Basic Blood Gas Interpretation

Basic Blood Gas Interpretation

Values Measured PaO2 – amount of oxygen in the arterial blood
SaO2 – percent saturation of the hemoglobin as measured by a CO-oximeter

Values Measured SpO2 – percent saturation of the hemoglobin as measured by a pulse oximeter Hb – amount of hemoglobin present

Values Measured Hct (Hematocrit) – percent of the blood that is composed of cells pH – concentration of hydrogen ions (H+) in the arterial blood

Values Measured PaCO2 – amount of carbon dioxide in the arterial blood
HCO3ˉ – amount of bicarbonate in the arterial blood

Values Measured B.E. (base excess/base deficit) – the total of all buffering systems in the arterial blood CO – amount of carbon monoxide present in the arterial blood

Determination of Oxygenation
Normal Values SaO2 93% – 97% 88-92% COPD/lung disease Hb Males: – 16.5 g/dl Females: 12 – 15 g/dl Hct Males: 42 – 54% Females: 38 – 47% PaO2 80 – 100 mmHg Mild hypoxemia 60 – 79 mmHg Moderate hypoxemia 40 – 59 mmHg Severe hypoxemia < 40 mmHg

Determination of Oxygenation
Normal Values CaO2 Males: – 21.7 ml/dl Females: – 19.7 ml/dl COHb <3%

Method of Determining Oxygenation
Evaluate the PaO2 80 – 100 mmHg: normal oxygenation > 100 mmHg: hyperoxygenation 60 – 79 mmHg: mild hypoxemia

Evaluate the PaO2 40 – 59 mmHg: moderate hypoxemia < 40 mmHg: severe hypoxemia

Evaluate the SaO2 > 93%: normal oxygenation < 93%: may be hypoxemic; examine the hemoglobin

Evaluate the Hb 12 – 16 g/dl: normal < 12 g/dl: anemic > 16 g/dl: polycythemic

Evaluate the CaO2 17 – 20 ml/dl: normal 15 – 17 ml/dl: mild hypoxia 12 – 14.9 ml/dl: moderate hypoxia < 12 ml/dl: severe hypoxia

Other factors in oxygenation Abnormal forms of hemoglobin: Detectable by CO-oximeter, not pulse oximeter

Estimate PaO2 Predicated PaO2 based on age= Estimated value of what there PaO2 might be PaO2= 110 – half the person’s age example: 20 year old. 110 – 10= PaO2 100 (PaO2 of 60 will equal approximately 90% saturation based on the oxyhemoglobin curve)

Determination of Acid Base Balance
pH is equal to the –log of the hydrogen ion pH = – Log [H+]

Henderson-Hasselbalch equation pH = pK + Log HCO3 H2CO3 pH = pK + Log HCO3 (Renal) Paco2 x0.03 (Lungs)

pH measures the blood’s acidity or alkalinity Must always be determined first when assessing acid- base balance

PaCO2 is our stimulus to breathe A high PaCO2 indicates that not enough carbon dioxide is being exhaled A low PaCO2 indicates that too much carbon dioxide is being eliminated

Evaluate base excess or bicarbonate (metabolic parameter) Is it acidic or alkaline mEq/L is normal > 26 Indicates metabolic alkalosis < 22 Indicates metabolic acidosis

Bicarbonate is the base or buffer that “neutralizes” hydrogen ions (HCO3) Bicarbonate is made in the red blood cells, liver, and kidney

When bicarbonate levels are elevated, an excess of alkalinity exists in the metabolic systems When bicarbonate levels are low, an excess of acidity exists in the metabolic systems

If the pH is not normal, identify whether it is acidic or alkaline

Identify whether the PaCO2 or bicarbonate disturbance (acidosis or alkalosis) matches The pH change (acidosis or alkalosis) This is the “cause” of the problem and represents where treatment should be directed. (e.g. if pH is acidic, PaCO2 is alkaline and bicarbonate is acidic, the problem is a metabolic acidosis)

If the pH is not 7.40 but within the normal range ( ), the disturbance is fully compensated If both the respiratory and metabolic parameters match the pH, the problem is a combined disturbance

A change in the opposite direction (acidosis or alkalosis) by the parameter (PaCO2 or bicarbonate) that does not match the pH is an attempt to restore the pH (referred to as partial compensation)

