The usefulness of this diagnostic tool depends on the ability to interpret the results correctly.
Arterial blood gas (ABG) analysis is essential for diagnosing and managing a patient’s oxygenation status and acid-base balance.
Acid-base balance disorders can create complications in many disease states, and sometimes the abnormality can be so severe that it becomes a life-threatening risk factor.
A thorough understanding of acid-base balance is mandatory for any physician and intensivist, and the anesthesiologist is no exception.
The three widely used approaches to acid-base physiology are HCO3- (in the context of pCO2), excess standard base (EBS), and vital ion difference (SID).
More than 20 years have passed since Stewart’s concept of big ionic difference was introduced, defined as the absolute difference between fully dissociated anions and cations.
According to electrical neutrality, this difference is balanced by weak acids and carbon dioxide.
Vital ion difference is defined as weak acids, and carbon dioxide has been re-designated as substantial ion difference (SIDe) identical to “buffer base.”
Similarly, Stewart’s original term for total weak acid concentration (ATOT) is now defined as dissociated anions (A-) plus undissociated soft acid forms (AH). In English).
This is known as an anion gap (AG) when the average concentration is caused by A-.
Therefore, all three methods produce nearly identical results when used to quantify the acid-base status of a given blood sample.
The arteries are the large vessels that carry oxygenated blood from the heart.
The distribution of the systemic arteries is like a branched tree, whose common trunk, formed by the aorta, begins in the left ventricle, while the smaller branches extend to the peripheral parts of the body and the contained organs.
Arterial blood gas sampling by direct vascular puncture is a procedure that is often practiced in the hospital setting.
The relatively low incidence of significant complications, its ability to be performed at the bedside, and its rapid analysis make it an essential tool.
It is used by physicians to direct and redirect the treatment of their patients, incredibly critically ill patients.
Arterial blood gas sampling provides valuable information about acid-base balance at a specific point in a patient’s disease.
It is the only reliable determination of ventilation success as evidenced by carbon dioxide content.
It is a more accurate measure of successful gas exchange and oxygenation. The arterial blood gas sample is the only way to determine the alveolar-arterial oxygen gradient accurately.
Because arterial blood gas sampling results only reflect the physiological status of the patient at the time of sampling, they must be carefully correlated with the evolving clinical scenario and any changes in the patient’s treatment.
Sampling and analysis
Arterial blood for blood gas analysis is usually drawn by a respiratory therapist and sometimes by a phlebotomist, nurse, paramedic, or physician.
Blood is most commonly drawn from the radial artery because it is easily accessible, can be compressed to control bleeding, and has less risk of occlusion.
The selection of the radial artery from which it is based on the result of an Allen test.
The brachial artery (or, less commonly, the femoral artery) is also used, especially in emergencies or for children. Blood can also be drawn from an arterial catheter already placed in one of these arteries.
There are plastic and glass syringes used for blood gas sampling. Most needles are prepackaged and contain a small amount of heparin to prevent clotting.
Other syringes may need to be heparinized, drawing a small amount of liquid heparin and squirting it again to remove air bubbles.
Once the sample is obtained, care must be taken to remove visible gas bubbles, as these bubbles can dissolve in the selection and produce inaccurate results.
The sealed syringe is taken to a blood gas analyzer.
If a plastic blood gas syringe is used, the sample must be transported, kept at room temperature, and analyzed within 30 minutes.
If extended delays (i.e., more than 30 minutes) are expected before analysis, the sample should be drawn into a glass syringe and immediately placed on ice.
Standard blood tests can also be performed on arterial blood, such as measuring glucose, lactate, hemoglobins, dishemoglobins, bilirubin, and electrolytes.
Derived parameters include bicarbonate concentration, arterial oxygen saturation (SaO2), and base excess.
The bicarbonate concentration is calculated from the measured pH and the partial pressure of carbon dioxide (PCO2) using the Henderson-Hasselbalch equation.
Arterial Oxygen Saturation is derived from measured PO2 and is calculated based on the assumption that all measured hemoglobin is normal hemoglobin (oxy or deoxy).
The machine used for the analysis draws this blood from the syringe and measures the pH and partial pressures of oxygen and carbon dioxide.
The bicarbonate concentration is also calculated. These results are usually available for interpretation within five minutes.
Two methods have been used in medicine to manage blood gases in hypothermic patients: the pH-stat method and the alpha-stat method. Recent studies suggest that the α-stat process is superior.
- pH-stat – pH and other ABG results are measured at actual patient temperature.
The goal is to maintain a pH of 7.40 and blood pressure of carbon dioxide (paCO2) at 5.3 kPa (40 mmHg) at the patient’s actual temperature. It is necessary to add CO2 to the oxygenator to achieve this goal.
- α-stat (alpha-stat): pH and other ABG results are measured at 37 ° C, regardless of patient temperature.
