Arterial Blood Gases

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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.

image
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. It is represented in the oxyhemoglobin dissociation curve as follows. When oxygen saturation is monitored during exercise, saturation is kept at or above 90%. As the curve denotes at a level of approximately 60 mm Hg, the saturation is about 90%. As the PO2 drops at the sharp part of the curve, there is a marked decrease in oxygen saturation for every mm Hg PO2.4

Acid-Base Balance

Normal body metabolism consists of the consumption of nutrients and the excretion of acid metabolites. Acid metabolites must be kept from accumulating in large amounts because the body’s cardiovascular and nervous systems operate in a relatively narrow free hydrogen ion (H+) range (narrow pH). Free H+ concentration is discussed as pH (−log [H+]). The maintenance of body systems requires an appropriate acid-base balance.5

Approximately 98% of normal metabolites exist in the form of CO2, which reacts readily with water to form carbonic acid.

< ?xml:namespace prefix = "mml" />CO2+H2OØ″H2CO3

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Carbonic acid (H2CO3) can exist as either a liquid or a gas. Because carbonic acid can change to CO2, much of the acid content can be excreted through the lungs during respiration. In this way, the lungs can help regulate the pH of the body.

The Henderson-Hasselbach equation demonstrates how the H+ concentration results from the dissociation of carbonic acid and the interrelationship of the blood acids, bases, and buffers.

H2O+CO2Ø″H2CO3H++HCO3−

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Blood Gas Interpretation

Consider the normal values for the arterial blood gas: pH, PCO2, PO2, O2 saturation, and base. With any given patient, there may be a range used (see below) that will be considered in light of the patient’s disease, current acute or chronic status, and signs and symptoms. At times, these values will be considered “acceptable ranges,” rather than ideal.

Once the norms for the pH, PCO2, PO2, and base have been established for a given patient, they should be considered first in the interpretation. Is the current value normal or not? Is the patient acute (uncompensated), partially compensated, or fully compensated (chronic)?

A stepwise interpretation can be helpful. Performing the assessment in the same order each time provides a comprehensive and standard overview.

It is important to note additional clinical considerations. Is the patient receiving oxygen or mechanical ventilation? If the patient is on increased inspiratory oxygen (FIO2) of 40% and the PO2 is not close to acceptable levels, it is more of a concern than if the patient is on a low FiO2 and the PO2 is within normal limits. Similarly, if the patient is on mechanical ventilation, the blood gases should be close to normal. The exception would be a patient who is a chronic CO2 retainer; that patient would be ventilated to his or her normal level (i.e., at the PCO2 level where the pH becomes closer to normal).

Acceptable Ranges of Arterial Blood Gases

At times, for various medical reasons, an acceptable range of blood gases will be used, rather than trying to get an ideal reading. The following ranges may be acceptable, depending on the physician and the patient’s particular circumstances.

The relationship between the pH and the PCO2 should be noted. In general, as the PCO2 rises, the pH will decrease. There is also predictable change caused by variation in carbonic acid. Some general rules of thumb include:

There is also a relationship between the PCO2 and the plasma bicarbonate.

By knowing these guidelines, the therapist can determine whether the changes in the arterial blood gases are in line with respiratory problems as opposed to metabolic problems such as acidosis due to diabetic ketoacidosis, in which the base deficit can be very low and the pH can be low (acidemic), but the PCO2 can be within normal limits.5,7

Respiratory Failure

Respiratory failure is defined as the inability of the pulmonary system to meet the metabolic demands of the body (i.e., ventilation and oxygenation).5 The blood gases usually show a pH below 7.30 and a PCO2 above 50. Generally, the patient is also hypoxemic.

During the acute phase, the kidneys have not started to compensate, and the base HCO3− is within normal limits. Later, a base excess can be noted as the kidneys try to compensate for the acidotic pH. Chronic respiratory failure can be noted by an increased PCO2, a pH within normal limits, and a low PO2.

In assessing the PO2, the following ranges are used: mild hypoxemia, less than 80 mm Hg; moderate hypoxemia, 60 to 80 mm Hg; and severe hypoxemia, below 40 mm Hg.