Evaluation of Compensation
pH Normal HCO3 Normal PaCO2 Normal Normal Acid-Base Balance PaCO2 Elevated HCO3 Elevated Fully Compensated Respiratory Acidemia or Fully Compensated Metabolic Alkalemia HCO3 Decreased PaCO2 Decreased Fully Compensated Respiratory Alkalemia of Fully Compensated Metabolic Acidemia

Arterial Punctures Indications1
Need to evaluate ventilation, acid-base balance, and oxygenation of blood Assess the patient’s response to therapy Monitor and assess the severity and progression of a disease process 1 Excerpt from the AARC Clinical Practice Guideline for Sampling for Arterial Blood Gas Analysis

Arterial Punctures Contraindications1 Negative Allen test
Presence of a surgical shunt proximal to the sample site 1 Excerpt from the AARC Clinical Practice Guideline for Sampling for Arterial Blood Gas Analysis

Dialysis shunt

Arterial Punctures Contraindications1
Presence of a lesion at the sample site Coagulopathy or medium to high dose anticoagulation therapy (relative contraindication) 1 Excerpt from the AARC Clinical Practice Guideline for Sampling for Arterial Blood Gas Analysis

Hematoma

Arterial Punctures Hazards and complications1 Arteriospasm
Air or clotted blood emboli Hematoma 1 Excerpt from the AARC Clinical Practice Guideline for Sampling for Arterial Blood Gas Analysis

Arterial Punctures Hazards and complications1 Hemorrhage Pain
Arterial occlusion 1 Excerpt from the AARC Clinical Practice Guideline for Sampling for Arterial Blood Gas Analysis

Arterial Punctures Hazards and complications1 Trauma to the vessel
Vasovagal response Patient or sample contamination Anaphylaxis (if local anesthetic used, Xylocain) 1 Excerpt from the AARC Clinical Practice Guideline for Sampling for Arterial Blood Gas Analysis

Arterial Punctures Assessment of need for arterial sample1
Initiation, change, or discontinuation of therapy (oxygen or ventilatory support) History and physical indicators 1 Excerpt from the AARC Clinical Practice Guideline for Sampling for Arterial Blood Gas Analysis

Arterial Punctures Assessment of need for arterial sample1
Presence of other abnormal diagnostic tests or findings Baseline study for pulmonary rehabilitation program 1 Excerpt from the AARC Clinical Practice Guideline for Sampling for Arterial Blood Gas Analysis

Arterial Punctures Frequency of monitoring1
Dependent upon clinical status of the patient and presence of indications If frequent monitoring required, use alternating sites or indwelling arterial catheter 1 Excerpt from the AARC Clinical Practice Guideline for Sampling for Arterial Blood Gas Analysis

Arterial Punctures Allen Test
Performed to determine presence of adequate collateral circulation in the hand Cannot be used with uncooperative or unconscious patients

Arterial Punctures Allen Test Procedure
Have patient clench hand into a tight fist Apply pressure to occlude flow through the radial and ulnar arteries Open hand; observe to ensure that the palm and fingers are blanched

Arterial Punctures Allen Test Procedure
Remove pressure from the ulnar artery Observe time necessary for flushing of hand Test is negative for collateral circulation if flushing does not occur within 20 seconds; an alternative site is chosen

The Modified Allen Test
The modified Allen test. A, The hand is clenched into a tight fist and pressure is applied to the radial and ulnar arteries. B, The hand is opened (but not fully extended); the palm and fingers are blanched. C, Removal of pressure on the ulnar artery should result in flushing of the entire hand.

Which Artery to Choose? The radial artery is superficial, has collaterals and is easily compressed. It should almost always be the first choice. Other arteries (femoral, dorsalis pedis, brachial) can be used in emergencies.

Preparing to perform the Procedure:
Make sure you and the patient are comfortable. Assess the patency of the radial and ulnar arteries.

