Arterial Blood Gases

Blood gas and pH analysis has more immediacy and potential impact on patient care than any other laboratory determination.

National Committee for Clinical Laboratory Standards ⁶⁹⁰

There is no substitute for PO₂, PCO₂, and pH when you are really in the dark about oxygenation status, acid-base, or ventilatory status.

Woody Kagler, M.D.¹⁰⁵

INTRODUCTION

The arterial blood gas report is the cornerstone in the diagnosis and management of clinical oxygenation and acid-base disturbances. An abnormal blood gas report may be the first clue to an acid-base or oxygenation problem: It may indicate the onset or culmination of cardiopulmonary crisis and may serve as a gauge with regard to the appropriateness or effectiveness of therapy. Thus, the arterial blood gas report plays a pivotal role in the overall care of cardiopulmonary disease. Using the arterial blood gas report as a reference point, this text explores the diagnosis, assessment, and intervention of clinical acid-base and oxygenation problems.

Over the past decade, the incidence of arterial blood gas sampling has decreased primarily for cost-containment reasons. Clinicians, casually and increasingly, rely on pulse oximetry as a complete substitute for arterial blood gas data. Although pulse oximetry is extremely valuable and provides us with real-time information, it provides only one small piece in oxygenation assessment. Furthermore, it has been shown that many junior physicians and nurses do not fully understand this technology and make serious errors in its interpretation.¹⁶⁷

More importantly, pulse oximetry provides absolutely no information regarding ventilation and acid-base balance. In one study, more than 50% of surgical patients who had arterial blood gases drawn manifested alkalemia at some point during their hospital stay.¹⁰⁶ With only pulse oximetry, these acid-base abnormalities may easily go unnoticed and untreated. In contrast to pulse oximetry, there are myriad reasons for arterial blood gas analysis. According to Clinical Guidelines published by the American Association for Respiratory Care (AARC), indications also include assessment of the adequacy of ventilation, acid-base evaluation, diagnostic evaluation, quantification of response to therapy, and monitoring of severity and progression of disease.¹⁵

Several studies have shown that by avoiding the use of blood gases, we may be delaying or preventing detection of serious oxygenation and acid-base disturbances.¹⁰⁸ ¹⁰⁹ ¹¹⁰ ¹¹¹ The real incremental cost of an arterial blood gas report is minuscule. The neglected and unquantifiable cost of overlooked clues in the diagnosis of life-threatening disturbances (i.e., acid-base, ventilation, and oxygenation) is immeasurable. Arterial blood gases remain the gold standard in comprehensive emergency and critical care assessment. Their value must be weighed against the potential for real, substantial cost savings and patient harm.

NORMAL BLOOD GAS VALUES

Indices

Table 1-1 shows the various indices that are typically reported when an arterial blood gas is acquired. Collectively, these indices give us valuable information about the important triad of patient oxygenation, ventilation, and acid-base balance.

Table 1-1

Normal Arterial Blood Gas Values

pH	7.35–7.45
PaCO₂	35–45 mm Hg
[BE]	0 ± 2 mEq/L
PaO₂	80–100 mm Hg
[HCO₃]	24 ± 2 mEq/L
SaO₂	97%–98%

Oxygenation

Arterial blood gases remain the indisputable gold standard for evaluation of arterial oxygenation.

Dean Hess, Ph.D., RRT., FAARC ¹⁶⁸

There are two indices shown in Table 1-1 (i.e., PaO₂ and SaO₂) that basically reflect the amount of O₂ present in the blood. Oxygen is carried in the blood in two forms, dissolved O₂ and combined O₂. The PaO₂ is the partial pressure of O₂ dissolved in arterial blood, whereas the SaO₂ is the oxygen saturation of arterial hemoglobin (an indicator of combined O₂).

Technically, the partial pressure of oxygen (denoted PO₂) is defined as the pressure of O₂ in both a gas phase and a solution in equilibrium.⁶ In contrast, oxygen saturation is the amount of oxyhemoglobin in a solution expressed as a fraction (%) of the total amount of hemoglobin able to bind oxygen.⁶ It is noteworthy that abnormal (inactive) forms of hemoglobin (dyshemoglobins) are not considered in this calculation.²⁴¹

The PaO₂ is directly measured and is the most sensitive indicator of oxygenation directly measured. The PaO₂ should be a focal point of every blood gas interpretation. The SaO₂ is a calculated value and a less sensitive indicator. There are times when a calculated SaO₂ may be misleading (e.g., burn patients) so it is sometimes not included with the routine blood gas report. Calculated SaO₂ should not be used for further clinical calculations such as shunt fraction because it may introduce significant error.²⁴¹ SaO₂ can actually be measured directly with co-oximetry (as opposed to calculated) in cases when this value is essential. Clinical and technical issues related to co-oximetry are discussed in Chapters 11 and 15.

