Assessment Description Mrs. P. has been in the ICU for several days, has made gradual progression, and appears to be doing well with labo

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CHAPTER 121 Intubation and Airway Support

Bisan A. Salhi, MD

Todd A. Taylor, MD

Douglas S. Ander, MD

INTRODUCTION Airway management can significantly affect outcomes for hospitalized critically ill patients. Failure to deliver adequate oxygen may cause irreversible brain damage or preclude successful resuscitation. Options for management may range from assisted ventilation with a bag-valve-mask (BVM) to noninvasive ventilation (NIV) support to endotracheal intubation (Table 121-1). A successful outcome in any intubation demands proficiency in patient assessment, knowledge of the equipment (basic and advanced), requisite technical skills, appreciation of individual limitations, and an alternative plan to deal with the difficult or failed airway.

TABLE 121-1 Overview of Emergency Airway Management

Technique Description Notes Rapid Sequence Intubation (RSI)

Defined by the simultaneous administration of a

Avoids insufflation of the stomach

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sedative and paralytic agent to assist in endotracheal intubation, usually via direct laryngoscopy

Minimizes risk of aspiration with assisted BVM ventilation

Bag-Valve-Mask (BVM) Ventilation

The ability to ventilate a patient can be an effective bridge prior to intubation and is a requirement prior to use of any paralytic agents

Prior to ventilating with a BVM, place an airway adjunct to maintain patent airway and to optimize ventilation: • Nasopharyngeal airway if

patient’s airway (gag) reflexes intact

• Oropharyngeal airway if absent airway reflexes

If patient has dentures, they should be left in place during BVM ventilation and removed just prior to insertion of laryngoscope If the operator is having problems maintaining a seal or ventilating, two-hand BVM should be attempted

Endotracheal Intubation Airway control established usually through direct laryngoscopy and orotracheal intubation

Any operator attempting intubation, particularly if using paralytic agents, should be very comfortable with the technique, equipment, rescue devices, and with other resources for assistance, have a plan to address any contingency

A small survey published in 2010 noted that individual hospitalists (n = 175) performed, on average, only 10 endotracheal intubations in the previous year with a range of 3 to 20. For those performing endotracheal intubation, it is important to maintain this essential skill, and to be aware of their own practices and skill limitations. Depending on their clinical environment and work setting, the expectations for different hospitalists in advanced airway management will vary. However, all hospitalists should be versed in initial airway management and stabilization, including effective use of oral and nasal airway and BVM devices.

Successful intubation requires not only knowledge of the basic procedural steps, but also knowledge of airway anatomy, landmarks, and locations of various airway structures relative to each other.

INDICATION FOR INTUBATION

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All indications for endotracheal intubation can be classified as (1) failure to maintain a patent airway, (2) failure of oxygenation and/or ventilation, and (3) anticipation of a rapidly deteriorating clinical course (Table 121-2).

TABLE 121-2 Indications for Intubation

Indication Suggestive Signs Comments Failure to maintain a patent airway

• Inability to phonate • Inability to swallow or handle

secretions • High risk of aspiration

• The presence/absence of a gag reflex does NOT effectively assess airway patency

• The gag reflex is physiologically absent in 20% of normal adults

• Stimulation of the gag reflex increases risk of vomiting/aspiration

Failure to oxygenate or ventilate

• Unresponsiveness to noninvasive oxygenation or ventilation methods

• Assess patient’s clinical appearance including vital signs, mentation

• Monitor oxygenation with continuous pulse oximetry and/or ABG analysis

• Monitor ventilation with capnography, ABG, or VBG analysis.

