CBC, BMP, CMP, coagulation studies, cultures, and critical lab values for respiratory care.
55 min estimated
The CBC provides critical information about oxygen-carrying capacity, infection, and coagulation.
Key values: - Hemoglobin (Hgb): M 13.5โ17.5 g/dL, F 12โ15.5 g/dL. Low = anemia โ reduced O2 carrying capacity โ increased work of breathing - Hematocrit (Hct): M 41โ53%, F 36โ46%. Rule of 3: Hct โ Hgb ร 3 - WBC: 4,500โ11,000/ฮผL. Elevated = infection/inflammation; neutropenia <1,500 = high infection risk - Platelets: 150,000โ400,000/ฮผL. <50,000 = bleeding risk; <20,000 = spontaneous bleeding - Bands/neutrophils: left shift (increased bands) = acute bacterial infection - Polycythemia: Hgb >17 (M) or >15 (F) โ seen in chronic hypoxia (COPD), sleep apnea
Clinical Tip
In a patient with Hgb of 7 g/dL and respiratory distress, adding supplemental O2 helps but treating the anemia (transfusion) will dramatically improve O2 delivery.
The Basic Metabolic Panel (BMP) includes sodium, potassium, chloride, bicarbonate, BUN, creatinine, and glucose.
Respiratory implications: - Potassium (K+): normal 3.5โ5.0 mEq/L. Hypokalemia (<3.5) causes respiratory muscle weakness and arrhythmias; worsened by albuterol (shifts K+ intracellularly). Hyperkalemia (>5.5) โ peaked T waves, arrhythmias - Sodium (Na+): normal 135โ145 mEq/L. Hyponatremia <120 can cause respiratory depression - Bicarbonate (HCO3): normal 22โ26 mEq/L โ reflects metabolic component of acid-base - Creatinine: normal 0.7โ1.3 mg/dL โ indicates renal function; elevated = AKI, reduced drug clearance - Magnesium: 1.7โ2.2 mg/dL; hypomagnesemia causes bronchospasm and arrhythmias
Clinical Tip
Check phosphate levels in ICU patients who are failing ventilator weaning โ severe hypophosphatemia causes diaphragm weakness and makes extubation impossible.
Identifying infection is critical in respiratory care. Key markers:
- WBC with differential: leukocytosis + left shift = bacterial; lymphocytosis = viral - Procalcitonin (PCT): bacterial infections โ elevated (>0.5 ฮผg/L suggests bacterial; >2 = likely); viral/non-infectious โ low. Used to guide antibiotic therapy duration - CRP (C-reactive protein): nonspecific inflammatory marker; useful for trending - Lactate: >2 mmol/L = hypoperfusion (sepsis screen); >4 = severe sepsis - Cultures: blood cultures ร2 before antibiotics; sputum culture (>25 PMNs, <10 squamous cells = adequate sample); BAL โฅ10โด CFU/mL = VAP - Urine Legionella antigen: for atypical pneumonia - BNP/NT-proBNP: elevated in heart failure โ differentiates cardiac vs pulmonary edema
Clinical Tip
In a patient with bilateral infiltrates and hypoxemia, a BNP level can help differentiate cardiogenic pulmonary edema (high BNP) from ARDS/pneumonia (normal or mildly elevated BNP).
Failing to Wean from Ventilator
A post-op patient on day 5 of mechanical ventilation fails repeated SBTs. She is awake, cooperative, FiO2 0.35, PEEP 5. Recent lab: Phosphate 1.0 mg/dL (normal 2.5โ4.5).
What lab abnormality is contributing to ventilator dependence and how should it be treated?
Hypophosphatemia causing respiratory muscle weakness. Treat with IV phosphate replacement.
Phosphate is essential for ATP production in muscle cells including the diaphragm. Severe hypophosphatemia (<1.0 mg/dL) causes profound respiratory muscle weakness, making SBT impossible. Replace IV phosphate (sodium or potassium phosphate) and recheck before next SBT attempt.
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