Central Venous Bicarbonate Versus Arterial Bicarbonate
Since bicarbonate generated during blood gases is calculated from pH and pCO2 , it would be expected that if central venous pH and pCO2 are clinically acceptable substitutes for arterial pH and pCO2,then central venous HCO3-, too, would be an acceptable substitute for arterial HCO3-.
This is borne out by the results of the four studies that compared central venous and arterial HCO3-. All studies indicate that mean central venous HCO3- concentration is slightly higher than mean arterial HCO3- concentration.
The magnitude of this negative bias ranged from 0.52 mmol/L in one study to 2.2 mmol/L in another . Of the four studies, three returned negative bias of < 1.2 mmol/L, which is clinically insignificant. Two studies provided 95 % LOA data.
The study showing best level of agreement with 95 % LOA 2.85 to +1.85 indicate that if measured central venous HCO3- is 25 mmol/L, then in 95 % of patients predicted arterial HCO3- would be in the range of 22 to 27 mmol/L with most close to 26 mmol/L.
There is general agreement that central venous bicarbonate is a clinically acceptable substitute for arterial bicarbonate, especially if the small systematic positive bias of ~1mmol/L is taken into account.
TABLE IV: Arterial versus central venous HCO3-
Peripheral Venous Blood Central Venous Blood And Mixed Venous Blood
Many clinical studies investigating the validity of using venous blood for BGA have been conducted using venous blood obtained by conventional venepuncture of a peripheral vein .
This article is concerned only with studies that have utilized central venous blood samples for comparison with arterial blood.
Central venous blood is the blood that is sampled via a central venous catheter . In addition to facilitating the means for easy sampling of venous blood for diagnostic testing, CVCs allow continuous monitoring of central venous pressure , and vascular access for administration of drugs, blood transfusion and other fluids.
Most patients in intensive care have an indwelling CVC, but CVC use is not confined to this patient population so these studies have relevance outside the intensive care unit, in emergency rooms, recovery rooms and some medical wards.
CVCs are usually inserted cutaneously via the jugular vein in the neck or subclavian vein in the upper chest to the superior vena cava, with the tip sited close to the point where the superior vena cava opens to the right atrium of the heart , so that the blood sampled is the mixed venous blood from the upper half of the body.
The inferior vena cava conveys mixed venous blood from the lower half of the body to the right atrium. Central venous blood is thus not truly mixed venous blood because it does not include that returning via the inferior vena cava.
Blood Gas Analysis And Pulse Oximetry
Arterial blood gas analysis is the most objective method for assessing and monitoring the physiologic effects of thoracic trauma . Clinical data in dogs reveal a high incidence of hypoxemia however, it is usually mild to moderate.6,7 This may be because many of the most severe cases will die before arriving at the veterinary clinic. Either hypocarbia or hypercarbia may be seen, depending on the severity of the parenchymal injury, the nature of concurrent thoracic injuries, and other factors such as pain, distress, and the effect of concurrent metabolic acid-base derangements.
In humans, the arterial oxygen tension/fractional concentration of inspired oxygen ratio is directly correlated with the volume of contused lung for the first 24 hours after injury, although this correlation is not consistent beyond 1 week.32 Whether this association exists in small animal patients is unknown.
Although pulse oximetry has some limitations, it may be a useful quantitative assessment of oxygenation in cases in which an arterial blood gas analysis is not possible . It is a less accurate indicator of impaired oxygenation, does not provide a measure of ventilation, and reliable measurements can be difficult to obtain in patients that are in shock. A pulse oximeter reading of less than 95% indicates hypoxemia and values less than 90% are consistent with severe hypoxemia.
In , 2016
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Alkalosis: Blood Ph Is More Than 745
Respiratory Alkalosis: In this condition, due to increased respiration, blood pH increases with reduced levels of carbon dioxide in the blood. It occurs in conditions such as stress, high altitudes, fever, hyperventilation, liver diseases and vocal cord paralysis.