Anticipate deterioration in clinical condition

• Patient must be unaccompanied for testing

• Patient unable to maintain current work of breathing

• Likely further studies or surgery etc

• Consider clinical factors such as severe metabolic acidosis with inadequate respiratory compensation; neuromuscular weakness (impaired maximal inspiratory pressure); etc

PREDICTORS OF A DIFFICULT AIRWAY A difficult airway refers to complex or challenging BVM or endotracheal intubation. Difficult oxygenation is the inability to maintain the oxygen saturation >90% despite using a BVM and 100% oxygen. A failed airway refers to the inability to either ventilate or intubate a patient after three intubation attempts by the same operator. A higher rate of poor clinical outcomes occurs when the airway is managed as an emergent (rather than elective) procedure. In addition, an increased number of airway attempts predicts poorer

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outcomes; thus, a backup plan is necessary if initial intubation attempts are not successfully executed. The LEMON rule is one popular rule for assessment for difficult intubation (Table 121-3).

TABLE 121-3 Assessment for Difficult Intubation (Mnemonic: LEMON)

Look Injury, large incisors, large tongue, beard, receding mandible, obesity, abnormal face or neck pathology or shape

Evaluate the 3-3-2 rule Mouth opening < 3 fingers, mandible length < 3 fingers, or larynx to floor of the mandible < 2 fingers

Mallampati Class III (see base of uvula) and class IV (soft palate is not visible)

Obstruction Any upper airway pathology that causes an obstruction (abscess, edema, masses, epiglottitis)

Neck mobility Limited mobility of neck (eg, trauma with cervical spine immobilization, arthritis, congenital defect)

BAG-VALVE-MASK (BVM) VENTILATION The most important skill required for inpatient clinicians in airway management is use of a BVM and airway adjuncts to ventilate and oxygenate the patient. BVM ventilation can effectively maintain airway patency while an alternative plan is developed and implemented. However, patients with a high risk of failing BVM ventilation may require more rapid and definitive airway evaluation and management. Predictors of difficult BVM are summarized in Table 121-4.

TABLE 121-4 Assessment for Difficulty with Bag-Valve-Mask (BVM) Ventilation (Mnemonic: MOANS)

Mask seal Inadequate mask seal (beard, blood/emesis, facial trauma, operator small hands)

Obesity BMI > 26 kg/m2

Age >55 y No teeth No teeth (impairs BVM effectiveness) Stiff ventilation Asthma, COPD, ARDS, term pregnancy

ARDS, acute respiratory distress syndrome; BMI, body mass index; BVM, Bag-valve-mask; COPD, chronic obstructive pulmonary disease.

PROCEDURAL STEPS Rapid sequence intubation (RSI) is now the predominant and preferred method in managing the emergent airway, precluding the apneic patient requiring a crash airway (ie,

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cardiac or respiratory arrest). Rapid sequence intubation is defined as the simultaneous administration of a sedative and paralytic agent to assist in endotracheal intubation, usually via direct laryngoscopy (Table 121-5). Central to the concept of RSI is the avoidance of assisted BVM ventilation to avoid insufflation of the stomach and minimize the risk of aspiration. Outcomes evidence supports RSI as a safe and effective technique for emergency airway management that maximizes the patient and physician likelihood of timely, successful airway management (Table 121-6).

TABLE 121-5 Equipment for Endotracheal Intubation

Endotracheal tubes (assortment of sizes) with stylet Intubation blade (direct or video) Oxygen Bag-valve-mask Suction with Yankuer tip Airway adjuncts (oral and nasal) Confirmation device (end-tidal CO2 detector) Stethoscope Lubricant

TABLE 121-6 Procedural Steps of Rapid Sequence Intubation (RSI)

Preparation Assessment of the airway, adequate IV access, continuous oxygenation monitoring, RSI medications (sedative and paralytic)

Equipment Laryngoscope with functioning light & blades of multiple sizes, working suction, oxygen, Medications (code cart nearby), Backup airway devices, BVM, Monitors (telemetry, pulse oximetry, BP)

Medical Team Engage team of appropriately trained staff; backup nearby, Utilize the assistance of respiratory therapists early, Call for help early

Preparation Anticipate a difficult airway (a complex or challenging intubation) with a backup plan such as fiberoptic intubation

Risk Factors Congenital Pierre Robin syndrome, Down syndrome, anterior epiglottis Acquired Ludwig angina, abscess, epiglottis; RA, AS, scleroderma, temporomandibular joint dysfunction;