Positional changes can affect the oxygenation status. For example, unilateral right lower lobe atelectasis with the patient lying on the right side causes increased blood flow to the right lung, which is collapsed. This causes increased shunting and a decrease in oxygenation (differential shunting). If the patient lies on the left side, the oxygenation will improve. When the patient lies in a supine position, a mixed PO2 can be observed.

When hypoxemia is noted, treatment consists of oxygen therapy, CPAP or biphasic positive airway pressure, and alleviation of the cause of hypoxemia, if possible. This may be achieved by means of airway-clearance techniques or medication, in addition to the oxygen therapy or mechanical ventilation. Oxygen therapy treats the hypoxemia, decreases the patient’s work of breathing, and decreases myocardial work.

Factors Affecting Arterial Blood Gases

Many normal causes affect arterial blood gases, such as extremes of age, from neonates to older adults. The neonate has many changes going on in the initial life process; fetal circulation changes dramatically in the first hours and days of life. In the older adult patient, decreases in cardiac output (CO), residual volume (RV) of the lungs, and maximal breathing capacity gradually lower PO2 over the course of the life cycle. It is estimated that after 60 years of age, the PO2 decreases by 1 mm Hg per year of age from 60 to 90 years.

Exercise or any increase in activity above rest may result in significantly increased oxygen consumption in patients with cardiopulmonary dysfunction. In the normal population, the human body compensates by increasing oxygen consumption to meet the workload. Usually, a plateau is reached and a constant oxygen consumption for that activity is achieved. In patients with cardiopulmonary dysfunction, oxygen consumption continues to increase, even at the same workload in untrained patients. It is important to monitor oxygen saturation to prevent desaturation in these patients.4 During pregnancy, hormonal and mechanical factors may have negative effects on cardiopulmonary function. In the last trimester, women commonly observe shortness of breath and difficulty in taking a deep breath because of hormonal issues and diaphragmatic encroachment.

In a small study by Sunyal and colleagues assessing oxygen saturation in pregnant women to try to evaluate the lung function in pregnancy, 32 women (25 to 35 years old) without any recent respiratory disease were monitored during each trimester. The study compared eight healthy nonpregnant women as an age-matched control group. In the pregnant women, the oxygen saturation levels increased progressively each trimester: first trimester (97.73% ± 0.30), second trimester (98.05% ± 0.54), and third trimester (98.40% ± .30). The oxygen saturation readings were higher in the pregnant women than in the nonpregnant women. The study conclusion was that the increased oxygen saturation during pregnancy was related to increased ventilation and a rising progesterone level, which reaches a peak in the later phases of pregnancy.9

During sleep there is a decrease in minute ventilation and a decreased responsiveness to CO2 and hypoxemia. Many patients who have undergone spinal cord injuries or cerebrovascular accidents, who have COPD, who have significant neuromuscular changes such as kyphoscoliosis, or who are morbidly obese have been noted to have apnea and hypoxemia during sleep. The possibility of these conditions should be considered in any patient with daytime somnolence who complains of difficulty sleeping or staying awake during the day or who has trouble following directions or is mentally confused. Often the patient’s spouse or partner will report that the patient is restless in bed and snoring increasingly. Some may actually notice that the patient stops breathing at times during sleep. These patients need to be referred for a sleep study to evaluate their breathing, oxygenation, and oxygen saturation at night. Physical therapists can help by monitoring these patient populations, by understanding and watching for the signs and symptoms listed above, and by communicating with the patient’s partner (if the patient has one) to ask about specific details in the patient’s sleeping patterns and whether there have been any noticeable changes in function.

Barometric Pressure Can Affect Oxygenation

Low barometric pressure associated with high altitude significantly decreases the amount of oxygen available to an individual. As noted before, the partial pressure of oxygen is dependent on the total atmospheric pressure. When the total pressure is reduced, less O2 is available. This is particularly important if a patient already has a decreased PO2 and is traveling to an area with lower barometric pressure. Patients on oxygen need an oxygen prescription that is based on the area in which they are living. Dr. Petty, who practiced in Colorado, urged his patients to buy an oximeter to carry with them. His motto was “titrate where you migrate.”