Procedure For Obtaining an Arterial Sample (Radius)
Confirm the order in the patient’s chart Note any contraindications and notify physician if any exist

Ensure that patient is in a steady state (no changes in oxygen status for at least twenty minutes) Obtain and assemble necessary equipment

Wash hands, don protective equipment, explain the procedure to the patient Position the patient correctly

Perform an Allen test and confirm collateral circulation Cleanse the site with 70% isopropyl alcohol or other antiseptic

Inject local anesthetic, if hospital protocol Heparinize the syringe, if not already heparinized

Palpate and secure the artery Insert the needle, bevel up, at a 45° angle through the skin until blood pulsates into the syringe

Withdraw the needle when sufficient sample is obtained Apply firm pressure to the puncture site using a gauze pad

Maintain pressure for a minimum of five minutes, longer if patient is on anticoagulant therapy Expel any air bubbles from the syringe

Mix the sample and label it Place the sample in an icy slush (only if a delay in running the sample is expected)

Dispose of any waste material in the appropriate container Document the procedure

Arterial Puncture

The Kit

Air bubbles Gas equilibration between ambient air (pO2 ~ 150, pCO2~0) and arterial blood. pO2 will begin to rise, pCO2 will fall Effect is a function of duration of exposure and surface area of air bubble. Effect is amplified by pneumatic tube transport.

Transport After specimen collected and air bubble removed, gently mix and invert syringe. Because the wbcs are metabolically active, they will consume oxygen. Plastic syringes are gas permeable. Key: Minimize time from sample acquisition to analysis.

Transport Placing the AGB on ice may help minimize changes, depending on the type of syringe, pO2 and white blood cell count. Its probably not as important if the specimen is delivered immediately.

Pre-Analysis Errors Presence of air in the sample
Recognized by presence of bubbles or froth, inconsistent PaCO2 Allows diffusion of CO2 into the air, lowering the PaCO2 As the CO2 diffuses, pH is raised

Pre-Analysis Errors Presence of air in the sample
In low PaO2 states, O2 diffuses into the blood, raising the PaO2 In high PaO2 states, O2 diffuses out of the blood, lowering the PaO2

Pre-Analysis Errors Venous admixture
Recognized by failure of syringe to fill by pulsations, inconsistent PaO2 Higher PaCO2 than expected

Pre-Analysis Errors Venous admixture Lower pH than expected
Lower PaO2 than expected (may be significantly lower)

Pre-Analysis Errors Metabolic effects
Caused by excessive time lag between sampling and analysis or improper storage of sample Increase in PaCO2 as cellular metabolism continues Decrease in pH secondary to increase in PaCO2 Decrease in PaO2 as cells use up oxygen

Pre-Analysis Errors Excessive anticoagulant in syringe
Caused by allowing excessive heparin to remain in syringe (dead space only should have heparin) Lowers PaCO2 Raises pH

Pre-Analysis Errors Excessive anticoagulant in syringe Raises low PaO2
Lowers high PaO2

Capillary Sampling Used as an alternative to arterial sampling in infants and small children. Sample may approximate pH and PaCO2 values; PaO2 value is generally inaccurate

Capillary Sampling Indications2
ABG analysis is indicated, but access is not possible Assessment of initiation, administration, or change in therapy is indicated Change in patient status is detected 2 Excerpt from the AARC Clinical Practice Guideline for Capillary Blood Gas Sampling for Neonatal and Pediatric Patients

Capillary Sampling Indications2
Monitoring severity or progression of disease is desirable Noninvasive monitor readings are abnormal 2 Excerpt from the AARC Clinical Practice Guideline for Capillary Blood Gas Sampling for Neonatal and Pediatric Patients

Capillary Sampling Contraindications2
Capillary punctures should not be performed at or through: The posterior curvature of the heel (Bone Damage) Fingers of neonates (nerve damage) 2 Excerpt from the AARC Clinical Practice Guideline for Capillary Blood Gas Sampling for Neonatal and Pediatric Patients

Capillary punctures should not be performed at or through: Heel of an infant who has begun walking Previous puncture sites 2 Excerpt from the AARC Clinical Practice Guideline for Capillary Blood Gas Sampling for Neonatal and Pediatric Patients

Capillary punctures should not be performed at or through: Inflamed, swollen, or edematous tissue Localized areas of infection Cyanotic or poorly perfused areas 2 Excerpt from the AARC Clinical Practice Guideline for Capillary Blood Gas Sampling for Neonatal and Pediatric Patients