Ventilation

The single best way to evaluate the adequacy of ventilation is via the PaCO₂ of an arterial blood gas. One cannot make a definitive evaluation of whether an individual is hypoventilating or hyperventilating by observation alone. Ventilatory status is best assessed via arterial blood gases.

Acid-Base Balance

The arterial pH is the single best indicator of global and blood acid-base status. In addition to providing definitive information about ventilation, the PaCO₂ also allows us to evaluate the respiratory component of acid-base balance. Thus, we can determine if a given acid-base problem is of respiratory system origin.

The remaining indices shown in Table 1-1 ([HCO₃] and [BE]) are “non-respiratory” acid-base indices. Non-respiratory indices are commonly referred to as metabolic indices. Metabolic indices will be abnormal when the patient has a so-called metabolic (non-respiratory) acid-base disturbance. Actually, the term metabolic is sometimes misleading because the patient often does not have a problem with metabolism per se, nevertheless, it is well ingrained in the acid-base lexicon and will be used in this text.

Although it is common practice, it is really unnecessary to include both [BE] and plasma [HCO₃] on a report. This practice originates from the Great Transatlantic Debate⁴ ⁵ between the Boston and Copenhagen schools of thought regarding acid-base diagnosis and treatment.

The Boston school has always advocated the use and application of plasma [HCO₃] as the most appropriate metabolic index. This index is calculated by most blood gas machines via the application of the well-known Henderson-Hasselbalch equation. The plasma [HCO₃] is also historically involved in the development of blood gas analysis because it was the first metabolic index to be routinely reported. Understanding of plasma [HCO₃] is also essential because it is the metabolic index most often used in respiratory care credentialling examinations.

The Copenhagen school, on the other hand, advocates the use of the [BE] as the primary metabolic indicator, purporting its superiority both diagnostically and therapeutically. The index of choice is really a matter of personal preference because the patient can be treated appropriately with use of either index. The level of understanding of the particular index being used has greater importance, because both indices may be misleading if their particular nuances are not well understood.

Historically, a variety of other metabolic indices have been reported (e.g., standard bicarbonate, total body buffer base, and CO₂ combining power ² ³), but none of these indices is currently well accepted. Furthermore, they provide us with no additional information necessary for optimum care of patients. They may, however, serve as a source of confusion and are probably best omitted from the blood gas report.

Normal Ranges

The various quantities shown in Table 1-1 are referred to collectively as arterial blood gases (ABGs). The values indicated in Table 1-1 are normal ranges for adults. Normal ranges are defined by the criterion that 95% of the normal population have values that fall within this range. Normal values for any laboratory measurement are established through measurements made on individuals assumed to have normal health. The average value is calculated as well as the dispersion of values around the average, which is described by a statistical term called the standard deviation.

A large number of measurements made on any normal population generally yields a distribution pattern similar to the one shown in Figure 1-1. The most frequent value observed would be identical to the arithmetic mean. As values deviate more and more from the mean, they occur less and less frequently. The curve represented in Figure 1-1 is referred to as the normal (or gaussian) distribution. In the normal distribution, 68% of the population has values that fall within 1 standard deviation and 95% of the values measured in the population fall within 2 standard deviations. Finally, 99.73% of measurements fall within 3 standard deviations.

Figure 1-1 **Normal distribution of laboratory values.** Of the normal population, 68% have measurements that fall within 1 standard deviation from the mean (). Ninety-five percent of the population values fall within 2 standard deviations, and 99.73% fall within 3 standard deviations. Normal laboratory values are considered to be ±2 standard deviations from the mean.

Normal laboratory values are generally considered to be within ±2 standard deviations from the mean because these values represent the vast majority of the population. The distribution pattern underlying the establishment of normal values is important to understand because 5% of the normal population has values that fall outside the normal range. Nevertheless, it is highly unlikely that an abnormal value reported in this population will deviate greatly from the normal range.