The goal is to maintain the arterial carbon dioxide pressure at 5.3 kPa (40mmHg) and the pH at 7.40 when measured at +37 ° C.
Both the pH-stat and alpha-stat strategies have theoretical drawbacks. The α-stat method is the method of choice for optimal myocardial function.
The pH-stat method can result in the loss of self-regulation in the brain (coupling cerebral blood flow with the metabolic rate in the brain).
By increasing cerebral blood flow beyond metabolic requirements, the pH-stat method can lead to brain microembolization and intracranial hypertension.
A 1 mmHg change in carbon dioxide blood pressure above or below 40 mmHg results in a 0.008 unit change in pH in the opposite direction.
Carbon dioxide blood pressure will decrease by approximately one mmHg for every one mEq / L reduction in [HCO – 3] below 24 mEq / LA, the change in [HCO – 3] of 10 mEq / L will result in a change in pH of about 0.15 pH units in the same direction.
Evaluate the relationship of the partial pressure of carbon dioxide to pH if the partial pressure of carbon dioxide and pH move in opposite directions, that is, the partial pressure of carbon dioxide ↑ when the pH is <7.4 or the partial pressure of carbon dioxide ↓ when pH> 7.4, It is a first-order respiratory.
If the partial pressure of carbon dioxide and pH move in the same direction, that is, the partial pressure of carbon dioxide when pH is> 7.4 or the partial pressure of carbon dioxide when pH is <7.4, it is a primary metabolic disorder.
The normal range for pH is 7.35–7.45. As the pH decreases (<7.35), it implies acidosis, while if the pH increases (> 7.45), it implies alkalosis. In the context of arterial blood gases, the most common occurrence will be respiratory acidosis.
Carbon dioxide dissolves in the blood as carbonic acid, a weak acid; however, it can drastically affect pH in large concentrations.
Whenever there is poor lung ventilation, carbon dioxide levels in the blood are expected to rise.
This leads to an increase in carbonic acid, which leads to a decrease in pH. The first pH buffer will be the plasma proteins, as they can accept some H + ions to maintain homeostasis.
As carbon dioxide concentrations continue to rise (oxygen partial pressure> 45 mmHg), a condition known as respiratory acidosis develops.
The body tries to maintain homeostasis by increasing the respiratory rate, a condition known as tachypnea.
This allows much more carbon dioxide to escape from the body through the lungs, thus increasing the pH by having less carbonic acid.
If a person is in a critical environment and intubated, one must increase the number of breaths mechanically.
Respiratory alkalosis (partial pressure of oxygen <35 mmHg) occurs when too little carbon dioxide is in the blood.
This may be due to hyperventilation or excessive breaths delivered through a mechanical ventilator in a critical care setting.
The action to be taken is to calm the person and reduce the number of breaths taken to normalize the pH.
The airway tries to compensate for the change in pH in 2 to 4 hours. If this is not enough, the metabolic pathway takes place.
Under normal conditions, the Henderson-Hasselbalch equation will give the pH of the blood.
- pH = 6.1 + log10 (HCO3)/0.03 x PaCO2
- At average body temperature, one is the acid dissociation constant (pKa) of carbonic acid (H2CO3).
- Bicarbonate (HCO3): is the concentration of bicarbonate in the blood in mEq / L.
- The partial pressure of oxygen is the partial pressure of carbon dioxide in the arterial blood in torr.
The kidney and liver are two main organs responsible for metabolic pH homeostasis.
Bicarbonate is a base that helps accept excess hydrogen ions when there is acidemia.
However, this mechanism is slower than the airway and can take anywhere from a few hours to 3 days to take effect.
In academia, the levels of bicarbonate increase so that they can neutralize the excess acid, whereas the opposite happens when there is alkalemia.
Thus, when an arterial blood gas test reveals, for example, an increase in bicarbonate, the problem has been present for a couple of days, and there was metabolic compensation for an academic issue in the blood.
It is much easier to correct acute pH disorder by adjusting your breathing. Metabolic compensations take place at a much later stage.
However, in a critical context, a person with an average pH, high carbon dioxide, and high bicarbonate means that although there is a high level of carbon dioxide, there is a metabolic offset.
As a result, one must be careful not to adjust the breaths to decrease carbon dioxide artificially.
In such a case, sharply reducing carbon dioxide means that the bicarbonate will be in excess and cause metabolic alkalosis.
In such a case, the carbon dioxide levels must be lowered slowly.
Indications for arterial blood gas sampling include the following:
- Identification of respiratory, metabolic, and mixed acid-base disorders, with or without physiological compensation, through pH ([H +]) and carbon dioxide levels (partial pressure of carbon dioxide).
- Measurement of the partial pressures of respiratory gases involved in oxygenation and ventilation.
- We monitor acid-base status in diabetic ketoacidosis (DKA) patients on an insulin infusion.