Barometric pressure increases, such as in a hyperbaric oxygen chamber with higher barometric pressure, can help deliver increased oxygen for a select group of patients. Wound-healing needs and carbon monoxide poisoning are two such indications. Hyperbaric chambers are not found in all hospitals, but when the need arises, a search can find the one closest for treatment.

Other Factors That Can Influence the Arterial Blood Gas

Increased temperature (a febrile state) can increase metabolism and therefore increase oxygen consumption, decrease PO2, and consequently, increase alveolar ventilation, which will decrease PCO2. Decreased temperatures can decrease oxygen consumption, as is seen in patients who have survived for several minutes in cold water and are then resuscitated or in patients who are kept in a state of hypothermia to decrease body functions and preserve oxygen while medical staff members do their best to stabilize the patient.

On the arterial blood gas report it is important to note the status of the patient at the time the blood gas is drawn. Usually, patients are at rest when the blood gas is drawn. If the PO2 is low at rest (60 mm Hg), it is close to the sharp part of the oxyhemoglobin dissociation curve, and the patient may desaturate quickly with an increase in exercise.

If the patient is on supplemental oxygen and the PO2 is only 55 mm Hg, the PO2 is still inadequate because of the additional O2.10–12 Any patient on oxygen at rest should be evaluated for appropriate oxygenation with exercise. Most patients’ blood gases are drawn at rest, not with exercise. Similarly, if a patient is on mechanical ventilation, the blood gases should be within or near normal limits.

Noninvasive Monitoring

Noninvasive monitoring of respiratory function is convenient, is accurate in showing trends, involves minimal complications, and causes little discomfort for the patient. The modalities are used as part of an ongoing examination, along with clinical monitoring. Noninvasive monitoring tools include pulse oximetry, transcutaneous oxygen (Ptc O2), transcutaneous PCO2 (Ptc CO2), and end-tidal CO2 using capnography.

Pulse Oximetry

Pulse oximetry provides estimates of arterial oxyhemoglobin saturation (SaO2) by utilizing selected wavelengths of light to noninvasively determine the saturation of oxyhemoglobin (SpO2).13

Precautions for Pulse Oximetry

If a patient needs ongoing measurement of pH, PCO2, and total Hgb, the presence of abnormal Hgb may be a “relative” contraindication for pulse oximetry.13 If the Hgb is low (i.e., 7-8, or virtually half the normal Hgb), the oxygen saturation may be above 90% at rest but as soon as exercise begins, the patient will usually desaturate very quickly. Thus pulse oximeter reading needs to be monitored before, during, and after exercise/activity to have a better overall view of the physiological response to the intervention. The other vital signs need to be evaluated as well.

A probe is attached to the patient’s finger, toe, or earlobe, and a measurement is made of the amount of light that is absorbed (i.e., relayed to a microprocessor that calculates SaO2). It can be read intermittently or displayed continuously. In addition, a pulse reading is given.

A reading taken at rest as an additional vital sign, along with HR, BP, and RR, provides a baseline before exercise. When an exercise workload is given, the parameters can be compared to see whether the patient has a normal or abnormal response to exercise. The recommendation is to keep the O2 saturation above 90% during exercise. Supplemental oxygen can be titrated to keep the O2 saturation within the appropriate range. This is done in conjunction with the physician because it is not in the scope of practice for physical therapists to change FIO2 levels without a consultation and physician’s order. Many hospitals that deal with critically ill patients as an interdisciplinary team will have standing orders to cover this situation. The orders may read, for instance, “titrate oxygen to keep the O2 sats above 90% with exercise.” In this case, limits to how high the FIO2 may be raised need to be considered, as well as changing the oxygen delivery system to get a more consistent level of oxygenation, especially during activity.

Situations exist in which pulse oximetry may not yield accurate results: abnormal Hgb, jaundice, anemia, low perfusion (i.e., diabetes), the use of intravascular dye such as methylene blue, deeply pigmented skin, and dark nail polish.15 In addition, movement artifacts and highly lit fluorescent lights can have an effect on the readings. These concerns must be appreciated by therapists, especially during exercise, to ensure proper readings and correct evaluation of responses to exercise.