Assessment Look for injury, large incisors, large tongue, receding mandible, obesity, abnormal face or neck pathology or shape Evaluate the 3-3-2 rule (LEMON rule) Mouth opening <3 fingers Mandible length <3 fingers

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Adenomas, goiter, lipoma, hygroma, carcinoma tongue, larynx, thyroid; Facial injury, cervical spine injury; Obesity, acromegaly; Active burns, inhalation injury; Subglottic stenosis

Larynx to floor of mandible <2 fingers Mallampati Class III (see base of uvula) Class IV (soft palate not visible) Obstruction Any upper airway pathology that causes an obstruction Neck Mobility Limited neck mobility

Preparation Assess difficulty with bag-valve-mask (BVM) ventilation

Five Independent Risk Factors (MOANS):

1. Inadequate mask seal (beard, blood, emesis, facial trauma, operator small hands)

2. Obesity (BMI > 26 kg/mm3)

3. Age > 55 y 4. Absence of teeth 5. Stiff ventilation (asthma,

COPD, ARDS, term pregnancy)

Difficult oxygenation—the inability to maintain oxygen saturation >90% despite BVM and 100% oxygen RSI by trained operators preferred, but other techniques and backup methods should be considered if difficulty with BVM is predicted

Preoxygenation 100% supplemental oxygen to induce nitrogen washout and maximize time for intubation without oxygen desaturation

Patients will have 7-9 min prior to desaturation in normal, healthy adult; less time in ill patient with comorbidity or critically ill

Premedication (Optional) Administration of drugs 3-5 min before induction and paralysis

To blunt effects of direct laryngoscopy, including bronchospasm and a strong sympathetic response. This step is often omitted

Paralysis Sedatives for induction; paralytics for intubation Sedative regimen should provide reliable amnesia; paralytics ↓ metabolic demands, ↓CO2 production, ↑chest compliance

Sedatives Etomidate …Onset 45-60 s for 5-10 min Propofol …Onset 45-60 s for 5-10 min …Short-acting, allows frequent monitoring of neurologic status Midazolam (Versed)

Side Effects Etomidate Minimal hypotension, possible adrenal insufficiency Propofol Hypotension, depresses myocardial contractility; ↑triglycerides, pancreatitis Midazolam (Versed)

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Unless contraindicated, commonly etomidate for sedation and succinylcholine for paralysis

…Often used in combination with fentanyl bolus …Anticonvulsant properties Fentanyl …Used to reduce pain from laryngoscopy, not always required …Higher potency, faster onset, shorter duration than morphine

Less hypotension than propofol, delirium, slower onset, respiratory depression, long half-life; contraindications: narrow-angle glaucoma Fentanyl Respiratory depression, constipation; contraindications: end-stage liver disease, severe respiratory disease if not intubated Succinylcholine Bradycardia, ↑ICP, histamine release; contraindications:

Paralytics Succinylcholine …First-line for RSI outside of ICU …Rapid onset, short acting Rocuronium …Alternative to succinylcholine …Rapid onset, minimal CV effects

…Hyperkalemia (ESRD, rhabdomyolysis, burns >10% BSA, crush injury) …Neurologic (stroke, spinal cord injury, ALS, MS,↑ICP, history of malignant hyperthermia, eye injury) …Prolonged immobility >48-72 h Rocuronium Caution with difficult airway: longer acting than succinylcholine

Proper Positioning to Optimize Visualization of Vocal Cords

Place a folded towel under the occiput to raise head by ~ 3-7 cm This “sniffing” position lines up the oral, pharyngeal, and laryngeal axes, thereby optimizing the view of the cords during laryngoscopy

Visualize the arytenoids and the vocal cords prior to insertion of endotracheal tube (ETT) by elevating epiglottis which lies just above larynx and vocal cords

Placement of ETT Typically advanced to 23 cm marker at the lip of adult male, 21 cm adult female

