If measured PCO2 < calculated PCO2, then concurrent respiratory alkalosis is present.
-
-
(a)
-
6.
In our patient, calculated PCO2 = (1.5 × 10) + 8 = 23 mmHg according to Winter’s formula.
Our measured PCO2 of 35 mmHg is higher than the calculated PCO2 of 23 mmHg, so our patient also has concurrent respiratory acidosis. Usually, metabolic acidosis is compensated by respiratory alkalosis, but due to somnolence in this patient, concurrent respiratory acidosis exists.
-
7.
Causes of anion gap metabolic acidosis are easily remembered by pneumonic MUDPILES [1].
-
M: methanol
-
U: uremia
-
D: diabetic ketoacidosis
-
P: paraldehyde
-
I: infection, INH therapy
-
L: lactic acidosis
-
E: ethanol, ethylene glycol
-
S: salicylates (aspirin)
Causes of non-gap metabolic acidosis:
-
Excessive administration of 0.9% normal saline
-
GI losses: diarrhea, ileostomy, neobladder, pancreatic fistula
-
Renal losses: renal tubular acidosis
-
Drugs: acetazolamide
-
-
8.
Respiratory acidosis which is from increased CO2 is due either to increased production or decreased elimination [2].
-
(a)
Increased production of CO2:
-
Malignant hyperthermia
-
Hyperthyroidism
-
Sepsis
-
Overfeeding
-
-
(b)
Decreased elimination of CO2:
-
Intrinsic pulmonary disease (pneumonia, ARDS, fibrosis, edema)
-
Upper airway obstruction (laryngospasm, foreign body, OSA)
-
Lower airway obstruction (asthma, COPD)
-
Chest wall restriction (obesity, scoliosis, burns)
-
CNS depression (anesthetics, opioids, CNS lesions)
-
Decreased skeletal muscle strength (myopathy, neuropathy, residual effects of neuromuscular blocking drugs)
-
Rarely, an exhausted soda–lime or incompetent one-way valve in an anesthesia delivery system can contribute to respiratory acidosis.
-
(a)
References
Miller RD, Pardo MC Jr. Basics of anesthesia. 6th ed. Philadelphia: Elsevier Saunders; 2011.
Morgan GE, Mikhail MS, Murray MJ. Clinical anesthesiology. 4th ed. New York: McGraw-Hill; 2006.
Emmett M, Narins RG. Clinical use of the anion gap. Medicine (Baltimore). 1977;56(1):38–54.
Barash PG, et al. Clinical anesthesia. 6th ed. Philadelphia: Lippincott Williams & Wilkins; 2009.
Feldman M, Soni N, Dickson B. Influence of hypoalbuminemia or hyperalbuminemia on the serum anion gap. J Lab Clin Med. 2005;146(6):317–20.
Hatherill M, Waggie Z, Purves L, et al. Correction of the anion gap for albumin in order to detect occult tissue anions in shock. Arch Dis Child. 2002;87(6):526–9.
Author information
Authors and Affiliations
Corresponding author
Editor information
Editors and Affiliations
Rights and permissions
Copyright information
© 2017 Springer International Publishing AG
About this chapter
Cite this chapter
Shah, T. (2017). Blood Gas I. In: Raj, T. (eds) Data Interpretation in Anesthesia. Springer, Cham. https://doi.org/10.1007/978-3-319-55862-2_36
Download citation
DOI: https://doi.org/10.1007/978-3-319-55862-2_36
Published:
Publisher Name: Springer, Cham
Print ISBN: 978-3-319-55861-5
Online ISBN: 978-3-319-55862-2
eBook Packages: MedicineMedicine (R0)