Interestingly, there are data that suggest that the mean normal pH is closer to 7.38 than 7.40.³ Nevertheless, there is little support or reason to change the accepted normal range because the difference is minimal and the range of 7.35 to 7.45 is well ingrained.

Regarding the normal blood gas values shown in Table 1-1, one study showed significantly lower values for arterial carbon dioxide tension (PaCO₂) in young women compared to values in young men.⁷ Mean arterial PCO₂ in the female group was 33 mm Hg. Lower arterial PCO₂ in women compared with men is also consistent with some earlier findings.⁸ Indeed, values of 30 to 46 mm Hg may more accurately characterize the normal range for the entire population, which is calculated from seven published studies.⁹ While keeping these issues in mind, the accepted normal range of 35 to 45 mm Hg is used in this text for standardization and to avoid confusion.

Normal values for [BE] and [HCO₃] may likewise be slightly (i.e., 1 to 2 mEq/L) lower in women than in men.⁷ Nevertheless, here again, a single accepted normal range of 24 ± 2 mEq/L for [HCO₃] and 0 ± 2 mEq/L for [BE] is used because the difference is slight and has little clinical significance.

The mean partial pressure of oxygen dissolved in arterial blood (PaO₂) in a normal young male is 97 mm Hg at sea level.¹⁰ Normal oxygen saturation of arterial hemoglobin (SaO₂) is 97.5%. Both PaO₂ and SaO₂ values tend to decrease with aging.

Oxygenation values may also differ slightly with body position; they are typically higher in the sitting position than in the supine (lying on the back) position particularly in the obese or elderly. Finally, altitude and the percentage of O₂ inspired also affect PaO₂. The effects of these variables on PaO₂ are discussed in Chapter 3. In this text, room air (21% oxygen) and sea level (760 mm Hg) are presumed unless otherwise noted.

The normal PaO₂ in the supine position for an adult 40 to 75 years old can be calculated specifically by the formula PaO₂ = 109 − (0.43 × age).²⁴² A PaO₂ within ±8 mm Hg of the predicted value is considered to be normal. Because the minimum normal PaO₂ at 40 years of age in the supine position is 80 mm Hg, most tables show the normal PaO₂ range as being approximately 80 to 100 mm Hg. Technically, however, a PaO₂ of 80 mm Hg in a 20-year-old individual is not normal.

Arterial PO₂ is approximately 5 mm Hg higher in the sitting position than in the supine position and the mean normal value at a given age can be calculated more precisely by the formula PaO₂ = 109 − (0.27 × age).¹¹ In general, the difference in PaO₂ associated with positional change is minimal in young adults but magnified in the elderly.

In clinical practice, it is not usually practical or expedient to calculate PaO₂ based on these formulas. An approximate rule of thumb is sometimes useful to estimate the minimum normal PaO₂ in adults of different ages. Minimum normal PaO₂should exceed 90 mm Hg if the patient is younger than 45 years old. Above the age of 45, PaO₂ generally decreases with age; however, low minimum normal PaO₂ should exceed 75 mm Hg regardless of age.²⁴² Interestingly, PaO₂ seems to progressively decrease between the ages of 45 to 75, then actually increases slightly and levels off beyond age 75.²⁴² This is contrary to earlier beliefs.

Units of Measurement

It is essential to have a clear understanding of the particular units in which any laboratory value is being measured. The pH value is dimension-less, and SaO₂ is measured as a percentage. The [HCO₃] and [BE] are usually reported in milliequivalents per liter (mEq/L). Nevertheless, because mEq/L is equal to millimoles per liter (mM/L) in ions with a univalent charge (e.g., HCO³⁻, Na⁺), mM/L may also be used as the units for these values.

The PaO₂ and PaCO₂ are measured in millimeters of mercury (mm Hg), a unit of pressure. The unit torr is synonymous with (mm Hg) and either may be substituted interchangeably. The International System of Units (SI) has attempted to standardize the reporting of all scientific data and has made recommendations with regard to the most appropriate units that should be used.

Table 1-2

Pressure Unit Conversion Factors

cm H₂O	mm Hg	kPa
1.0	0.736	0.098
1.359	1.0	0.133
10.197	7.501	1.0

From Burke, J.F.: Surgical Physiology. Philadelphia, W.B. Saunders, 1984.