Multiple methods to confirm correct placement: …Condensation of ETT …Bilateral breath sounds …Absence of breath sounds over epigastrium …End-tidal CO2 detection

Gold standard for confirming appropriate tube placement: use of end-tidal carbon dioxide detection

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…CXR (ETT tube 2-3 cm above carina)

Postintubation Care Proper stabilization of ETT to prevent movement or accidental dislodgment. Patients who are medically paralyzed require sedation and pain control

Placement of a nasogastric or orogastric tube can help decompress any insufflated air that occurred during BVM use and will help ↓risk of emesis and aspiration

In general, RSI is safer and more successful than awake intubation. However, RSI is not advised if difficulty with BVM is predicted or the ability to intubate via direct laryngoscopy is in question (eg, upper airway obstruction, stridor, angioedema, head and neck cancers). In select patients in which awake intubation is indicated, it should be approached with caution, and may require backup rescue airway methods and/or the involvement of consultants (eg, anesthesiology or otolaryngology).

COMPLICATIONS OF RSI (TABLE 121-7)

TABLE 121-7 Endotracheal Intubation Complications

Directly Related to Laryngoscopy Notes Hemodynamic changes including hypertension, hypotension, tachycardia, and bradycardia

A pneumothorax needs to be considered in a patient with hypoxia and hypotension and should be evaluated for all patients with postprocedure chest radiography

Hypoxemia Routine preoxygenation with high flow oxygen via non-rebreather mask is standard in healthy, nonobese adults. Consider using noninvasive ventilation in critically ill patients with ill patient with compromised lungs or abnormal body habitus. Consider apneic oxygenation

Airway trauma/perforation Laryngospasm and bronchospasm Trauma to teeth, lips, and tongue

Proper technique is essential to avoid any local trauma to oral anatomy and airway structures

Right mainstem bronchus intubation

Evaluate with postprocedure chest radiography

Raised intracranial and intraocular pressure

Unclear clinical significance

Esophageal intubation Prompt recognition of an esophageal intubation will allow immediate removal of the ETT and reventilation and oxygenation with a BVM prior to reattempting intubation

Failed intubation Clinicians should assess patients for a difficult

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airway in an effort to prevent a failed intubation attempt. When a difficult intubation is predicted, consultation should be initiated early and backup equipment should be readily available

Related to Endotracheal Intubation Tension pneumothorax A pneumothorax needs to be considered in a patient

with hypoxia and hypotension and should be evaluated for all patients with postprocedure chest radiography

Aspiration Placement of a nasogastric (NG) or orogastric (OG) tube following endotracheal intubation can help decompress any insufflated air that occurred during BVM use and will help reduce the risk of emesis and aspiration

Obstruction of endotracheal tube Suction the endotracheal tube Accidental extubation Accidental dislodgment of the ETT should be

avoided by proper stabilization of the tube with appropriate sedation of the patient

Various complications can occur during the course of accessing an advanced airway in a patient.

CONTRAINDICATIONS (TABLE 121-8)

TABLE 121-8 Contraindications to Endotracheal Intubation

Notes Absolute Total airway obstruction

(eg, angioedema) Total loss of facial or oropharyngeal landmarks (eg, blunt or penetrating trauma to the face)

During cardiac or respiratory arrest, oxygenation and ventilation are of paramount importance, and therefore the use of BVM, intubation, or both should be attempted despite any contraindications. In these patients it is advised to perform an early cricothyrotomy as endotracheal intubation will be extremely difficult

Relative Anticipated difficult airway

If a difficult airway is anticipated early consultation is strongly advised. Other options include awake intubation, video laryngoscopy or use of the difficult airway adjuncts

Contraindications to endotracheal intubation can be divided into either absolute or relative but these need to be tailored to the specific clinical situation.

