Pediatric Respiratory Failure Workup

Arterial blood gas (ABG) measurement can be used to define acute respiratory failure. Arbitrary definitions include a partial pressure of CO₂ (PaCO₂) greater than 50 mm Hg, a partial pressure of oxygen (PaO₂) less than 60 mm Hg, or arterial oxygen saturation less than 90%. An elevated serum bicarbonate level suggests metabolic compensation for chronic hypercapnia.

A complete blood count (CBC) may be helpful. Polycythemia suggests chronic hypoxemia.

Electrolyte abnormalities can contribute to weakness; hypokalemia, hypocalcemia, and hypophosphatemia can impair muscle contraction.

Calculate the alveolar-arterial oxygen difference ([A-a]DO₂), which is the difference between the alveolar PAO₂ and the arterial PaO₂. This value is an index of the efficiency of gas exchange by the lungs.

The alveolar gas equation is used to calculate the PAO₂ on the basis of the relationship between the pressure of oxygen in inspired gas (PiO₂), the PaCO₂, and the respiratory quotient (RQ), as follows: PAO₂ = FiO₂ (Pb - P_H 2O) - (PaCO₂/RQ).

PiO₂ is a function of the fractional concentration of inspired oxygen (FiO₂), the barometric pressure (Pb), and the partial pressure of water vapor (P_H 2O) in humidified air.

RQ is the ratio of the volume of carbon dioxide expired to the volume of oxygen consumed by an organism. The body normally produces approximately 200 mL of carbon dioxide per minute and consumes approximately 250 mL of oxygen per minute; therefore, RQ is 0.8. Different fuel sources produce different RQ values: the RQ for carbohydrates is 1; protein is 0.8; and fat is 0.7.

In children, (A-a)DO₂ is normally 5-10 and reflects venous admixture from anatomic right-to-left shunts, which include the bronchial circulation, thebesian veins, and small arteriovenous anastomoses in the lung.

The PaO₂/FiO₂ ratio is a commonly used indicator of gas exchange. A PaO₂/FiO₂ less than 200 is correlated with a shunt fraction greater than 20%. For ventilated patients, a similar calculation is called the oxygen index, calculated by (PaO₂ x FiO₂/mean airway pressure) x 100. These numbers are used to quickly communicate the severity of respiratory failure and can provide some diagnostic and therapeutic guidance (eg, when to start inhaled nitric oxide).

Imaging studies may include plain radiography or computed tomography (CT), or magnetic resonance imaging (MRI) scans. Fluoroscopy is valuable to evaluate the movement of the diaphragms and dynamic obstructive lesions of both the extrathoracic and intrathoracic airway. Ventilation/perfusion (V/Q) scanning can predict a probability of V/Q mismatch secondary to a pulmonary embolism.

Radiography

Lateral and anteroposterior (AP) radiographs of the neck can reveal a radiopaque foreign body or soft-tissue structures encroaching on the lumen of the airway, such as in acute epiglottitis.

Chest radiographs may yield helpful findings (see examples in the images below).

Evaluate for abnormalities that require immediate intervention (eg, malpositioned endotracheal tube, pneumothorax).

Common findings associated with respiratory failure include the following:

• Focal or diffuse pulmonary disease (eg, pneumonia, ARDS)
• Bilateral hyperinflation (eg, asthma)
• Asymmetric lung expansion suggesting a bronchial obstruction
• Pleural effusion
• Cardiomegaly

If hypoxemia is present but the chest radiograph is clear, this finding could suggest cyanotic congenital heart disease, pulmonary hypertension, or pulmonary emboli.

CT and MRI

Chest CT scanning can be performed when sophisticated diagnostic images are needed. It can further define radiopacities due to vascular, pleural, interstitial, or airway lesions.

Airway CT scanning, MRI, and/or angiography can be used to differentiate deep-tissue structures, bone lesions, and vascular abnormalities.

Useful information may be provided by determination of dead-space volume to tidal gas volume (V_D/V_T) and determination of the intrapulmonary shunt fraction (Q_s/Q_t).

Determination of dead-space volume to tidal gas volume

V_D/V_T is based on the difference between PaCO₂ and the CO₂ in exhaled gas (PeCO₂). PeCO₂ is measured by collecting expired gas in a large collection bag and using an infrared CO₂ analyzer to measure the PCO₂ in a sample of gas.

In a normal lung, the capillary blood equilibrates fully with alveolar gas; therefore, the PeCO₂ approximates the PaCO₂. As V_D/V_T increases, the PeCO₂ falls below PaCO₂.

Reference range V_D/V_T is approximately 0.30.

V_D/V_T = (PaCO₂ - PeCO₂)/PaCO₂

Determination of the intrapulmonary shunt fraction

Q_s/Q_t is the ratio of shunted flow (Q_s) to the total flow or cardiac output (Q_t). It is derived by the relationship between the oxygen content in arterial blood (CaO₂), mixed venous blood (CvO₂), and pulmonary capillary blood (CcO₂) while breathing FiO₂ that equals 1.

Arterial oxygen content (in mL O₂/dL) = [1.34 mL O₂/g hemoglobin × hemoglobin (in g/dL) × SpO₂] + [PaO₂ (in mm Hg) × 0.003 mL O₂/dL/mm Hg].

Directly measuring pulmonary capillary blood (CcO₂) is difficult; therefore, CcO₂ is assumed to be 100% when FiO₂ equals 1.

The normal intrapulmonary shunt is less than 10%.

Q_s/Q_t = (CcO₂ - CaO₂)/(CcO₂ - CvO₂)

Bronchoalveolar lavage (BAL) is performed to identify a specific infectious pulmonary pathogen; bacterial, viral, and acid-fast bacillus (AFB) cultures and silver stains can be performed. BAL can also be performed to isolate lipid-laden macrophages (suggestive of recurrent aspiration) or pulmonary hemorrhage.

In an intubated patient, samples can be obtained blindly or bronchoscopically.

BAL is indicated in critically ill children to guide antimicrobial therapy and in children whose conditions have deteriorated during therapy.

Lung biopsy may be indicated if BAL does not reveal a pathogen, especially in immunocompromised hosts; it can identify Aspergillus species or Pneumocystis jiroveci. Lung biopsy is also helpful in the diagnosis of sarcoidosis and other granulomatous conditions.

Electromyography (EMG) or nerve conduction testing can help determine the etiology for neuromuscular weakness leading to respiratory pump failure.

Fiberoptic and rigid bronchoscopy can be performed to assess large and small airways for anatomic abnormalities or foreign bodies.

Nasal airflow tracings coupled with chest-movement recordings (pneumograms) have a specific role in identifying sleep-associated extrathoracic airway obstruction and respiratory control abnormalities.

Thoracentesis is used in patients with pleural effusions, to check the cell count and protein level to determine whether pleural fluid is an exudate or transudate. Other pleural fluid studies include measurement of triglycerides, to determine whether the effusion is chylous, and bacterial and acid-fast bacterial (AFB) cultures. Cytology is used to evaluate for malignant effusions.

Test of respiratory mechanics and lung-volume measurements are most beneficial in following the progression of disease and the effects of treatment over time. Many infants and children cannot cooperate with traditional pulmonary function measurements. Many contemporary pediatric ventilators incorporate sophisticated sensors and software that measure inhaled and exhaled breaths and can display pulmonary flow loops and other pulmonary parametrics. This provides valuable information regarding real-time, as well as trended, pulmonary dynamics.