Blood Gases


بسم الله الرحمن الرحيم

إِنَّ فِي ذَلِكَ لَآيَاتٍ لِّأُولِي النُّهَى

             صدق الله العظيم

طه (54)

Blood Gases

Specimen Collection

● Also known as/ Arterial blood gases (ABGs), Venous blood gases, Capillary blood gases.

● Whole blood (arterial, venous, capillary).

● Blood is most commonly drawn from the radial artery in the wrist  (فى المعصم) because it is easily accessible, can be compressed to control bleeding, and has less risk for occlusion.

● The femoral artery (or less often, the brachial artery الشريان العضدى) is also used, especially during emergency situations or with children. Blood can also be taken from an arterial catheter (قسطره شريانيه) already placed in one of these arteries.

● Sometimes a blood sample drawn from a vein in the arm; capillary blood from a heelstick may be used for babies.

● The syringe is pre-packaged and contains a small amount of heparin, to prevent coagulation or needs to be heparinised, by drawing up a small amount of heparin and squirting it out again. Once the sample is obtained, care is taken to eliminate visible gas bubbles, as these bubbles can dissolve into the sample and cause inaccurate results. The sealed syringe is taken to a blood gas analyzer. If the sample cannot be analyzed within 10 –15 minutes, it must be placed on ice for valid results. Even when placed on ice, samples should still be analyzed within 1 hour.

● Excessive amounts of Heparin in the sample may falsely decrease pH, pCO2 and pO2.

● Citrates should never be used as an anticoagulant, as it will cause a marked decreasing in pH.

● After an arterial blood draw, pressure must be firmly applied to the site for at least 5 minutes. Since blood pumps through the artery, the puncture will take a while to stop bleeding. If you are taking blood thinners or aspirin, it may take as long as 10-15 minutes to stop bleeding.

● Contamination of the sample with room air will result in abnormally low carbon dioxide and possibly elevated oxygen levels, and elevation in pH.

● Delaying analysis (without chilling the sample) may result in inaccurately low oxygen and high carbon dioxide levels as a result of ongoing cellular respiration.

http://www.youtube.com/watch?v=YuFK22n-tvI

Test Preparation

● Typically, none. However, if you are on oxygen therapy, the O2 may either be turned off for 20 to 30 minutes before the collection for a “Room Air” test or, if this cannot be tolerated or if the doctor wants to check your oxygen levels with the O2 on, the amount of oxygen being taken will be recorded.

Physiologic Bases

● You get tested To determine if your lungs are functioning well enough to exchange O2 and CO2 if you are having symptoms of a respiratory problem; to determine if you have an imbalance in the amount of oxygen gas (O2) or carbon dioxide gas (CO2) in your blood or an acid-base imbalance, which may indicate a respiratory, metabolic, or kidney disorder.

● Measurements of Blood gases are commonly reported as:

* pH (which is No. of free hydrogen ions in the body)

* Partial pressure of carbon dioxide in blood (pCO2)

* Partial pressure of oxygen in blood (pO2)

* Bicarbonate (HCO3), O2 saturation

* base excess (BE) or base defect (BD) … which reflects the No. of anions available in the blood to help buffer changes in pH.

● The main buffer system in the body is the … bicarbonate – carbonic acid system, (in which bicarbonate is an important alkaline ion that participates with other anions such as hemoglobin, proteins and phosphates to neutralize acids … carbonic acid is the main acid in the acid – base system which can be/ metabolic or respiratory component).

● A pH less than 7.35 indicates Acidosis, while pH greater than 7.45 indicates Alkalosis.

● For body to keep the acid – base balance, there must be a ratio of 20 parts bicarbonate to 1 part carbonic acid (20:1).

● Carbonic acid is indirectly measured, but can be estimated as it is 3% of pCO2… While Bicarbonate can be measured indirectly by total carbon dioxide content (tCO2) or can be estimated directly when obtained Carbonic acid value because of the ratio (20:1).

EX:

pCO2 = 40… Then carbonic acid is 3% of 40, that is equal to 1.2 … and so bicarbonate would be 20 * 1.2 = 24

● (BD) or negative (BE) indicates metabolic acidosis, while positive (BE) indicates metabolic alkalosis.