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NONINVASIVE VENTILATION (TABLE 121-9)

TABLE 121-9 Noninvasive Positive Pressure Ventilation (NIPPV)

Indications Contraindications Notes COPD (moderate to severe exacerbation) Acute CHF exacerbation, Asthma, Pneumonia in some selected cases

Impending circulatory or pulmonary arrest Altered mental status Inability to handle secretions Recent facial trauma or surgery Recent upper airway or GI surgery (gastric distention) Inability to properly fit mask Inability to adequately monitor patient for decompensation

BIPAP preferred to intubation: ↓need for intubation, ↓LOS, ↓mortality BIPAP, CPAP ↓wall stress, ↓afterload, ↑oxygenation ↓mortality (likely due of ↓VAP) NIPPV not shown to be helpful and may be harmful in following situations: ALI, ARDS Postextubation respiratory failure (↑mortality by delaying intubation) Failure of ABG to improve after 1 h of therapy also highly predictive of subsequent impending respiratory failure

In select patients, NIPPV may result in decreased need for intubation, serious complications, decreased hospital length of stay, and/or improved likelihood of survival to hospital discharge.

SUGGESTED READINGS Bair AE, Filbin MR, Kulkarni RG. The failed intubation attempt in the emergency

department: analysis of prevalence, rescue techniques, and personnel. J Emerg Med. 2002;23:131-140.

Caplan RA, Benumof JL. Practice guidelines for management of the difficult airway: an updated report by the American Society of Anesthesiologists Task Force on management of the difficult airway. Anesth. 2004;101:565.

Cattano D, Paniucci E, Paolicchi A, Forfori F, Giunta F, Hagberg C. Risk factors assessment of the difficult airway: an Italian survey of 1956 patients. Anesth Analg. 2004;99:1774- 1779.

Kheterpal S, Han R, Tremper KK, et al. Incidence and predictors of difficult and impossible mask ventilation. Anesthesiology. 2006;105:885-891.

Masip J, Roque M, Sanchez B, Fernandez R, Subirana M, Exposito JA. Noninvasive ventilation in acute cardiogenic pulmonary edema: systematic review and meta- analysis. JAMA. 2005;294:3124-3130.

Pistoria M, Amin A, Dressler D, McKean S, Budnitz T. Core competencies in hospital medicine. J Hosp Med. 2006;1(S1):87.

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Ram FSF, Picot J, Lightowler J, Wedzicha JA. Non-invasive positive pressure ventilation for treatment of respiratory failure due to exacerbations of

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CHAPTER 122 Arterial Blood Gas and Placement of A-

line

Joseph J. Miaskiewicz, Jr., MD

Critically ill patients require arterial blood gas (ABG) analysis to assess oxygenation and ventilation due to limitations of noninvasive oximetry measurements. Below a pO2 of 60 mm Hg corresponding to an O2 saturation of 80%, the oxyhemoglobin saturation curve is steep and large changes in oximetry may mean small changes in oxygenation. Below this level oximetry may not correlate with oxygenation, and an arterial blood gas (ABG) should be obtained (Table 122-1).

TABLE 122-1 Obtaining an Arterial Sample and Placement of an Arterial Line

  ABG A-Line Indications In hospitalized medical patients,

an ABG is primarily obtained to confirm the severity and likely cause of the disturbance • Level of oxygenation, especially

in settings when the oximeter measurements are thought to be unreliable or difficult to obtain

• Need for intubation: refractory hypoxemia (pO2 < 55 on 100% O2

Usually in the ICU setting for • Frequent ABG sampling • Continuous blood

pressure monitoring in use of inotropic or vasopressor agents

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NRB mask) or hypercapnic respiratory failure (pCO2 > 55 with acidemia pH < 7.25)

• Severity metabolic acidosis and adequacy of respiratory compensation when ↑ work of breathing

• Contribution of ↑pCO2 versus other causes in somnolent patient

Contraindications Impaired collateral circulation • Raynaud • Thromboangiitis obliterans • Cyanosis

Impaired collateral circulation

Preparation Allen test: occlusion of the radial and ulnar arteries by firm pressure while the fist is clenched followed by opening of the hand and release of the arteries one at a time to assess adequacy of returning blood flow to the hand