ARTERIAL VERSUS VENOUS BLOOD

Blood vessels that carry blood away from the heart are classified anatomically as arteries, whereas vessels that return blood to the heart are called veins. Arterial blood in the systemic circulation (Fig. 1-2) provides more information than systemic venous blood with regard to ventilation and oxygenation assessment. Arterial blood is a uniform substance presented to all organs for their metabolic needs.

Figure 1-2 **Pulmonary and systemic circulation.** Blood from the pulmonary circulation flows back to the left heart after gas exchange in the lungs. Oxygenated blood flows from the left heart to the systemic circulation.

An important concern in oxygenation assessment is the adequacy of O₂ delivery to all human cells. To assess delivery, one must analyze arterial blood en route to the cells. The PO₂ of peripheral venous blood, on its journey back to the heart from the cells, provides little information concerning O₂ delivery.

Arterial blood also provides direct information with regard to lung function and the adequacy of CO₂ excretion. When PaCO₂ levels are excessive, the ventilatory system has failed to perform one of its primary functions—namely, CO₂ regulation in the blood. The venous PCO₂ level, on the other hand, is primarily a result of local metabolic rate and perfusion. Either an increase in local metabolism or a decrease in local perfusion elevates venous PCO₂. Thus, venous PCO₂ varies in different areas of the body and provides little useful information regarding the adequacy of pulmonary ventilation.

Finally, arterial blood is superior to peripheral venous blood in both acid-base and oxygenation assessment because it reflects overall blood or body conditions. Arterial blood gases are uniform regardless of the specific artery from which the sample was drawn. This is true because arterial blood, after being well mixed in the heart, does not change appreciably in O₂ or CO₂ composition until it reaches the systemic capillaries. The systemic capillaries are the small vessels between arteries and veins within which gas exchange takes place between blood and body tissues. In general, samples of blood gases taken from any artery should have identical blood gas values.

Peripheral venous blood, on the other hand, reflects only localized conditions. The O₂ and CO₂ levels in a given peripheral vein depend on the metabolic rate and perfusion of the tissue traversed earlier. Because local metabolism may vary widely, venous blood gas samples acquired simultaneously from different peripheral veins likewise vary substantially. The different PvO₂ levels in various peripheral veins are discussed later in Chapter 7 and are shown in Table 7-1.

Although less accurate than arterial samples, venous samples from a well-perfused patient may provide a gross indication of acid-base balance,⁵ electrolyte levels, or abnormal hemoglobins.²⁴¹ Likewise venous blood pH appears to correlate well with arterial blood in patients with uremic acidosis and diabetic ketoacidosis.¹¹² One should also realize that any difference between arterial and venous blood will be exaggerated when the general or local circulation is impaired.¹⁰⁷ Thus, measurement of arterial blood gases is the gold standard in the diagnosis and clinical management of oxygenation and acid-base disturbances.

TECHNIQUE

The collection of arterial blood is not only technically difficult but can be painful and hazardous to the patient. Therefore, it is essential that individuals performing arterial puncture be familiar with the proper techniques, with the dangers of the procedure, and with necessary precautions.

National Committee for Clinical Laboratory Standards ¹⁰⁷

Compared with the acquisition of venous blood, arterial sampling is technically more difficult and has greater potential for serious complication. The higher arterial pressure can make bleeding complications more profuse. Furthermore, large clot formation or prolonged spasm in an artery could cut off vital supply of O₂ to the tissue. Arterial blood gas samples are also very vulnerable to improper handling technique because of their high gas content. Arterial blood is one of the most sensitive specimens sent for clinical laboratory analysis.¹⁰⁷

Despite these drawbacks, after appropriate training, arterial blood sampling may be accomplished simply, safely, accurately, and expediently by respiratory therapists, laboratory technologists, nurses, or physicians. The following section involves pre-analytical considerations when preparing to draw an arterial blood sample; this is followed by a description of a technique of arterial puncture and specimen collection.

Preparation and Pre-analytical Considerations

Status of Patients and Control of Infection

Before attempting to perform an arterial puncture, the clinician should always be aware of the patient’s primary diagnoses and current status. A brief review of the chart, inspection of the patient, and observation for respiratory care modalities (e.g., O₂ therapy, mechanical ventilation) are essential. This initial evaluation may alert the clinician to a potential complication or suggest that the sample should be drawn at a later time. When the sample is to be drawn by a non-physician, the first step is to verify that a written order is documented in the patient’s chart. The chart should then be evaluated for factors (e.g., medications) that might suggest the need for special precautionary measures.