● O2 saturation – the percentage of hemoglobin that is carrying oxygen.

● Bicarbonate (HCO3) – the main form of CO2 in the body, it can be calculated from the pH and PCO2. It is a measurement of the metabolic component of the acid-base balance. HCO3 is excreted and reabsorbed by the kidneys in response to pH imbalances and is directly related to the pH level; as the amount of HCO3 rises, so does the pH.

● Base excess/base deficit – a calculated number that represents a sum total of the metabolic buffering agents (anions) in the blood; these anions include hemoglobin, proteins, phosphates, and HCO3 (bicarbonate, which is the dominant anion). The anions try to compensate تعادل for imbalances in the blood pH.

● Extreme acidosis (generally more life threatening than alkalosis), can be developed very quickly (as in cardiac arrest) or over longer time (as in renal failure).

● Testing on specimens other than arterial blood is often ordered when O2 measurements are not needed, or when it can be obtained by noninvasive techniques like Pulse Oximetry (قياس مستوى الاكسجين خلال النبض).

● Arterial blood is required to accurately measure pO2 and oxygen saturation, while Capillary blood is enough for most purposes of pH and pCO2 but for pO2 it’s limited to the exclusion of hypoxia (نقص ارتباط الاكسجين بالدم).

 

 So, how Acid-Base imbalance occurred??!!!

● The normal range for pH is 7.35-7.45. As the pH decreases (<7.35), it infers acidosis, while if the pH increases (>7.45) it infers alkalosis.

● In arterial blood gases disorders, the most common occurrence will be the respiratory pathway: As CO2 concentrations continue to increase (PaCO2 > 45 mmHg), a condition known as respiratory acidosis occurs.

● Carbon dioxide is dissolved in the blood as carbonic acid, a weak acid; however, in large concentrations, it can affect the pH drastically. Whenever there is poor pulmonary ventilation ,(التهويه الرئويه) the carbon dioxide levels in the blood are expected to rise. This leads to a rise of carbonic acid, leading to a decrease in pH.

● The first buffer of pH will be the plasma proteins, since these can accept some H+ ions trying to maintain homeostasis, The body tries to maintain homeostasis by increasing the respiratory rate, a condition known as tachypneoa (سرعة معدل التنفس) which allows much more CO2 to escape the body through the lungs, thus increasing the pH by having less carbonic acid.

● On the other hand, respiratory alkalosis (PaCO2 < 35mmHg) occurs when there is too little carbon dioxide in the blood. This may be due to hyperventilation or else excessive breaths given via a mechanical ventilator in a critical care setting.

● The action to be taken is to calm the patient and try to reduce the number of breaths being taken to normalise the pH. The respiratory pathway tries to compensate for the change in pH in a matter of 2-4 hours. If this is not enough, the metabolic pathway (acidosis or alkalosis)takes place.

● The kidney and the liver are two main organs responsible for the metabolic homeostasis of pH.

● In acidaemia, the bicarbonate levels rise, so that they can neutralise the excess acid, while the contrary happens (بينما يحدث العكس) when there is alkalaemia.

● However, this mechanism is slower than the respiratory pathway and may take from a few hours to 3 days to take effect.

● Thus when an arterial blood gas test reveals, for example, an elevated bicarbonate, the problem has been present for a couple of days, and metabolic compensation took place over a blood acedemia problem.

● The metabolic compensations take place at a much later stage.

EX./ However, in a critical setting, a patient with a normal pH, a high CO2, and a high bicarbonate means that, although there is a high carbon dioxide level, there is also metabolic compensation.

● As a result one must be careful as to not artificially adjust breaths to lower the carbon dioxide. In such case, lowering the carbon dioxide abruptly means that the bicarbonate will be in excess and will cause a metabolic alkalosis. In such a case, carbon dioxide levels should be slowly diminished.

Applications

● This group of tests are used to assess conditions such as: Asthma (الربو), Chronic Obstructive Pulmonary Disease (COPD) and Hypoxia.

● Also, can be used to assist in the diagnosis of respiratory failure (which is pO2 less than 50 mm Hg and pCO2 greater than 50 mm Hg.