Metabolic Alkalosis: This condition occurs when there is an increase in bicarbonate levels in the blood. It is seen in cases of vomiting and dehydration, from diuretic therapy and so on.
Medically Reviewed By
Dr. Himanshi is a Homoeopathic consultant and currently working as a lecturer in Post-graduate faculty of Homeopathy, Parul University, Vadodara. Completed BHMS and MD in Homeopathy in January 2018 and also has a clinical experience of about 6 years. Personal interests include reading, spending time with family and traveling.
Goals Of Arterial Blood Gas Analysis
For the purpose of this guide, we have set three goals that we need to accomplish when interpreting arterial blood gases. The goals are as follows:
We need to keep these goals in mind as theyll come up later in the steps for the ABG interpretation technique.
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Normal Values Of Arterial Blood Gas
The result of an Arterial Blood Gas Test shows values for pH, Oxygen, Carbon dioxide, Bicarbonates, Lactic acid levels and oxygen saturation. Some ABG results also show hemoglobin and serum electrolyte values.
Why The Test Is Performed
The test is used to evaluate respiratory diseases and conditions that affect the lungs. It helps determine the effectiveness of oxygen therapy or non-invasive ventilation . The test also provides information about the body’s acid/base balance, which can reveal important clues about lung and kidney function and the body’s general metabolic state.
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Why Is A Blood Gas Test Done
A blood gas test provides a precise measurement of the oxygen and carbon dioxide levels in your body. This can help your doctor determine how well your lungs and kidneys are working.
This is a test that is most commonly used in the hospital setting to determine the management of acutely ill patients. It doesnt have a very significant role in the primary care setting, but may be used in a pulmonary function lab or clinic.
Your doctor may order a blood gas test if youre showing symptoms of an oxygen, carbon dioxide, or pH imbalance. The symptoms can include:
- shortness of breath
These symptoms may be signs of certain medical conditions, including asthma and chronic obstructive pulmonary disease .
Your doctor may also order a blood gas test if they suspect youre experiencing any of the following conditions:
- metabolic disease
- head or neck injuries that affect breathing
Identifying imbalances in your pH and blood gas levels can also help your doctor monitor treatment for certain conditions, such as lung and kidney diseases.
Interpreting Arterial Blood Gas Imbalances
Interpreting arterial blood gases is used to detect respiratory acidosis or alkalosis, or metabolic acidosis or alkalosis during an acute illness. To determine the type of arterial blood gas the key components are checked. The best way of interpreting arterial blood gas is by using the tic-tac-toe method below:
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S For Performing A Modified Allen Test:
1. Have the Patient Make a Fist
Instruct the patient to clench their fist in order to enhance the circulation within the arteries. If the patient cannot do so, you can close their hand manually.
2. Locate the Radial and Ulnar Arteries
Face the patient and locate the radial and ulnar arteries. The radial artery is located on the thumb side of the wrist on the underside of the forearm. The ulnar artery is on the pinky side of the wrist. Make sure to locate both the radial and ulnar pulses.
3. Grab the Patients Hand
Using your right hand, slowly grab your patients left hand. Depending on your preference, you can also use your left hand to grab your patients right hand.
4. Locate the Pulse
Place your middle finger on top of the radial pulse and your pointer finger on the ulnar pulse of the patient.
5. Apply Pressure to Both Arteries
When the pulses can be felt, apply occlusive pressure to both the ulnar and radial arteries to temporarily stop blood circulation of the hand. Be sure to tell the patient to relax their hand while performing this step.
6. Have the Patient Open Their Hand
This is done to check whether the palm and fingers have blanched. Blanching means that you have completely occluded the radial and ulnar arteries with your fingers. The hand should now have a white-ish appearance in color.
7. Slowly Release the Pressure on the Ulnar Artery
This indicates that collateral circulation is present, and you may proceed to stick the radial artery of that hand.
