The Massachusetts General Hospital Handbook of Neurology

Authors: Flaherty, Alice W.; Rost, Natalia S.

Title: Massachusetts General Hospital Handbook of Neurology, The, 2nd Edition

Copyright 2007 Lippincott Williams & Wilkins

> Table of Contents > Medicine > Lungs

Lungs

A. Blood gases

• A-a gradient: Alveolar-to-arterial difference in oxygen tension. Lowered by diffusion defects, R-L shunts (these do not correct on 100% O₂), ventilation-perfusion (V/Q) mismatch.

  • Definitions: PA O₂ = alveolar oxygen tension, PaO₂ = arterial oxygen tension, pCO₂ = arterial carbon dioxide tension, FiO₂ = fractional concentration of oxygen in inspired gas, RQ = respiratory quotient = ratio of CO₂ out to O₂ in (normally 0.8).

  • A-a = PAO₂ - PaO₂. Normal = 3 16 mm Hg.

    • Age correction: A-a should be < 2.5 + 0.25 (age). If pt. <30 years old, A-a should be <8 mm Hg.

  • PAO₂ = (FiO₂)(barom. - water vapor pressure) pCO₂/RQ, so: PAO₂ = (FiO₂)(713) pCO₂/0.8

    • On room air: (FiO₂)(713) = 143; nl PAO₂ = 100 mm Hg.

    • On 100% ventilation: nl PAO₂ = 673 mm Hg. This does not apply to 100% oxygen by face mask.

    • PaO₂ >70 gives full hemoglobin saturation.

  • FiO₂ increase of 10% adds 5 to PaO₂.

• Respiratory effects on pH: See also Electrolytes, p. 194.

  • High pH: Suggests hyperventilation. Rule out CHF, PTX, PE.

  • Normal pH + high pCO₂: Suggests chronic CO₂ retention.

  • Low pH: Suggests acute hypoxia or sepsis.

P.218

B. Acute dyspnea or hypoxia

• H&P: Check VS, temperature, alertness, respiratory rate and pattern, listen to lungs, heart; assess jugular distension, ankle swelling, ask about pleuritic pain.

• DDx: Pulmonary edema (cardiac, vasogenic, neurogenic), COPD, MI, PE, PTX, effusion, upper airway obstruction, lung hemorrhage, hypoventilation from drugs or CNS problem, hyperventilation from acidosis or anxiety.

• Tests:

  • Blood gas: On room air if possible. See above for interpretation. Hct performed on blood gas is sometimes inaccurate.

  • D-dimer: Will be high in PE, but very nonspecific.

  • Portable CXR.

  • EKG: Look for ischemia; signs of PE (see below).

  • Lung scan: Consider PE-CT, V/Q scan (if CXR clear), or Pagram plus possible IVC filter placement.

• Rx: L-M-N-O-P : Lasix, Morphine, Nitrates, Oxygen, and Posture. Avoid low BP, especially in pts. with cerebrovascular dz.

  • Lasix: For exacerbation of chronic CHF, but not for flash pulmonary edema.

  • Morphine: For hyperventilation in pain and decreased preload in CHF. Beware of somnolence.

  • Nitrates: For pulmonary edema and cardiac ischemia.

  • Oxygen: Avoid causing CO₂ retention in COPD. Consider mechanical ventilation (see p. 220).

  • Posture: Have pt. sit up, with legs dangling.

  • Benzodiazepines: For anxiety. They can help prevent breath stacking in COPD, but beware of somnolence. If quaaludes were still used for anxiety, the mnemonic could have been L-M-N- O-P-Q.

C. COPD flare

• H&P: Baseline FEV₁ (see pFTs, p. 219), functional limitations, pack-years of smoking, home oxygen, previous intubations. Lung and heart exam.

• DDx: See Acute dyspnea (see above).

• Tests: ABG, CXR, EKG, K, Mg, CBC, theophylline level. Sputum for gram stain, culture. If suspect TB: PPD, sputum for AFB 3. Consider sputum for cytology 3.

• Drugs: Write order for inhalers, not nebulizers, if pt. intubated.

  • Albuterol nebulizer 0.5 cc in 2.5 cc NS q2-4h.

  • Ipratropium inhaler 2 puffs qid or glycopyrrolate nebulizer 0.8 cc in 2.5 cc NS q6-8h.

  • Methylprednisolone: 60 mg IV q6h, then taper (eventually switch to inhaled steroids). Ranitidine or sucralfate while on steroids.

  • Empiric Abx: If signs of airway infection, e.g., increased mucus, fever, etc. See Infection, p. 212.

• Other orders: Guaiac stools if on steroids. Chest physical therapy. Check peak flow.

D. Lung mass workup

Chest CT through the adrenals; consider needle biopsy or bronchoscopy.

E. Pulmonary function tests (PFTs)

P.219

F. Pulmonary embolus (PE)

• Risk factors: Trauma, surgery, prior PE, bed rest, cancer, CHF, hypercoagulable states, oral contraceptives.

• Prophylaxis: If not already anticoagulated. Heparin 5000 U SQ bid-tid or enoxaparin 40 mg SQ qd for low risk, 30 mg SQ bid for high risk, and pneumo-boots. If pt. has been in bed more than 24 h, check venous ultrasound before starting boots.

• H&P: Dyspnea, anxiety, tachycardia, pleuritic pain, cough.

• DDx of PE:

  • Small PE: Pneumothorax, hyperventilation, asthma, MI, CHF, serositis.

  • Big PE: RV infarct, tamponade, air embolism.