● Blood gases Values are used to determine acid-base status and the type of imbalance as the following:

* When pH values are moving in the same direction (increasing or decreasing in pCO2 or HCO3) then the imbalance is Metabolic.

* When pH values are moving in the opposite direction from pCO2 or HCO3, then the imbalance is Respiratory.

● The doctor will look at the HCO3 and base excess/deficit results to evaluate the total buffering capacity when deciding on a treatment to correct an imbalance.

● Blood gases can be valuable in management of patients on ventilators (اجهزة التنفس الصناعى).

● In medicine: 2 methods have been used in the management of blood gases of patients in hypothermia:

pH-stat method and alpha-stat method .

* pH-stat: the arterial carbon dioxide tension (paCO2) is maintained at 5.3 kPa (40 mmHg) and the pH is maintained at 7.40 when measured at the actual patient temperature.

* α-stat (alpha-stat): the arterial carbon dioxide tension (paCO2) and the pH are maintained at 5.3 kPa (40mmHg) and 7.40 when measured at 37°C.

● Recent studies suggest that the α-stat method is superior, as When a patient is cooled down (hypothermia), the pH-value will increase and the pCO2-value and the pO2-value will decrease

Results Interpretation

● Acid – base imbalance is determined by evaluating (pH, pCO2 and HCO3) values, in which pH less than 7.35 represent acidic state, while pH greater than 7.45 represent alkalosis state.

● Both pCO2 and HCO3 determine whether the imbalance is respiratory or metabolic (non-respiratory).

● Respiratory conditions that interfere with normal breathing (Respiratory imbalance) will cause CO2 to be retained in the blood, which resulting in increasing of circulating carbonic acid with decreasing in pH (respiratory acidosis).

* Acute respiratory acidosis can be occurred in cases of pulmonary edema, severe respiratory infections, bronchial obstruction (انسداد الشعب الهوائيه), pneumothorax (معالجة السل الرئوى), open chest wounds, respiratory depressant drug therapy and inhalation of air with high CO2 content.

* Chronic respiratory acidosis can be seen in patients with asthma, pulmonary fibrosis, emphysema

(انتفاخ الرئه), bronchiectasis (توسع القصبات).

● Respiratory conditions that increasing the breathing rate will cause CO2 to be removed from alveoli more rapidly than it’s being produced, which results in alkaline pH (respiratory alkalosis).

* Acute respiratory alkalosis may be seen in cases of hysteria, anxiety (القلق الشديد), hyperventilation

(فرط التنفس), pulmonary embolus (انسداد رئوى) and with increasing in artificial ventilation.

* Chronic respiratory alkalosis may be seen in cases of hypoxia, high fever, hepatic coma, central nervous system (CNS) lesions, injuries that result in stimulation of the respiratory system.

● Metabolic (non-respiratory) conditions (Metabolic Imbalance) that cause excessive formation or decreased excretion of organic or inorganic acids result in metabolic acidosis.

* Some of these cases include {ingestion of salicylates, ethylene glycol and methanol}, uncontrolled diabetes, starvation, shock and renal disease.

● Metabolic conditions that increasing pH results in metabolic alkalosis.

* Some of these cases include excessive intake of antacids to treat gastritis or peptic ulcer

(مضادات الحموضه لعلاج التهاب القناه الهضميه او القرحه المعديه), loss of stomach acid that caused by protracted vomiting (القىء الممتد), cystic fibrosis (التليف المثانى) and K+ & Cl deficiencies.

Combinations of results that may be seen in certain conditions are summarized below:

pH result

HCO3 result

PCO2 result

Condition

Common causes

Less than 7.4

Low

Low

Metabolic acidosis

Kidney failure, shock, diabetic ketoacidosis

Greater than 7.4

High

High

Metabolic alkalosis

Chronic vomiting, low blood potassium

Less than 7.4

High

High

Respiratory acidosis

Lung diseases such as pneumonia, COPD

Greater than 7.4

Low

Low

Respiratory alkalosis

Hyperventilation, pain, anxiety

Helpful Guidelines

  1. A 1mmHg change in PaCO2 above or below (40 mmHg) results in 0.008 unit change in pH in the opposite direction.
  2. The PaCO2 will decrease by about 1 mmHg for every 1 mEq/L reduction in [HCO3] below 24 mEq/L
  3. A change in [HCO3] of 10 mEq/L … will result in a change in pH of approximately 0.15 pH units in the same direction.

