Introduction to respiratory medicine

Differentials of respiratory distress

• Most common
  • Obstructive airway disease
  • CCF
  • Ischaemic heart disease (unstable angina/MI)
  • Pneumonia
  • Psychogenic
• Most immediately life-threatening
  • Upper airway obstruction
  • Tension PTX
  • PE
  • Neuromuscular: GBS, myaesthenic crisis, botulism
  • Fat embolism

Differentials of hypoxia

• Hypoventilation (normal A-a)
• Shunt
• VQ mismatch
• Diffusion abnormality
• Low atmospheric O2

Differentials of hypercarbia

• Depressed central drive
  • Structural CNS disease e.g. tumour
  • Drug depression
  • Endogenous toxins e.g.tetanus
• Thoracic cage disorders
  • Kyphoscoliosis
  • Morbid obesitsy
• Neuromuscular impairment
  • Neuromuscular disease: Myaesthenia gravis, GBS
  • Neuromuscular toxin: Botulism, organophosphate poisoning
• COAD
• Upper airway obstruction

Respiratory fatigue

• Examination
  • Rapid shallow respirations (decreases respiratory work for given minute volume)
  • Paradoxical respirations (abdomen draws in on inspiration)
  • Respiratory alternans (abdomino-diaphragmatic alternation)
  • Increased PCO2
  • Bradypnoea as late sign

Maximise neuromuscular function

• Avoid neuromuscular blocking agents e.g. aminoglycosides
• Correct hypo- or hyperkalaemia
• Treat severe hypo- or hypermagnesaemia
• Treat dehydration
• Aminophylline improves respiratory muscle function
• Positioning

Nasal prongs

• Flow
  • 1L/min = 0.24
  • 2L/min = 0.28
  • 3L/min = 0.32
  • 4L/min = 0.36
  • 6L/min = 0.5

Hudson mask

• Claustrophobic and hot
• Flow rate minimum 6L/min to prevent re-breathing
• Flow
  • 6L/min = 0.45
  • 7L/min = 0.50
  • 8L/min = 0.55
  • 9L/min = 0.60
  • 10L/min = 0.65

Non-rebreather

• Delivers 60-80% FiO2
• Minimum flow 10L/min

Complications of O2 therapy

• CO2 retention
• Reperfusion injury
• Diffusion atelectasis
  • Loss of N2 splinting
• ARDS
  • High O2 concentrations can damage mucosal surfaces, decrease macrophage function, decrease surfactant production and increased pulmonary capillary permeability due to endothelial damage
  • Risk if FiO2 >0.8 for >12 hours or FiO2 >0.5 for >48 hours
• Retrolental fibroplasia (Retinopathy of prematurity)
• Augments paraquat/bleomycin toxicity
• May reduce CO if increased vascular resistance

Dyspnoea

• 2/3 have cardiac or pulmonary disorder
• Orthopnoea most commonly seen with LV failure (but also COAD and diaphragmatic paralysis)
• Trepopnoea is dyspnoea in only one of several recumbent positions
  • Seen in unilateral diaphragmatic paralysis, ball-valve obstruction or surgival pneumonectomy
• Platypnoea
  • Dyspnoea in upright position seen with loss of normal abdominal wall tone or PFO
• Hyperpnoea
  • Hyperventilation

Differentiating cardiac vs. pulmonary

• S3 gallop, pulmonary venous congestion/interstitial oedema on CXR all strongly support CCF
• Physician gestalt, raised JVP and alveolar oedema on CXR support CCF
• Wheeze, dyspnoea on exertion, orthopnoea, PND and leg oedema are not useful in discrimination
• Absence of above also does not rule out CCF
• Only BNP <80 is useful in ruling out CCF

Acute Cough

• Acute cough = <3 weeks
• Usually associated with self-limiting infectious upper respiratory or bronchial infections, LRTI or allergic reactions
• May be rhinorrhoea, pharyngitis, sinusitis, laryngitis with cough due to drainage onto cough receptors in pharynx/larynx
• Productive cough is hallmark of acute bronchitis
• Pnuemonia may produce a cough but not necessarily productive
• DDx
  • Asthma, environmental irritants, transient airway hyperresponsiveness, airway FB
• Consider CXR, pertussis PCR

Acute cough treatment

• Disease-specific treatment
• Antitussives (codeine/dextromethorphan)
  • Useful in dry cough but no evidence that causes harm in productive cough
  • For intractable coughing paroxysms in ED, can try 4mL of 1% lignocaine nebulised
• Treat pertussis is suspected

Subacute cough

• 3-8 weeks
• Mostly post-infectious cough
• Post-viral upper airway inflammation with bronchial hyperresponsiveness, mucus hypersecretion, upper airway cough syndrome (aka post-nasal drip) or asthma

Chronic cough

• >8 weeks
• Most commonly smokers cough (chronic bronchitis), upper airway cough syndrome (formerly postnasal discharge), asthma, GORD, ACEi
• Smokers cough usually worse in the morning and productive (chronic bronchitis)
• Asthma usually worse at night, exacerbated by irritants and episodic wheezing/dyspnoea
• Cough with GORD is typically worse on supination and improves with antacid therapy
• ACEi/ARB cough
  • 10-12% incidence due to accumulation of bradykinin
  • Can be 1 week to 1 year after initiation