Capillary Sampling Contraindications2 Contraindicated:
In patients less than twenty-four hours of age When there is a need for direct analysis of oxygenation When there is a need for direct analysis of arterial blood 2 Excerpt from the AARC Clinical Practice Guideline for Capillary Blood Gas Sampling for Neonatal and Pediatric Patients

Capillary Sampling Contraindications2 Relatively contraindicated:
When peripheral vasoconstriction is present In the hypotensive patient In polycythemia 2 Excerpt from the AARC Clinical Practice Guideline for Capillary Blood Gas Sampling for Neonatal and Pediatric Patients

Capillary Sampling Precautions and/or complications2
Contamination and infection of the patient Inappropriate management through use of capillary rather than arterial samples 2 Excerpt from the AARC Clinical Practice Guideline for Capillary Blood Gas Sampling for Neonatal and Pediatric Patients

Capillary Sampling Precautions and/or complications2 Burns
Bone calcification Bruising 2 Excerpt from the AARC Clinical Practice Guideline for Capillary Blood Gas Sampling for Neonatal and Pediatric Patients

Capillary Sampling Precautions and/or complications2 Pain Hematoma
Nerve damage 2 Excerpt from the AARC Clinical Practice Guideline for Capillary Blood Gas Sampling for Neonatal and Pediatric Patients

Capillary Sampling Precautions and/or complications2 Scarring Bleeding
Tibial artery laceration 2 Excerpt from the AARC Clinical Practice Guideline for Capillary Blood Gas Sampling for Neonatal and Pediatric Patients

Capillary Sampling Assessment of need2
Should be performed only when a documented need exists and arterial access is unavailable or contraindicated 2 Excerpt from the AARC Clinical Practice Guideline for Capillary Blood Gas Sampling for Neonatal and Pediatric Patients

Capillary Sampling Assessment of need2
When initiation, administration, or change in therapy occurs When noninvasive monitoring and assessment indicates a change in condition has occurred 2 Excerpt from the AARC Clinical Practice Guideline for Capillary Blood Gas Sampling for Neonatal and Pediatric Patients

Procedure for Obtaining a Capillary Sample
Verify the order and the need for a capillary sample Note any complications and notify the physician if any exist

Ensure that patient is in a steady state (no changes in oxygen status for at least twenty minutes) Wash hands and don protective equipment

Select the site Warm the site up to 42° C For ten minutes using a compress, heat lamp or commercial hot pack

Cleanse the skin with an antiseptic solution Puncture the skin (less than 2.5 mm) with a lancet

Wipe away the first drop of blood and observe free flow (do not squeeze) Fill the sample tube from the middle of the drop until it is full

Place the flea in the tube and seal the ends Tape sterile cotton or a bandage over the puncture site

Mix the sample by moving the magnet back and forth along the tube Place the sample in an icy slush

Dispose of waste materials properly Document the procedure

Capillary Tubes

Neonatal Puncture Site

Capillary Sampling Finger Stick Heel Stick

Capillary Blood Gas

Cord Gas

Arterial Lines

Indications The arterial line with transducers is usually used to obtain accurate blood pressure readings every few seconds. This is especially important in monitoring the hemodynamic status of a critical patient. With an arterial line, the immediate effects of medication can be seen. Both systolic, diastolic and mean pressures can be monitored immediately. This is especially important when pressors such as Nipride, dopamine or Levophed are being used. Another advantage of using an arterial line is that frequent blood samples can be obtained.

A-line monitoring This system consists of
arterial line connected by saline filled non-compressible tubing to apressure transducer. This converts the pressure waveform into an electrical signal which is displayed on the bedside monitor pressurized saline for flushing

A-Line Monitoring Sources of error
failure of any one of the components in system transducer position pressure displayed is pressure relative to position of transducer in order to reflect blood pressure accurately transducer should be at level of heart. Over-reading will occur if transducer too low and under-reading if transducer too high transducer must be zeroed to atmospheric pressure damping. Important to have appropriate amount of damping in the system. Inadequate damping will result in excessive resonance in the system and an overestimate of systolic pressure and an underestimate of diastolic pressure. The opposite occurs with overdamping. In both cases the mean arterial pressure is the most accurate. An underdamped trace is often characterized by a high initial spike in the waveform.