Parameters and reference ranges

Analyte

Range

Interpretation

pH

7.34 – 7.44

The pH or H+ indicates if a patient is acidemic (pH < 7.35; H+ >45) or alkalemic (pH > 7.45; H+ < 35).

H+

35–45 nmol/L

See above.

Arterial oxygen partial pressure (PaO2)

11 – 13 kPa or

75 – 100 mmHg

A low PaO2 indicates that the patient is not oxygenating properly, and is hypoxemic. (Note that a low PaO2 is not required for the patient to have hypoxia.) At a PaO2 of less than 60 mm Hg, supplemental oxygen should be administered. At a PaO2 of less than 26 mmHg, the patient is at risk of death and must be oxygenated immediately.

Arterial carbon dioxide partial pressure (PaCO2)

4.7 – 6.0 kPa or

35 – 45 mmHg

The carbon dioxide partial pressure (PaCO2) is an indicator of CO2 production and elimination: for a constant metabolic rate, the PaCO2 is determined entirely by its elimination through ventilation. A high PaCO2 (respiratory acidosis, alternatively hypercapnia زيادة ارتباط ثانى اكسيد الكربون بالدم) indicates hypoventilation (or, more rarely, a hypermetabolic disorder), a low PaCO2 (respiratory alkalosis, alternatively hypocapnia نقص ارتباط ثانى اكسيد الكربون بالدم) indicates hyperventilation.

HCO3

22–26 mEq/L

The HCO3- ion indicates whether a metabolic problem is present (such as ketoacidosis). A low HCO3 indicates metabolic acidosis, a high HCO3 indicates metabolic alkalosis. As this value when given with blood gas results is often calculated by the analyzer, correlation should be checked with total CO2 levels as directly measured (see below).

SBCe

21 to 27 mmol/L

the bicarbonate concentration in the blood at a CO2 of 5.33 kPa, full oxygen saturation and 37 Celsius.

Base excess

−2 to +2 mmol/L

The base excess is used for the assessment of the metabolic component of acid-base disorders, and indicates whether the patient has metabolic acidosis or metabolic alkalosis. Contrasted with the bicarbonate levels, the base excess is a calculated value intended to completely isolate the non-respiratory portion of the pH change.

Total CO2 (tCO2)

23 – 30 mmol/L or 100 – 132 mg/Dl This is the total amount of CO2, and is the sum of HCO3 and PCO2 by the formula:
tCO2 = [HCO3] + α*PCO2, where α=0.226 mM/kPa, HCO3 is expressed in millimolar concentration (mM) (mmol/l) and PCO2 is expressed in kPa

O2 Content

(CaO2, CvO2, CcO2)

vol% (mL oxygen/dL blood) This is the sum of oxygen dissolved in plasma and chemically bound to hemoglobin as determined by the calculation:

CaO2 = (PaO2 * 0.003) + (SaO2 * 1.34 * Hgb), where hemoglobin concentration is expressed in g/dL.

● These are typical reference ranges, although various analysers and laboratories may employ different ranges.

References

http://labtestsonline.org/understanding/analytes/blood-gases/tab/test

http://en.wikipedia.org/wiki/Arterial_blood_gas

Aboutable Pocket Guide to Common Laboratory TestsSelection and Interpretation

Page No. 49-55.

تم بحمد الله و فضله …

 

أضف تعليقاً

إملأ الحقول أدناه بالمعلومات المناسبة أو إضغط على إحدى الأيقونات لتسجيل الدخول:

WordPress.com Logo

أنت تعلق بإستخدام حساب WordPress.com. تسجيل خروج   / تغيير )

صورة تويتر

أنت تعلق بإستخدام حساب Twitter. تسجيل خروج   / تغيير )

Facebook photo

أنت تعلق بإستخدام حساب Facebook. تسجيل خروج   / تغيير )

Google+ photo

أنت تعلق بإستخدام حساب Google+. تسجيل خروج   / تغيير )

Connecting to %s

%d مدونون معجبون بهذه: