Revisiting respiratory failure

Respiratory failure continues to be a challenging condition for physicians. Some issues include:

  • Distinction between, and clinical diagnostic criteria for, acute and chronic respiratory failure
  • Recognition of an acute exacerbation of chronic respiratory failure
  • Classification of acute respiratory failure as hypoxemic or hypercapnic
  • Identification of respiratory failure as a post-procedural complication
  • Necessity of precise diagnostic terminology for the correct coding of acute and chronic respiratory failure
  • Numerous clinical and non-clinical consequences of documenting the diagnoses of acute and chronic respiratory failure
Image by Thinkstock
Image by Thinkstock.

Respiratory failure is commonly defined as respiratory dysfunction resulting in abnormalities of oxygenation and/or carbon dioxide (CO2) elimination and is classified as either hypoxemic (type I) or hypercapnic (type II), or a combination of both. These distinctions are clinically important and have diagnostic and therapeutic implications, but current coding rules consider them “non-essential” terms that do not affect the code assigned. Physicians won't be required to use them with ICD-10, either, though the coding system will allow for these distinctions.

Respiratory failure occurs frequently in association with chronic obstructive pulmonary disease (COPD), heart failure, pneumonia, and sepsis and after cardiac arrest. The correct diagnosis is essential to accurately portray a patient's severity of illness and influences quality scores, performance indicators, clinical outcome measures and hospital revenue. Even chronic respiratory failure contributes to severity classification.

However, non-specific terms (such as hypoxia, severe dyspnea, respiratory insufficiency or distress) result in the assignment of codes that do not reflect any significant respiratory problem even when, as an extreme example, endotracheal intubation is necessary.

Chronic respiratory failure

Chronic respiratory failure is characterized as a combination of hypoxemia, hypercapnia and hypercarbia (renal compensation for hypercapnia by retention of bicarbonate). In the absence of an acute exacerbation, the pH level will be normal (7.35-7.45). The treatment of chronic respiratory failure includes chronic supplemental oxygen therapy (“home O2”), so the diagnosis should apply to all patients who receive this.

It's important to document chronic respiratory failure for coding purposes because it is always considered a significant comorbidity contributing to a patient's severity of illness, even if it seems clinically intrinsic to another condition like severe COPD and treatment is unaffected.

Any degree of respiratory acidosis (hypercapnia with pH <7.35) indicates an acute decompensation in these patients— so-called acute-on-chronic respiratory failure. The latter term is preferred for severity classification, because “respiratory acidosis” results in an incorrect code that will not reflect the presence of respiratory failure.

Acute respiratory failure

Acute respiratory failure is defined by any one of the following:

  • pO2 <60 mm Hg or SpO2 (pulse oximetry) <91% breathing room air
  • pCO2 >50 and pH <7.35
  • P/F ratio (pO2 / FIO2) <300
  • pO2 decrease or pCO2 increase by 10 mm Hg from baseline (if known)

A pO2 less than 60 mm Hg measured by arterial blood gas (ABG) on room air is the “gold standard” for the diagnosis of acute hypoxemic respiratory failure (excluding patients with chronic respiratory failure whose baseline pO2 is often less than 60 mm Hg). On the normal oxygen/hemoglobin dissociation curve, a pO2 less than 60 mm Hg is equivalent to oxygen saturation less than 91%. While the saturation measured by pulse oximetry (SpO2) is less precise than on the ABG (SaO2), it may be used as the only practical surrogate for serial monitoring of oxygenation.

It is also well known that many metabolic factors influence (“shift”) the oxygen/hemoglobin curve, altering the relationship between pO2 and oxygen saturation. Nevertheless, in the absence of serial ABGs, the SpO2 represents the only available measurement for estimating the pO2. These limitations should be recognized and acknowledged, but do not invalidate the clinical utility, and practice, of applying the SpO2 as an indicator of the degree of hypoxemia (pO2 equivalency).

Acute hypercapnic respiratory failure is defined by a pCO2 greater than 50 mm Hg in association with a pH less than 7.35. It occurs commonly in COPD patients and others with impaired respiratory drive (such as head trauma and drug overdose) sometimes even without hypoxemia. The acidosis is caused by the acute retention of carbon dioxide (increasing pCO2). On the other hand, an elevated pCO2 with normal pH is characteristic of stable chronic respiratory failure.

Management requiring endotracheal intubation and mechanical ventilation or initiation of bilevel positive airway pressure nearly always means the patient has acute respiratory failure, but this is obviously not required for the diagnosis. Similarly, providing 40% or more supplemental oxygen implies that the physician is treating acute respiratory failure since only a patient with that disorder would need that much oxygen.

There ought to be some indication in the record that a patient with acute respiratory failure has, for example, respiratory distress (even if mild), tachypnea (normal respiratory rate is generally 8-16), dyspnea, shortness of breath, wheezing, etc. Physicians, however, often use the unnecessary and frequently contradictory term “no acute distress” (“NAD”) to describe these and other acutely ill patients. Doing so can create an appearance of being out of touch with the patient's condition and serve as grounds for auditors to dispute the diagnosis. Think too of the risk one may be taking by using “NAD” when something goes wrong. If the patient truly has “no distress,” why is he an inpatient at all? Why is she not on observation? The term probably should be abandoned altogether, since today it's important to focus on the severity and acuity of a patient's condition.

In summary, always consider acute respiratory failure in patients who have any difficulty breathing, especially those with COPD, congestive heart failure or pneumonia. Document the diagnosis when one of the four clinical criteria is present. Identify chronic respiratory failure, usually in patients with COPD on “home” oxygen. Avoid describing acutely ill patients as having “no acute distress” (“NAD”).

Next month's column will deal with the importance and power of the P/F ratio, the diagnosis of acute-on-chronic respiratory failure, the nuances of post-procedural respiratory failure, and the methods and relationships of supplemental oxygen administration.