Mixed venous blood:
Obtain from the pulmonary arter If there is infusion fluid:
Remove thrice the catheter dead space before sampling. Aspirate 1 m1/5 sec to avoid back mixing of oxygenated pulmonary capillary blood.
The specimen reflects:
Mixed venous PO2
Arterio-venous difference in O2 i.e. CO2 (a) – CO2 (v)
It is useful for detecting shunts between arterial and venous blood.
Peripheral venous blood:
The cubital vein or the back of the hand is used. The specimen is suitable for the determination of electrolytes, bicarbonate and base excess, haemoglobin and haematocrit and ionised calcium. The sample is taken anaerobically without tourniquet and without muscular
This is a mixture of blood from arterioles, venules, capillaries: interstitial and intracellular fluids: Avoid pressure to surrounding tissues to avoid excess of tissue fluid.
. pH and PCO2 values correlate with the arterial values.
. PO2 value differs because arterial is mixed with venous blood’
. Ionised calcium values are similar in arterial and capillan’ blood.
. Potassium value increases having escaped from cells to tissue and blood.
. Erythrocyte fragility and haematocrit values increase in the newborn.
by skin puncture in neonates. children or ambulatory adults,(industrial or sports medicine ) from medial or lateral heel surface of distal portion of finger or from ear lobe.
1. Lubricate inner wall of syringe with 0.3-0.5 ml anticoagulant solution.
2. Replace aspiration needle with canula
3. Expel air and excess anticoagulant: note the hazards of arterial blood collection.
4. check blood collected for air bubbles and expel air immediately.
5. Replace needle by a tight fitting cap.
6. Invert syringe 5 times to mix.
7. Roll between palms for 5 seconds and measure or store in ice. Mix to measure.
If pre-prepared syringes are used. follow the manufacturer’s instructions..
If sampling from capillaries:
. Warm the skin at 39-420 C for 3 minutes
. Clean and dry.
. Puncture the skin with a lancet
. Remove first drop and allow free flow
. Place capillary tube tip in blood and fill gently
. Seal end which was in blood.
. Insert the metal mixing bar and seal opposite end.
. Mix with magnet, moving the bar 5 times before measurement
Storage and Transport:
If transportation is delayed,
glycolysis in the red blood cells:
1. produces lactic acid
2. shifts pH,bicarbonate and base excess to range of metabolic acidosis
Oxygen consumption in platelets and leucocytes:
causes a fall in Po2 and increase in pco2 the fall in po2 is accelerated if the original pO2 in sample is high; e.g
In a sample at room temperature for 30 minutes:
. pH decreases by 0.021 + or – 0.008
. PCO2 increases by 0.11 + or – 0.14 kPa (0.86 + or – l.02mm Hg)