• Tests:

  • General labs: ABG, D-dimer often high if recent trauma or surgery (low D-dimer is highly specific; high D-dimer is not), CXR.

  • EKG: Often see only sinus tachycardia. See also Electrocardiogram, p. 207.

  • Venous ultrasounds (LENIs): 97% sensitive for thrombus above the knee, 80% below. Not good for pelvic clots.

  • Ventilation-perfusion (V/Q) scan: Consider going straight to pulmonary angiogram if high risk. V/Q scan may be useless if CXR is abnormal or there has been a previous PE.

  • Pulmonary angiogram: Pulmonary HTN and renal failure are relative contraindications.

• Rx of PE:

  • Heparin: (Unless contraindication, e.g., recent brain surgery) With bolus, for 5-7 days, then warfarin, or SQ heparin bid for at least 3 months, or longer if idiopathic or recurrent.

  • Thrombolyse: If life-threatening PE.

  • Inferior vena cava filter: For recurrent PE or pt. who cannot be anticoagulated.

G. Stridor or laryngeal edema

See Allergy, p. 191.

H. Ventilators and oxygen delivery

• Nonendotracheal oxygen delivery systems:

  • Passive ventilation: Nasal cannula, face mask or face tent, 100% nonrebreather.

  P.220

  • Partially assisted ventilation:

    • Continuous positive airway pressure (CPAP): Indications: sleep apnea, or to keep alveoli open in COPD.

    • Biphasic positive airway pressure (BIPAP): AKA intermittent positive pressure breathing. Indications: chronic neuromuscular dz; not great for COPD.

• Endotracheal intubation: Call anesthesia, respiratory therapy.

  • Indications: To maintain oxygenation or ventilation, protect airway or manage secretions, or to allow adequate sedation. Remember that intubation makes it hard to follow pts. neuro exam.

  • Anesthetics for intubation: Most wear off fast, but neurological abnormalities may linger longer. see p. 158 and 173.

  • Stat CXR after intubation: Endotracheal (ET) tube should be below thoracic inlet and >2 cm above carina. Check CXR daily.

• Ventilator setting orders: Specify FiO₂, TV, IMV, PEEP, PS, and type of oxygen monitoring (e.g., continuous saturation monitor).

  • Typical ventilator settings:

    • Oxygen (FiO₂): 40% (start at 100%).

    • Tidal volume (TV): 600-800 cc (7-10 cc/kg). More if COPD, less if restrictive lung dz.

    • Respiratory rate (RR): 8-12. Less if COPD, more if restrictive dz.

    • Positive end expiratory pressure (PEEP): Start at 5.

    • Pressure support (PS): 10-15. PS ~8 just overcomes resistance of ET tube.

• Ventilator modes:

  • Intermittent mandatory ventilation (IMV, SIMV): For pts. without adequate spontaneous minute ventilation. Specify RR and TV. Can be used with backup pressure support for pts. spontaneous breaths.

  • Pressure support (PS, PSV): For pts. with adequate spontaneous respiratory drive. Specify PS only; adjust so that TV is 7-10 cc/kg. Mode has apnea alarm so pt. will get a backup IMV breath if necessary.

  • Pressure control (PC, PCV): For severe COPD, adult respiratory distress syndrome. Set RR, insp. pressure, and insp. time. It is like PS, but maintains a minimum RR. Pt. usually needs sedation and paralysis to tolerate PC.

• Changing ventilator settings: In some hospitals, only respiratory technicians are allowed to do this.

  • Ventilation increases (and thus pCO₂ decreases) with increased PS, TV, RR.

  • Oxygenation increases with increased PEEP, FiO₂, or insp. time unless there is a significant pulmonary or cardiac shunt).

    • To follow oxygenation across different FiO₂: Use pO₂/FiO₂ as index. Normal = 100/0.2 = 500.

  • PEEP vs. PS: PEEP is like CPAP, PEEP + PS is like BIPAP.

• Ventilator complications:

  • Infection: Most pts. get colonized; do not treat presence of bacteria in sputum with Abx unless abundant PMNs in sputum, fever, high WBC, or copious secretions.

  P.221

  • Tracheal stenosis: Associated with prolonged intubation. Generally plan tracheostomy after 21 days.

  • Oxygen toxicity: Try to keep FiO₂ below 50%.

  • Gastric distension: From ET tube leak. Place NG tube.

  • Blood pressure changes: High intrathoracic pressure lowers BP.

    • Intubation: Treat with IV fluids but remember to diurese pt. at extubation to avoid fluid overload.

    • COPD: Inadequate exhalation (breath stacking) causes auto-PEEP and may be helped by decreasing RR.

  • Barotrauma: From high peak inspiratory pressure.

  • Pneumothorax.

• Extubating:

  • Criteria: Pt. on room air, PEEP (= CPAP) 5, no IMV, should have RR <30, few secretions, PIP <40, normal blood gas.

  • Preparation: Empty stomach. HOB 45 degrees. Tell the respiratory technician. Extubate to 100% O₂, continuous O₂ sat monitor. Watch for laryngeal edema (see p. 191).

• Tracheostomy wean: Consult anesthesia or surgery, speech path.

  • Indications: Pt. can tolerate breathing off the ventilator, and no need for frequent suctioning.

  • Methylene blue test: Put dye in tube feeds to r/o aspiration.

  • Tracheostomy types: Usually pts. first trach is an 8 Portex tube, sutured. This may later change to a 4 cuffless Shiley tube with Passy-Muir valve. Consider a fenestrated tracheostomy.

I. Button tracheostomy

If pt. tolerates the tracheostomy wean. If there is no need for suctioning, you may eventually decannulate.