- Arterial blood gas and venous blood gas (VBG) could be obtained simultaneously for comparison, with a venous blood gas sample subsequently used for further monitoring.
- Evaluation of the response to therapeutic interventions such as mechanical ventilation in a patient with respiratory failure.
- Determination of arterial respiratory gases during diagnostic evaluations (such as evaluating the need for home oxygen therapy in patients with advanced chronic lung disease).
- The quantification of oxyhemoglobin, which, combined with the measurement of arterial oxygen pressure (PaO2), provides valuable information on the oxygen-carrying capacity of the patient.
- Quantification of dishemoglobin levels (such as carboxyhemoglobin and methemoglobin).
- Obtaining a blood sample in an acute emergency setting when venous sampling is not feasible (many blood chemistry tests could be performed on an arterial example).
The American Association for Respiratory Care (AARC) has published a clinical practice guideline on blood gas analysis and hemoxymetry.
Absolute contraindications to arterial blood gas sampling include the following:
- An abnormal modified Allen test, in which case the possibility of trying the puncture at a different site should be considered.
- Local infection or distorted anatomy at the potential puncture site (from previous surgeries, congenital or acquired malformations, or burns).
- The presence of arteriovenous fistulas or vascular grafts, in which arterial vascular puncture should not be attempted.
- Known or suspected severe peripheral vascular disease of the affected limb
Relative contraindications include the following:
- Severe coagulopathy
- Anticoagulation therapy with warfarin, heparin, and derivatives, direct thrombin inhibitors, or factor X inhibitors; aspirin is not a contraindication for arterial vascular sampling in most cases.
- Use of thrombolytic agents, such as streptokinase or tissue plasminogen activator.
Arterial gas sampling can be challenging in uncooperative patients or in whom pulses cannot be easily identified.
Challenges arise when healthcare personnel cannot position the patient properly for the procedure.
This situation is commonly seen in patients with cognitive impairment, advanced degenerative joint disease, or essential tremors.
The amount of subcutaneous fat in obese and overweight patients can limit access to the vascular area and obscure anatomical landmarks.
Peripheral artery arteriosclerosis, as seen in elderly patients and patients with end-stage renal disease, can cause increased stiffness in the vascular wall.
Arterial blood gas sampling is generally performed in the radial artery because the superficial anatomical presentation of this vessel makes it easily accessible.
However, this should be done only after it has been shown that there is sufficient collateral blood supply on hand.
In cases where distal perfusion is compromised, and distal pulses decrease, a femoral or brachial artery puncture may be performed.
The brachial artery begins at the lower margin of the teres central tendon. As it passes through the arm, it ends about 1 cm below the bend of the elbow, where it branches into the radial and ulnar arteries.
The radial artery begins at the bifurcation of the brachial artery and passes along the radial side of the forearm to the wrist.
The following suggestions can improve the performance of arterial blood gas sampling:
Patients with poor distal perfusion (such as those in hypovolemic states, advanced heart failure, or vasopressor therapy) may not have a solid arterial pulsation.
The operator may need to withdraw the plunger from the arterial blood gas syringe to obtain a blood sample, although this increases the risk of venous blood sampling.
If arterial blood flow is not obtained, the operator may slowly withdraw the needle; The needle may have gone through the vessel.
Initial arterial flow can be lost later if the needle is moved out of the vessel lumen.
An attempt could be made to re-identify the arterial pulse, using the non-dominant middle and index fingers, and reposition the needle in the vessel’s direction; avoid blind movement of the hand while it is inserted deep into the patient’s body.
Pull it back to a point just under the skin and redirect it towards the arterial pulse that is felt with the other hand.
A lack of pulsatile flow or dark-colored blood can determine the puncture of venous structures. However, arterial blood in patients with severe hypoxemia can also appear dark.
If venous blood is obtained, it may be necessary to remove the needle from the patient to expel the venous blood from the syringe.
Excess skin and abundant soft tissue can obstruct the puncture site; the operator can use the non-dominant hand to smooth the skin, or an assistant can remove the subcutaneous tissue from the field of the puncture site.
Incomplete removal of the heparin solution from the syringe could cause falsely low values for the partial pressure of carbon dioxide; To avoid this, the operator must expel all of the heparin solutions from the needle before arterial puncture.
Incomplete removal of air bubbles can cause falsely high values for the partial pressure of oxygen.
To avoid this, the operator must ensure that air bubbles are completely removed from the syringe (vented plungers have an advantage over standard needles in this regard).
Avoid puncturing the brachial or femoral artery in patients with absent or decreased distal pulses; the absence of distal pulses may indicate severe peripheral vascular disease.
When considering brachial or femoral artery puncture, ultrasound guidance during needle passage helps provide an accurate route map to the vessel and helps minimize inadvertent arterial injury.
Although patients with severe coagulopathy are at increased risk of bleeding complications, there is no clear evidence of the safety of arterial puncture in the setting of coagulopathy.