Adjudication Of The Final Diagnosis
After patient discharge from the hospital, the final diagnosis was independently adjudicated in a blinded fashion by two internal medicine specialists who were not involved in the care of the patients during the hospitalization according to current recommendations on the basis of available medical records, including B-type natriuretic peptide levels, the results of all diagnostic investigations, the response to treatment, and autopsy data in those patients who died . The physicians adjudicated the final diagnosis by choosing one or more diagnoses from a pre-specified list that included the following items: AHF, exacerbated COPD/asthma, CAP/bronchitis, PE, hyperventilation from anxiety disorder , other, or unknown. If more than one cause for acute dyspnea was identified, the leading disorder responsible for the current episode of acute dyspnea was determined. When there was disagreement about the final diagnosis, cases were reviewed and adjudicated in conjunction with a third internal medicine specialist who was considered an expert in the field.
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What Are The Contraindications For Sticking An Abg
Not all patients are potential candidates for an ABG test. Here are the contraindications for when you would not want to stick an arterial blood gas:
- An abnormal Modified Allen Test
- Blood clotting problems
- Local infection or damage at the injection site
- Patients who are receiving anticoagulation therapy
- Patients who are taking thrombolytic agents
- The presence of a disease that affects the blood vessels
- The presence of arteriovenous fistulas or vascular grafts
Determine If Its Compensated Or Uncompensated
After identifying whether the blood gas is Acidosis or Alkalosis and whether its a Respiratory or Metabolic issue, we must now observe the compensatory component of the ABG results.
Here are two things that you should remember:
- When there is a Respiratory Problem , our body will compensate with Bicarbonate.
- When there is a Metabolic Problem , our body will compensate with Carbon Dioxide.
For example, when we have Respiratory Acidosis the body will try to compensate by increasing the amount of Bicarb in our system.
Bicarbonate is a base, so one of its functions is to neutralize the acid that is causing the problem. When we have Respiratory Alkalosis, it will to do the opposite by decreasing the amount of Bicarb.
To conclude that there is compensation, the increase or decrease in HCO3- has to go outside the normal range. In other words, it has to be lower than 22 or higher than 26.
If the Bicarb value is still within normal limits, you can conclude that there is no compensation.
As we have already learned using the previous steps, we can conclude that the pH is Acidosis because it is less than 7.35.
Now we need to identify if there is a Respiratory or Metabolic problem. The PaCO2 is increased above the normal range, which indicates that there is a Respiratory issue.
Interpretation: Respiratory Acidosis
Next, we need to look at the Bicarb to determine if its compensated or uncompensated.
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Are There Any Risks To An Arterial Blood Gas Test
Theres little risk associated with getting an arterial blood gas test when the respiratory therapist performs the procedure correctly. Arteries vary in size from one person to another and from one side of your body to the other, so taking blood from some people may be more difficult than from others.
Risks associated with having an arterial blood gas blood draw are rare, but may include:
- Fainting or feeling lightheaded.
Patients In Severe Circulatory Failure
The studies discussed thus far have confirmed that the normal arterio-venous difference for pH and pCO2 are maintained within broadly clinically acceptable limits for the generality of patients requiring BGA.
That is not the case for patients with severe circulatory failure . Adrouge et al found much larger A-V differences in this small subset of very critically ill patients.
His study revealed that mean difference between arterial pH and central venous pH ranged from 0.10 to 0.35 pH units depending on the severity of the circulatory failure, rather than ~0.03 pH units.
Mean difference between arterial pCO2 and central venous pCO2 for the same group ranged from 3.2 to 7.4 kPa, rather than 0.6kPa. According to the authors of this report assessment of acid-base status in these patients requires consideration of both arterial and central venous blood gas results.
Two further studies confirm the much larger difference between arterial and central venous pH and pCO2 for patients in circulatory collapse.