Subacute and chronic cough management

• Determine if post-infectious and reassure if so
• If not, use sequential approach (95% successful at resolving cough)
  • Reduce exposure to irritants/ACEi/ARB/Beta-blockers
  • Treat for post-nasal discharge with antihistamines/decongestants +- IN steroid
  • Evaluate and treat for asthma
  • Obtain chest and sinus imaging
  • Evaluate and treat GORD
  • Refer for respiratory physician for CT +- bronchoscopy

Hiccups

• Acute benign
  • Gastric distension
  • Alcohol intoxication
  • Excessive smoking
  • Abrupt change in temperature
  • Psychogenic
• Chronic, persistent, intractable
  • Usually injury or irritation to vagus or phrenic nerves
  • One rare but treatable cause is a hair/FB in the external auditory canal up against TM and stimulating the auricular branch of the vagus nerve
  • Steroids and benzodiazepines have weak evidence of inciting hiccups
  • CNS structural lesions must be considered
  • Uraemia, hyperglycaemia, general anaesthesia, thoraco-abdominal surgery, prostate and urinary tract surgery and craniotomy

Hiccups

• Resolution during sleep suggests psychogenic cause but is not absolute
• Carefully examine auditory canal
• CXR for intrathoracic pathology
• Full neuro exam and consider CT head +- MRI
• Unilateral diaphragmatic movement suggests focal phrenic/vagal nerve pathology
• Swallowing a teaspoon of dried sugar is as effective as any other method
• Chlorpromazine and metoclopramide have the most evidence for benefit and take 30 minutes to work

Cyanosis

• Central cyanosis – Cyanosis of mucous membranes and tongue due to inadequate pulmonary oxygenation or abnormal Hb
• Peripheral cyanosis – Cyanosis of fingers or extremities due to vasoconstriction and diminished peripheral blood flow
• All conditions that cause central cause peripheral
• Traditionally >5g/dL (but may be much lower than this in some cases)
• In methaemoglobin or carboxyhaemoglobin states
  • Pulse oximetry overestimates SpO2 and reflects diminished response to supplemental oxygen
  • In methaemoglobinaemia will read 80-85% regardless of degree of methaemoglobinaemia, thereby possibly overestimating the true oxygen saturation
  • In carboxyhaemoglobin, oximeter reads carboxyHb as oxyHb
  • Need co-oximetry which will show normal PaO2, normal calculated oxygen saturation but reduced measured oxygen saturation (as less binding sites)

Pleural effusion

• Transudates
  • Heart failure
  • Cirrhosis with ascites
  • Peritoneal dialysis
  • Nephrotic syndrome
  • PE
• Exudates
  • Cancer
  • Parapneumonic effusion
  • PE
  • Viral/fungal/parasitic infection
  • SLE, RA
  • Uraemia
  • Pancreatitis
  • Post-cardiac surgery
  • Amiodarone

Pleural effusion

• Diagnosis
  • In adults, need 150-200mL of pleural fluid in hemithorax to produce signs on upright CXR
  • Supine CXR may show veil
  • A significant pleural effusion is large enough to produce a pleural fluid strip >10mm wide on lateral decubitus views or by USS
• Indications for diagnostic thoracocentesis
  • If no clearly identifiable cause
  • To confirm suspected diagnosis
  • To detect pleural space infection
  • If therapeutic drainage indicated
  • If suspected CCF, warrants treatment for 3-4 days and if no resolution perform drainage to confirm no alternative diagnosis
    • Diuretic therapy resolves 75% of effusions in CCF within 2-3 days

Pleural fluid

• Indications for therapeutic thoracocentesis
  • Dyspnoea at rest – Remove 1-1.5L
  • Greater volumes associated with re-expansion pulmonary oedema
• Indications for parapneumonic effusion drainage
  • Comorbid disease
  • Failure to respond to antibiotics
  • Pleural fluid pH <7.10
  • Effusion involving >50% of hemithorax or air-fluid level of CXR

Pleural effusion

• Lights criteria
  • 98-99% sensitive for exudate
  • 65-86% specific
• Diuretic use causes water reabsorption > protein with resultant effusion going from ‘transudative to exudative’ as per Light’s criteria
• Exudate = one or more of:
  • Pleural fluid/serum protein ratio >0.5
  • Pleural fluid/serum LDH ratio > 0.6
  • Pleural fluid LDH >2/3 ULN for serum LDH

Pleural fluid

• Extra tests on exudate effusions
  • MCS
  • Cell count
    • Neutrophil predominance: Bacterial empyema/parapneumonic, PE, pancreatitis
    • Lymphocyte predominance: TB, malignancy, post-cardiac surgery
  • Glucose
    • Low glucose in parapneumonic, malignant, TB, RA
  • Cytology
    • Highest yield for adenocarcinoma (much lower for squamous, mesothelioma, lymphoma)
  • pH
    • In parapneumonic effusion, pH <7.10 predicts empyema or persistence and indicates need for drainage
  • Amylase; Pancreatitis/oesophageal rupture
  • Mycobacterial/fungal stains
  • TB markers: PCR, adenosine deaminase, IFN-gamma

Hb-O2 dissociation curve

• Important values
  • P50: Normally PaO2 27mmHg
  • SpO2 90% = PaO2 60mmHg
  • SpO2 98% = PaO2 100%

Last Updated on October 28, 2020 by Andrew Crofton