Assess collateral circulation with Allen test Avoid brachial and femoral arteries (inadequate collateral supplies)

Technical Tips The radial artery at the wrist best site (near the surface, relatively easy to palpate, and stabilize with good ulnar collateral supply)

Apply local anesthetic with 1% lidocaine in the conscious patient Immobilize hand on a wrist board or towel and dorsiflex wrist

Same as for ABG If lose ability to palpate pulse, likely arterial spasm precluding successful cannulation. Wait until subsides or choose another site If unsuccessful, apply pressure for several minutes to avoid hematoma formation (which will make subsequent attempts more difficult) and consider use of ultrasound to visualize vessel Reassess perfusion of hand after placement

Complications Transient obstruction of blood flow may ↓ arterial flow in distal tissues unless adequate collateral arterial vessels available in the setting of

Remove catheter immediately if any sign of vascular compromise

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• Spasm • Intraluminal clotting • Bleeding and hematoma

formation

Use nondominant hand preferred

By measuring both oxygenation and ventilation ABG analysis assesses the effects of the cardiopulmonary system in oxygen delivery. ABG analysis directly measures the pH, pCO2, and pO2. The normal range for the pH is between 7.36 and 7.44 corresponding to a normal range of 36 to 44 torr for the pCO2. The normal range for the pO2 is between 80 and 100 torr. However, age and the pCO2 also determine alveolar O2.

Oximetry does not measure pCO2 and does not reflect ventilation or acid-base status. Ventilation may be defined in terms of movement of a volume of air into and out of the lungs, removing carbon dioxide from the blood and providing oxygen. Alveolar ventilation is defined in terms of ventilation of CO2. High oxygen saturation may be falsely reassuring in patients whose respiratory drive is compromised by an increase of oxygenation due to supplemental O2. Assessment of alveolar ventilation is the key to determining whether a patient is receiving enough oxygen. A raised PaCO2 reflects reduced alveolar ventilation. See Chapter 238 (Acid Base Disorders). An approach to interpreting arterial blood gases is essential when caring for hospitalized patients (Table 122-3).

Respiratory failure is classified as hypoxemic respiratory failure (hypoxemia without carbon dioxide retention [SaO2 < 95%, PaO2 < 80 on room air]) or hypercarbic respiratory failure (pCO2 > 45 mm Hg). Calculation of the gradient between the alveolar and arterial oxygen tensions (the A-a gradient) in respiratory failure will help to determine whether the patient has associated lung disease or just reduced alveolar ventilation (Table 122-2). See Chapter 138 (Acute Respiratory Failure).

TABLE 122-2 Calculation of the A-a Oxygen Gradient from the ABG

The Alveolar-Arterial Oxygen Gradient The A-a oxygen gradient = PAO2 – PaO2 Estimated normal gradient ∼ (Age/4) + 4

The Alveolar Gas Equation PAO2 = (FiO2 × [Patm – PH2O]) – (PaCO2/R) • Inspired air at sea level, the FiO2 of room air = 0.21 • Atmospheric pressure, Patm = 760 mm Hg • PH2O at 37 F = 47 mm Hg • Respiratory quotient, R = 0.8 Hypoxemic Respiratory Failure with Normal A-a Oxygen Gradient • Alveolar hypoventilation (oversedation, obesity hypoventilation syndrome, muscular

weakness, neurologic disease) • High altitude (low inspired FiO2)

Hypoxemic Respiratory Failure with ↑ A-a Gradient

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• Ventilation-perfusion mismatch (pulmonary embolism, COPD, ARDS, pulmonary artery vasospasm)

• Right-to-left shunt (anatomic: cardiac, pulmonary AVM, hepatopulmonary syndrome; physiologic due to fluid preventing ventilation of perfused alveoli: pneumonia, atelectasis)

Disorders of the lung structure reduce the efficiency of oxygen transfer and widen the A-a gradient. The prolonged respiratory depression may lead to collapse of some areas of lung and an increase in the A-a gradient. Hypercarbic Respiratory Failure, Hypoxemia from Impaired Ventilation with Normal A-a Oxygen Gradient • Inadequate alveolar ventilation (without shunting from fluid or collapse of alveoli) • Ventilatory pump failure (respiratory muscle weakness, neurolgic disease, thoracic cage

issues)

TABLE 122-3 Blood Gas Interpretation

Step 1: Acid-base (ventilation) pH PaCO2 Interpretation

↓ ↑ In acute respiratory failure the change in pH will be accounted for by the high carbon dioxide concentration.