Documentation of patients’ responses to exercise can be included with vital sign notations: ECG, HR, BP, RR, and SpO2 may be recorded. The readings taken at rest, during exercise, and at the end point of exercise are important, as well as the time it takes for the patient to recover (to return to baseline). This information documents the physiological response to exercise.

Transcutaneous Carbon Dioxide

Transcutaneous carbon dioxide (Tc CO2) measurements are used in the neonatal ICU, where continuous monitoring is indicated, especially if the baby is not intubated or being mechanically ventilated. The Tc CO2 has a skin sensor and is most effective in monitoring hemodynamically stable patients. The sensor must be calibrated and must be rotated frequently to prevent burns. A relatively close correlation exists between arterial PCO2 and Pt CO2, and the latter has the additional benefit of requiring fewer needle sticks.

A study by Sandberg and colleagues performed in the neonatal intensive care unit (NICU) evaluated the accuracy of transcutaneous (Tc) blood gas monitoring in newborn infants, including infants with extremely low birth weights. The measurements were taken under “stable conditions.” Some differences were found in arterial measurements versus Tc measurements, related to body weight, postnatal age, and the amount of oxygen each neonate required. Overall, Sandberg and colleagues concluded that there was good agreement between the two methods and that it was clinically acceptable to use Tc in their NICU. They added that although it would not replace the need for doing arterial blood gases, Tc monitoring would be a good supplement to use in assessing neonates.16

End Tidal Carbon Dioxide

Capnography uses infrared spectroscopy or mass spectrometry reading of expired CO2 to analyze CO2 content in a continuous reading. There is also a wave display of the breath-to-breath readings. The waveform can be divided into segments that represent various phases of the respiratory cycle. In the beginning of a normal exhalation, gas is expelled from the anatomical dead space and has a low CO2 reading. As more alveoli empty, there is an increasing proportion of alveolar gas in relationship to dead space gas and thus a greater concentration of CO2. A plateau level is reached when there is a nearly constant CO2 concentration (alveolar plateau). At the alveolar plateau, the end tidal CO2 concentrations closely approximate the arterial PCO2. Then with inspiration, the CO2 concentration decreases rapidly to baseline.

The end tidal CO2 readings help monitor changes in the ventilatory status and assist in determining the need for changes in the ventilator settings to improve alveolar ventilation.17 Most notable is the ability to see trends in these readings and maintain an ongoing monitoring system that can allow for early detection of complications, such as pneumothorax, hypoventilation, pulmonary embolism, or fat embolism (any increase in dead space). The end tidal CO2 readings have also been used to determine the proper positioning of feeding tubes and endotracheal tubes.18,19

Summary

This chapter discusses the normal arterial blood gases and the significance of their values. A guide to the interpretation of blood gases is proposed. The relationships between pH and PCO2, between PCO2 and HCO3−, and between O2 saturation and PO2 are examined. Predictable changes that are caused by respiratory changes are described. In addition, it is noted that metabolic changes can have marked effects on blood gases. Oxygen therapy and airway-clearance techniques can improve hypoxemia.

Position changes can be detrimental, causing differential shunting, or can improve the PO2 by means of better ventilation-perfusion matching. For example, if the affected lung that has pneumonia is placed down (i.e., lying on the side of the pneumonia), there will be more perfusion passing by the poorly ventilated lung and the PO2 will decrease. If the well ventilating lung is placed down the PO2 will improve.

Noninvasive monitoring has been a great advance; it is now possible to see ongoing trends in a patient and assess complications early so that patients can be moved forward more safely and efficiently. Noninvasive monitoring can also be an adjunct to exercise, as will be discussed in Chapter 43.

Physical therapists must have a thorough understanding of the various respiratory monitors so that they are better equipped to provide comprehensive evaluation and assessment, thus allowing them to safely encourage patients to progress to their optimal rehabilitation outcomes.

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