The Modified Bohr Equation

PICU Doc On Call

Hosts:

• Pradip Kamat, Children’s Healthcare of Atlanta/Emory University School of Medicine
• Rahul Damania, Cleveland Clinic Children’s Hospital

Case Introduction:

• 6-year-old patient admitted to PICU with severe pneumonia complicated by pediatric Acute Respiratory Distress Syndrome (pARDS).
• Presented with respiratory distress, hypoxemia, and significant respiratory acidosis.
• Required intubation and mechanical ventilation.
• Despite initial interventions, condition remained precarious with persistent hypercapnia.

Physiology Concept: Dead Space

• Defined as the volume of air that does not participate in gas exchange.
• Consists of anatomic dead space (large airways) and physiologic dead space (alveoli).
• Physiologic dead space reflects ventilation-perfusion mismatch.

Pathological Dead Space:

• Occurs due to conditions disrupting pulmonary blood flow or ventilation.
• Common in conditions like pulmonary embolism, severe pneumonia, or ARDS.

Clinical Implications:

• Increased dead space fraction (DSF) in PARDS is a prognostic factor linked to severity and mortality.
• Elevated DSF indicates worse lung injury and inefficient gas exchange.
• DSF can be calculated using the formula: DSF = (PaCO2 – PetCO2) / PaCO2.

Practical Management:

• Optimize Mechanical Ventilation
• Enhance Perfusion
• Consider Positioning (e.g., prone positioning)

Summary of Physiology Concepts:

• Bohr equation for physiologic dead space.
• Importance of lung-protective ventilation strategies.
• Monitoring and trending dead space fraction.
• Strategies to improve airway patency and mucociliary clearance.

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• Hosted by Dr. Pradip Kamat and Dr. Rahul Damania.

Reference:

• Yehya N, Bhalla AK, Thomas NJ, Khemani RG. Alveolar Dead Space Fraction Discriminates Mortality in Pediatric Acute Respiratory Distress Syndrome. Pediatr Crit Care Med. 2016 Feb;17(2):101-9. doi: 10.1097/PCC.0000000000000613. PMID: 26669646; PMCID: PMC4740261.