Arterial Blood Gases

Published on 13/02/2015 by admin

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Arterial Blood Gases

Donna Frownfelter

Key Terms

An arterial blood gas is a physiological assessment tool that measures a patient’s acid-base balance, alveolar ventilation, and oxygenation status.1 It is a valuable resource for obtaining information from respiratory monitoring in the intensive care unit, for conducting follow-up of outpatients, and for evaluating the treatment and the progress of their diseases. There has been a trend toward finding more noninvasive techniques to serve these purposes. This trend has been motivated by the desire to spare patients from being “stuck” so frequently and to avoid the risk for infection posed by indwelling cannulas in patients who already are ill and have weakened immune systems. Physical therapists frequently use oxygen saturation for assessment, but this is insufficient in many patients. When a patient has been thoroughly examined and it is determined that the arterial blood gas values are relatively stable, the physical therapist should compare those values to subsequent readings on the oximeter. Therapists need to consider the blood gas trends, acid/base status, and alveolar ventilation and then correlate those findings to the patient’s signs and symptoms, the physiological monitoring of HR, BP, and RR, and the perceived exertion with exercise. In many cases we need to consider a multisystem approach without examination and evaluation. Oximetry readings are important but must be evaluated in light of all body systems, not just the oximetry readings.

Arterial blood gas evaluation and interpretation can be confusing. The purpose of this chapter is to provide the background needed to understand what the arterial blood gas measures and how it can be applied clinically to improve the evaluation and assessment of patients. It cannot be overemphasized that the arterial blood gas is an essential part of the total examination and evaluation. In the case of a complex patient who has both respiratory and metabolic issues and there is no straightforward or “easy” interpretation, interdisciplinary team members often collaborate to evaluate the patient’s arterial blood gases. It is extremely valuable for a new physical therapist to make ward rounds with the pulmonary physicians, nurses, and respiratory care practitioners, learning about physiological assessment through discussions and questioning during these rounds.

A further goal of this chapter is to help the clinician evaluate and interpret arterial blood gases more effectively and to integrate the information into treatment planning and the patient’s overall progress. However, it is important to remember that blood gases provide a single, static picture of a patient’s condition at a given time; as such, they are important in the management of acutely ill, unstable patients. As a patient’s condition changes, new arterial blood gas results must be obtained and evaluated to determine whether the patient is improving or declining.

Noninvasive Monitoring

Noninvasive monitoring has become more readily available and is used to measure expired carbon dioxide (Pet CO2) and transcutaneous CO2 (T CO2) in neonatal and pediatric patients. New studies are showing that a universal pulse oximetry screening of all neonates before they are discharged would improve the detection rate of congenital heart disease. In a survey conducted in the United Kingdom by Kang and colleagues, it was found that 209 (93%) of the 224 units contacted did not use pulse oximetry routinely. In the neonatal units that did use pulse oximetry, if the saturation was abnormal, they also did echocardiography. Two other units obtained not only echocardiogram, but also x-rays and ECG. The conclusion was that this should be used with national guidelines to supplement the postnatal examination.2

A similar study was conducted by Arlettaz and colleagues to determine whether pulse oximetry could detect congenital heart disease in newborns. It was noted that about half of all newborns who had congenital heart disease were asymptomatic in the first few days of their life. However, early detection of this condition is important because treatment outcomes are related to the time of the diagnosis, and early identification leads to best outcomes. The authors studied the effectiveness of oximetry on the first day of life for early detection of congenital heart disease in neonates who appeared normal. They also examined whether pulse oximeter screening in addition to the clinical examination of the patient would yield more results in the diagnosis of congenital heart disease than would examination alone. If pulse oximetry was less than 95%, echocardiography was done. Of the 3262 newborns screened, 24 infants had repeated oxygen saturations of less than 95%. Of these neonates, 17 had congenital heart disease and the other 5 had persistent pulmonary hypertension. Arlettaz and colleagues concluded that pulse oximetry screening in the first few days of life is an effective tool to identify cyanotic congenital heart disease in otherwise healthy newborns.3

Pulse oximetry has also been used to measure trends in oxygen saturation in patients before, during, and after exercise. For patients who are nervous about exercising, the oximeter can be used to check their saturation level and assure them that they are safe as they exercise. If they are feeling short of breath during exercise, they can stop to look at the value on the oximeter, which will usually return to baseline within a couple of minutes. This gives patients more confidence to exercise. These measures have resulted in fewer invasive arterial sticks and in the ability to monitor patients in an ongoing manner; in addition, they have made it possible to improve the care of patients with cardiovascular and pulmonary illness and to administer the most appropriate ventilation and oxygenation support. Many pulmonary physicians are encouraging patients to obtain a less expensive model that they can carry with them. Such a device will let patients follow their trends and help them to be safer. If more desaturation than normal is measured or anything out of the ordinary occurs, patients are instructed to call their doctor. An early visit may help to prevent exacerbations of their disease.

Partial Pressure of Gases

To better understand blood gases, it is important to remember the properties of gases. The earth’s atmosphere consists of gas molecules that have mass and are attracted to the earth’s center of gravity. At the surface, this atmospheric weight exerts a pressure that can support a column of mercury 760 mm high.

Dalton’s law states that in a mixture of gases, the total pressure is equal to the sum of the partial pressures of the separate gases. Oxygen is 20.9% of the atmosphere, so it has a partial pressure of 159 (760 × 20.9% = 159). Nitrogen is 79% of the atmosphere; thus it has a partial pressure of 600 (760 × 79% = 600). Other gases make up 0.1% of the atmosphere.