↓ ↓ A severe metabolic acidosis or some limitation on the ability of the respiratory system to compensate.

Normal ↑ Alveolar hypoventilation (raised PaCO2) with a normal pH most likely a primary ventilatory change present long enough for renal mechanisms to compensate. Increased serum bicarbonate may also be a clue of chronic CO2 retention. A similar picture may result from carbon dioxide retention due to reduced ventilation compensating for a metabolic alkalosis, although such compensation is usually only partial.

Normal ↓ A primary metabolic acidosis in which the respiratory system has normalized the pH. Calculate the anion gap.

↑ ↓ Acute alveolar hyperventilation if the pH is appropriately raised for the reduction in PaCO2. Chronic alveolar hyperventilation if the pH is between 7.46 and 7.50 as the renal system seldom compensates completely for an alkalosis.

Step 2: Oxygenation (pO2, %saturation) pO2 PaCO2 pH   Normal Normal ↑ A primary metabolic alkalosis

to which the ventilatory system has not responded.

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↓ Normal ↓ Hypoxemia: when patients with chronic CO2 retention increase usual level of ventilation (acute pulmonary embolism in chronic lung disease).

Step 3: Calculate the A-a gradient to determine whether carbon dioxide retention is related to an intrapulmonary cause A-a Explanation Etiology Gradient Calculating the A-a gradient is

most useful for determining the severity of the underlying disorder and whether there is a component of hypoventilation.

Especially for hospitalized patients who are prescribed medications that may suppress respiration, the A-a gradient is used to determine the relative contribution of hypoventilation to hypoxia due to underlying lung disease.

Normal A normal A-a gradient is ∼10- 15 torr. Advancing age results in increases of the normal A-a gradient. A-a gradient = 2.5 + 0.21 × age in years.

The ABG abnormality is all due to hypoventilation.

Elevated An elevated A-a gradient represents ↑ difficulty in getting O2 from the alveoli to the blood. A higher FiO2 disproportionately increases the PAO2 more than the PaO2.

• Diseases that affect the pulmonary interstitium including interstitial lung disease, pneumonia, and CHF.

• Pulmonary vascular disease: pulmonary emboli, shunts, pulmonary hypertension.

• Ventilation/perfusion mismatches of large vessels (pulmonary or tumor emboli) and small vessels (pulmonary hypertension, vasculitis, interstitial lung disease and emphysema).

• When breathing 100% oxygen, older patients may normally have an A-a gradient as high as 80 torr and younger patients as high as 120 torr.

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Step 4: Does the result correlate with the clinical setting? Possible Source of Error Prevention Presence of heparin in syringe

Express any heparin out of syringe prior to sampling

Air bubbles (resulting in equilibrium between air and arterial blood: ↓PaCO2, ↑PaO2

Inspect sample and remove air bubbles

Inadequate sample Obtain at least 3 mL aterial blood Metabolically active cellular constituents of blood (resulting in changing arterial gas tensions over time)

Cool sample on ice Analyze sample within 1 h

Sampling of venous blood

Pay attention to technique

Neither oximetry nor ABGs will detect the presence of a reduced O2-carrying capacity because anemia, and carbon monoxide (CO) poisoning, and methemoglobinemia do not affect the alveolar pO2. When there is CO poisoning, the oximeter cannot differentiate between hemoglobin molecules with CO attached and those with O2 attached and will report normal O2 saturation. ABGs will also report normal values because the PaO2 is a measurement of the oxygen dissolved in the blood and not the number of

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