A-line Monitoring

A-line Complications distal ischemia arterial thrombosis embolism.
infection hemorrhage disconnection around line accidental drug injection damage to artery eg aneurysm

Drawing Blood from an A-line

A-line Placement 1. Prepare a 500 ml bag of normal saline. Most institutions no longer use a heparinized bag. Spike the bag with the transducer administration set. Remove all air from the tubing and transducer set. Pay particular attention to the transducer part of the Tubing and the flush port. The smallest air bubble must be removed to insure transducer accuracy. The easiest way to do this is to pressurize the bag up to 300 mm Hg, then invert the bag, and fast flush it to remove all air from the bag. 2. Pressurize the pressure bag to 300 mm Hg. The purpose of this is to provide backpressure to prevent blood from contaminating the transducer. 3. With the transducer connected to the monitor, select arterial monitor, and perform a transducer check by fast flushing the line. As you do this, you should see a change in the waveform. This is called a square wave test.

A-line Placement 4. Zero the transducer and monitor by placing the transducer at the phlebostatic axis of the patient. Close the line off to patient and open to air. Press zero on the monitor. To monitor pressure, close the port off to an air and open to patient. 5. At this point the patient catheter is ready to be connected. Connect the catheter and fast flush to clear the catheter of blood. 6. You should now see an arterial waveform on the monitor with arterial blood pressure and mean should be on the monitor screen. Check for good waveform

MORE ABG PRACTICE

Can You Interpret this Blood Gas?
Blood Gas Drawn on Room Air pH 7.39 PaCO2 42 mmHg HCO3 23 mEq/L PaO2 97 mmHg

Blood Gas Drawn on Room Air pH 7.39 PaCO2 42 mmHg HCO3 23 mEq/L PaO2 97 mmHg Normal acid-base balance with normal oxygenation

Blood Gas Drawn on Room Air pH 7.32 PaCO2 49 mmHg HCO3 26 mEq/L PaO2 60mmHg

Blood Gas Drawn on Room Air pH 7.32 PaCO2 49 mmHg HCO3 26 mEq/L PaO2 60mmHg Uncompensated or acute respiratory acidemia with mild hypoxemia

Blood Gas Drawn on Room Air pH 7.31 PaCO2 60 mmHg HCO3 29 mEq/L PaO2 58 mmHg Partially compensated respiratory acidemia with moderate hypoxemia

Blood Gas Drawn on Room Air pH 7.50 PaCO2 60 mmHg HCO3 19 mEq/L PaO2 60 mmHg Lab error

Blood Gas Drawn on Room Air pH 7.35 PaCO2 63 mmHg HCO3 33 mEq/L PaO2 60 mmHg Compensated respiratory acidemia with mild hypoxemia

Blood Gas Drawn on Room Air pH 7.28 PaCO2 28 mmHg HCO3 8 mEq/L PaO2 104 mmHg Partially compensated metabolic acidemia with hyperoxygenation

Blood Gas Drawn on Room Air pH 7.36 PaCO2 24 mmHg HCO3 18 mEq/L PaO2 109 mmHg Fully compensated metabolic acidemia with hyperoxygenation

Blood Gas Drawn on Room Air pH 7.45 PaCO2 48 mmHg HCO3 33 mEq/L PaO2 79 mmHg Fully compensated metabolic alkalemia with mild hypoxemia

Blood Gas Drawn on Room Air pH 7.30 PaCO2 30 mmHg HCO3 35 mEq/L PaO2 81 mmHg Lab error

Blood Gas Drawn on Room Air pH 7.49 PaCO2 47 mmHg HCO3 35 mEq/L PaO2 81 mmHg Partially compensated metabolic alkalemia with normal oxygenation

Blood Gas Drawn on Room Air pH 7.24 PaCO2 77 mmHg HCO3 7 mEq/L PaO2 28 mmHg Combined acidemia with severe hypoxemia

Blood Gas Drawn on Room Air pH 7.36 PaCO2 45 mmHg HCO3 25 mEq/L PaO2 108 mmHg Lab error ( PaCO2 and PaO2 cannot total more than 140 on room air)

Basic Blood Gas Interpretation

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