Anticoagulants/Bleeding Disorders

Pharmacologic therapy should be reviewed to ascertain whether the patient is undergoing anticoagulant or thrombolytic therapy. Commonly prescribed anticoagulants include heparin, warfarin (Coumadin), and dipyridamole. The mild anticoagulant effect of aspirin may be of lesser importance.¹⁵ Anticoagulant therapy is associated with an increased likelihood of bleeding complication after puncture, and additional preventive measures should be taken. Consideration may be given to scheduling the arterial puncture approximately 30 minutes before the next scheduled dose of anticoagulant, if feasible.¹⁶

Thrombolytics (e.g., streptokinase, tissue plasminogen activator) differ from anticoagulants in that they are administered to break down (lyse) blood clots rather than simply to prevent clotting. Nevertheless, excessive bleeding after arterial puncture may also occur when these drugs are being administered.¹⁵

When evaluating the patient’s history and progress notes, the clinician should be especially alert for documentation of blood coagulation disorders (coagulopathy). Hemophilia, a genetic disorder found in men, is characterized by a prolonged blood clotting time and, therefore, a predisposition to bleeding complications. Similarly, a low platelet count or a prolonged bleeding time on laboratory reports should also be noted. Identification of any coagulation disorder should activate implementation of special bleeding precautions.

Infection Control

The clinician should always be cognizant that infectious diseases may be transmitted by contact with blood. The disease foremost on our minds in this regard is the acquired immunodeficiency syndrome (AIDS). AIDS is caused by the human immunodeficiency virus (HIV), formerly known as the human T-lymphotropic virus type III–lymphadenopathy-associated virus. This viral disease has essentially no cure as yet and may be contracted through intimate contact with the body secretions of an infected individual.

The body secretions that contain the greatest amount of the virus are blood, semen, and vaginal secretions.¹⁷ The virus may be transmitted by sexual contact, percutaneous (through the skin) exposure, absorption through mucous membranes (e.g., mouth, eyes), and through non-intact mucous membranes or skin (e.g., cuts, open wounds).¹⁷ The risk that healthcare workers may acquire the disease is related to the potential for percutaneous exposure or mucous membrane contact with contaminated body secretions.

A major problem in controlling the spread of this disease is the fact that individuals infected with the virus are asymptomatic early in the disease while at the same time they are contagious. Thus, all blood samples must be treated as though infectious and handled with standard precautions.¹⁰⁷ Standard precautions are new guidelines that include the major features of universal fluid precautions and body substance isolation procedures.¹⁰⁷ Standard precautions are more comprehensive than universal precautions, which only account for bloodborne pathogens. Standard precautions address the transmission of all bloodborne pathogens.¹⁰⁷ Both standard precautions and universal precautions are available through the Centers for Disease Control.

Other infectious disorders that may be acquired through blood contact include viral hepatitis, syphilis, Jakob-Creutzfeldt disease, and septicemia. Viral hepatitis is a generalized inflammation of the liver caused by hepatitis virus A, B, or C. Hepatitis vaccination, which prevents hepatitis B on a long-term basis, is available and healthcare workers who routinely perform arterial puncture should receive it. There is also an injection available to prevent hepatitis A in the short term; however, no protection is available for hepatitis C. Syphilis is a chronic infectious venereal disease that may also be transmitted through the blood. Jakob-Creutzfeldt disease is a rare, fatal neurologic disorder that is transmitted by a virus. Septicemia is a systemic infection in which pathogens are present in the blood.

In the past, samples obtained from individuals with any of the above disorders were marked as precaution samples, and special procedures were implemented to minimize the risk of infection to the healthcare worker. Today, however, all blood samples should be handled as if they are infected, because most individuals who have HIV are not diagnosed and are asymptomatic.

Standard precautions require diligent handwashing and use of gloves when the hands are likely to come into contact with body secretions (e.g., during arterial blood gas sampling ¹⁴). The Centers for Disease Control also recommends the use of masks and protective eyewear (to avoid contact with mucous membranes) if a procedure is likely to generate droplets of blood and aprons or gowns if blood is likely to be splashed during a procedure.¹⁹

Handwashing is critical between examinations of patients and immediately after any direct contact with blood. Gloves should always be worn when acquiring an arterial blood sample, and the gloves should be changed before contact with each new patient. Remember, however, that gloves are an adjunct to, but not a substitute for, handwashing.