The diffusion of gases across semipermeable membranes shows gradients from higher concentrations to lower concentrations. Each gas moves independently from the others.

During respiration, oxygen and CO2 exchange across the alveolar capillary membrane. Special situations may affect the normal progress of respiration and gas exchange.

Normally, alveolar units ventilate and capillary units bring oxygenated blood to the tissues, excreting CO2 back into the alveoli to be removed through the lungs. Some abnormal situations may occur, however, such as shunts and dead-space units. In a shunt unit, the alveoli have collapsed, but blood flow continues and is unable to pick up oxygen. An example of this is atelectasis, in which a lung segment or part of a segment has retained secretions and lung tissue distal to the mucous plug collapses. Circulation continues but oxygenation does not occur, and the PO2 decreases. On the other hand, a dead-space unit can have ventilation but not perfusion. This occurs with pulmonary embolism, when a blood clot obstructs the circulation. The oxygen is available in the ventilated alveoli, but with no circulation, a dead unit is credited. Figure 10-1 demonstrates the regional differences seen in respiratory units.

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Fig. 10-1 A, Normal alveolus. B, Dead space. C, Shunt.

Normal Blood Gas Values

The acid-base balance is denoted by the pH scale; it ranges from 1 to 14, where 1 is the most acidic and 14 the most basic. The arterial pH values are normally 7.35 to 7.45. If the pH is below 7.35, the patient is considered to be in a more acidotic state. If the pH is above 7.45, the patient is considered to be in a more alkalotic state.

Alveolar ventilation is reflected in the partial pressure of carbon dioxide (PCO2). Normal PCO2 values are 35 to 45 mm Hg. If the PCO2 is below 35 mm Hg, the patient is said to be hyperventilating (having increased ventilation, blowing off more CO2 than normal). If the PCO2 is above 45 mm Hg, the patient is hypoventilating (having decreased alveolar ventilation, not blowing off enough CO2 to maintain normal alveolar ventilation).

Arterial oxygen is measured as PO2, the partial pressure of oxygen. Normal values are 80 to 100 mm Hg. If the PO2 is below 80 mm Hg in someone younger than 60 years of age, the patient is hypoxemic. A value of 60 to 80 mm Hg is considered mild hypoxemia; 40 to 60 mm Hg is considered moderate hypoxemia, and less than 40 mm Hg is severe hypoxemia.1

Base Excess/Base Deficit

The blood normally has the capacity to buffer acid metabolites. The normal level of base HCO3− in the blood is 22 to 26 millimoles per liter (mmol/L). This buffering capacity diminishes in the presence of acidemia or alkalemia. Acidosis is an abnormal acid-base balance in which the acids dominate. Alkalemia is an abnormal acid-base balance in which the bases dominate. When there is a decrease in HCO3−, it is seen in a negative base excess and referred to as a base deficit, which is usually seen as a negative number on the blood gas report, such as −3.

As one looks at the base excess and the base deficit (BE/BD), it is helpful to determine whether the patient’s condition is acute or chronic. This state can also be thought of as uncompensated (acute); partially compensated; or completely compensated (chronic). The pH is the key to making this determination. If the pH is not in the normal 7.35 to 7.45 range, the patient is in an acute state. As the balance progresses back toward normal, it may be partially compensated. When a normal pH exists, it is compensated, or chronic. The interpretation comes from looking at the BE/BD and then at the pH. Having a series of arterial blood gases for comparison is helpful, for example, in a patient with respiratory failure and on a ventilator; the patient’s improvement can be documented.

Another example is that of a patient in the chronic state who has chronic obstructive pulmonary disease (COPD) and is retaining CO2 at a level of 55 mm Hg. If the pH reads 7.25, this would be considered an acute state. As the body retains base, the pH will rise back toward normal. If the pH is 7.32 with a PCO2 of 55 and +3 base excess, the status would be partially compensated. If the pH is within normal limits, with a PCO2 of 55 and a pH of 7.35, the status would be compensated or chronic. This is a means of identifying people who are chronic CO2 retainers.

Hemoglobin

Hemoglobin (Hgb) is the main component of the red blood cell. It is crucial for oxygen transport. The normal Hgb range is 12 to 16 g/100 mL blood. In patients who have lost blood through surgical procedures or disease, the decreased hemoglobin can account for their extreme weakness as a result of decreased total oxygen transport capacity. Some of the patients at highest risk are those who have just undergone joint replacement in which there has been much blood loss. Often their Hgb levels are only 8, and because they are considered “orthopedic” patients, it is thought that the low levels can be overlooked. In addition, patients with advanced COPD can at times desaturate (pulse oximetry below 90%) with exercise when the PO2 is low (i.e., PO2 55-60). Pulse oximetry is essential for monitoring the exercise of patients with low PO2, low Hgb, or both.

Cyanosis, the presence of a bluish color to the skin, mucous membranes, and nail beds, is indicative of an abnormal amount of reduced Hgb concentration, usually greater than 5 g of reduced Hgb. The presence of cyanosis suggests a high probability of hypoxemia; however, hypoxemia can occur without cyanosis. Two examples may be cited, one in which an anemic patient with hypoxemia can have little cyanosis, and the other in which a patient with polycythemia can have cyanosis with minimal hypoxemia.

There is a predictable relationship between the arterial oxygen saturation of the Hgb and the PO2

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