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Diagnostic Accuracy Of Arterial Blood Gas Parameters
Values of ABGA parameters at presentation varied widely in all underlying disorders . Acidosis was found in 88 patients at presentation. Of those, 9 had metabolic acidosis, 66 respiratory acidosis, and 13 mixed-type acidosis. In 142 patients alkalosis was present at presentation. Six patients showed metabolic, 108 respiratory, and 65 mixed-type alkalosis. Hypoxemia was found in 377 patients .
Values of arterial blood gas parameters at presentation. Boxplots of pH , partial pressure of arterial carbon dioxide , partial pressure of arterial oxygen , and partial pressure of bicarbonate at presentation in patients with acute heart failue , excerbated chronic obstructive pulmonary disease or asthma , community-acquired pneumonia , pulmonary embolism , hyperventilation from anxiety , and other disorders.
Arterial Blood Gas Analysis
ABG analysis is useful for determining whether a suspected cardiac abnormality has led to significant right-to-left shunting of blood. Right-to-left shunting allows deoxygenated venous blood to enter the aorta without passing through the lungs, reducing the partial pressure of oxygen below reference values. Affected animals have a decreased activity level or tolerance for exercise. To differentiate shunting from other causes of hypoxemia, presupplemental and postsupplemental oxygen concentrations may be compared shunts prevent an increase in PaO2, whereas oxygenation increases with most other causes of hypoxemia. Serial ABG analyses may be used to track failure, as some animals with left-to-right shunts or a mixture of oxygenated and deoxygenated blood entering the aorta will have a worsening of hypoxemia as left heart failure progresses.
ABG analysis is also a useful diagnostic test in poor-doing animals, particularly neonates and juveniles. Some of these animals will have a covert cardiac defect or acquired valvular disease that does not cause a prominent murmur. Finding hypoxemia, particularly when it does not respond to supplemental oxygen, may be the first clue to direct the diagnostic efforts toward the heart.
Techniques for obtaining arterial samples are described in Chapter 37.
R Eddie Clutton, in, 2007
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Interpreting A Blood Gas Result
The automated analysers measure the pH and the partial pressures of oxygen and carbon dioxide in arterial blood. Bicarbonate is also calculated . These measurements should be considered with the patient’s clinical features .
HCO3 + H+
For any disturbance of gas tensions in arterial blood, a compensatory system exists to maintain homeostasis. In a metabolic disorder, where HCO3 may be retained or excreted by the kidneys, respiratory compensation can occur almost immediately to alter the rate and depth of ventilation to retain or remove CO2. This occurs due to the exquisite sensitivity of chemoreceptors in the medulla to carbonic acid or H+. Renal compensation in response to a respiratory disorder takes much longer, sometimes between three and five days, to retain or remove HCO3 as required.
As a general rule, when compensation is present the arterial blood gas result shows two imbalances derangement of both HCO3 and PaCO2. A clue to which imbalance is the primary disturbance is obtained from the pH. If pH is leaning toward acidosis or alkalosis, then the parameter that matches the pH trend is the primary problem and the other is due to compensation.
Acidosis: Blood Ph Becomes Less Than 735
Metabolic Acidosis: A condition where there is excess acid in the bloodstream. This condition occurs when the body is unable to flush out toxins from the body.
Commonly occurs in diabetes, lactic acidosis, renal failure and intoxication of organic acid, isoniazid, ethanol, sulfates and metformin.
Respiratory Acidosis: It is considered to be a medical emergency where, in cases of decreased ventilation, blood pH decreases and carbon dioxide concentration increases. Respiratory acidosis may be acute or chronic.
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Why Is An Abg Done
ABGs are drawn for a variety of reasons. These may include concerns for:
- Registered Nurse
- Respiratory Therapist
During the blood draw, the healthcare provider will locate the radial artery in either the right or left arm. The radial artery is the preferred artery of choice because it is easier to locate and is superficial. Then a small needle is inserted into the artery and blood is withdrawn. Approximately 1 ml is required. Once the blood is taken, it is sent to a specialized laboratory to determine the values.
Alternative sites for drawing an ABG include the brachial or femoral artery however, these do have disadvantages because they are,
- Harder to locate