Furthermore, needles must be handled carefully to prevent accidental puncture. Needle sticks are the most frequent source of transmission of bloodborne diseases in healthcare workers.¹⁹ ²⁰ Needles should not be purposely bent or broken by hand, removed from syringes, or manipulated by hand in any way. Specimen sampling devices in which the needle retracts after use or use of some other device to assure that inadvertent puncture cannot occur is essential. After use, needles should be placed in puncture-resistant containers that are located as close as is practical to the area where they are being used.

Patient Identification and Assessment

Identification of the correct patient is extremely important. NCCLS document H3—Procedure for the Collection of Diagnostic Blood Specimens by Venipuncture—describes this in more detail.⁶⁹¹

Likewise, the patient and the clinical indication for the sample should be assessed before acquisition. Knowledge of current vital signs and a general awareness of the patient’s background and psychological status may also contribute to acquisition of the sample smoothly and efficiently. The more information the clinician has with regard to a particular patient, the more prepared he or she is to care for that patient most effectively. Notwithstanding, the review of the chart is most often brief in clinical practice due to time constraints and the need for efficiency.

Steady State

When oxygen therapy or mechanical ventilation is used, a period of time is required before the complete effect of the therapy is reflected in the arterial blood specimen. Similarly, the same principle is true when therapy is changed or discontinued and following exercise. Because blood gases are often the major criteria on which major therapeutic decisions are made regarding oxygenation and acid-base disturbances, it is crucial that the blood gas results provide us with an accurate and current reflection of the patient’s status.

During this period of adjustment to a change in therapy, blood gas values are in a dynamic, changing state. In time, the entire cardiopulmonary system reaches a new equilibrium or steady state. Blood gas values remain relatively constant from this point on, and the complete impact of the therapy is reflected in the arterial blood.

Arterial blood samples must always be drawn only when the patient is in a steady state. The actual time required for the attainment of a steady state differs slightly with the patient’s pulmonary status. In patients free of overt pulmonary disease, a steady state is likely achieved in as few as 1 to 3 minutes ¹ ²¹ and almost certainly within 10 minutes.²² ²³

In patients with chronic airway obstruction, up to 24 minutes after a change in therapy may be necessary.²⁴ In clinical practice, a 20- to 30-minute waiting period is usually recommended before sampling arterial blood after a change in oxygen therapy or ventilation.¹ ¹⁵ ²⁵ As described previously, however, only 3 to 10 minutes is necessary to achieve steady-state conditions in the absence of pulmonary disease.

Ideally, a patient who is breathing spontaneously should also be at rest for at least 5 minutes before sample acquisition.¹⁴ Likewise, temporary fluctuations in therapy also compromise steady-state conditions, which may occur if the patient removes his or her oxygen mask or must be suctioned for excessive pulmonary secretions. The clinician drawing the sample is responsible for ensuring that the patient is in a steady state before arterial puncture. When a sample is thought to represent non–steady-state conditions, a repeat puncture with related pain, risks, and cost is probably necessary. Worse yet, if the non–steady state goes unnoticed, incorrect or inappropriate therapy may be prescribed. Thus, before arterial puncture, the patient must be carefully assessed to ensure steady-state conditions.

Samples drawn to assess response to exercise require special considerations. They are best drawn at peak exercise, however, samples drawn within 15 seconds of termination of exercise are acceptable.¹⁵ Outside this time range, samples may yield false-negative results for hypoxemia.¹⁵

Mild to moderate pain may accompany arterial puncture.²⁶ The clinician should be aware that pain and anxiety associated with arterial sampling may in itself cause changes in ventilation that, in turn, alter blood gas results. Thus, the patient should be approached calmly with a quiet voice and reassurance to promote physical and mental comfort¹⁴; and the sample should be obtained as quickly as possible. Some suggest the use of numbing agents before the actual puncture²⁶ and this issue is discussed later in this chapter.

Spontaneous Variability of PaO₂

The clinician should also appreciate that some studies have shown considerable spontaneous variability in PaO₂ in apparently stable patients.¹⁶⁹ This variability may be as much as 10% and may be due to patient or machine issues.¹⁶⁸ The important point here is that changes in PaO₂ of as much as ±8 mm Hg should be viewed with skepticism because they are commonly a result of spontaneous variability.