Lungs

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


Table 60. Pulmonary function tests.

TLC (total lung capacity) ~
RV (residual volume) ~
FRC (functional residual capacity) ~
VC (vital capacity) ~
FEV1 (forced exp. vol. in 1sec) ~ ~
MIP (mean inspiratory pressure) ~ ~
MEP (mean expiratory pressure) ~ ~

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.



The Massachusetts General Hospital. Handbook of Neurology
The Massachusetts General Hospital Handbook of Neurology
ISBN: 0781751373
EAN: 2147483647
Year: 2007
Pages: 109

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