Rosen & Barkin's 5-Minute Emergency Medicine Consult (761 page)

Deep Venous Thrombosis

CODES

• 453.40 Acute venous embolism and thrombosis of unspecified deep vessels of lower extremity
  
• 454.9 Asymptomatic varicose veins
  
• 459.81 Venous (peripheral) insufficiency, unspecified
  

BASICS

• Mechanical ventilation is machine generated flow of gas into and out of the lungs that acts as a substitute for normal respiratory function
  

Basic Concepts: Physiology and Pulmonary Mechanics

• Mechanical ventilation is
  positive pressure ventilation
  indicating that forced gas delivery generates positive pressure during inspiration
  
• Negative pressure ventilation:
  
  • Natural respiratory pattern
    
  • At rest (functional residual capacity) surface tension of alveoli is balanced by elastic recoil of chest wall; alveoli pressure equals atmospheric pressure at this point
    
  • In inspiration, lungs expand causing alveolar pressure to become negative compared with atmospheric pressure and air travels down pressure gradient into lungs
    
  • Exhalation is normally passive, but can be made active with the use of accessory muscles in the setting of airway obstruction/increased airway resistance
    
• Minute ventilation (MV):
  
  • Total volume of breaths in 1 min
    
  • Breaths in 1 min is respiratory rate (RR)
    
  • Standard breath is called tidal volume (TV)
    
  • MV = TV × RR: Each component can be adjusted to control ventilation
    
• Oxygenation is controlled with adjusting fraction of inspired oxygen (FiO
  ₂
  ) and positive end-expiratory pressure (PEEP)
  
• Compliance:
  
  • Describes lung distensibility
    
  • Defined as change in volume with given change in pressure
    
  • Decreased lung compliance can be caused by problems with the lung parenchyma (i.e., pneumonia, ARDS) or problems with the chest wall/pleura (i.e., abdominal distension)
    
  • Lung compliance determines
    plateau pressure
    :
    
    • Plateau pressure is the steady state pressure; represents the attenuated pressure that is distributed to the small airways and alveoli during positive pressure ventilation
      
    • Goal ≤30 mm Hg
      
• Resistance:
  
  • Defined as change in pressure with given flow
    
  • Main determinant is airway radius
    
  • Increased resistance can be caused by problems with the airways (i.e., bronchospasm), problems with the endotracheal tube (i.e., secretions), or problems with ventilator tubing
    
  • Resistance determines
    peak pressure
    :
    
    • Peak pressure is the pressure seen in the larger airways before delivered volume is distributed to smaller airways and alveoli
      
    • Also determined by TV delivered
      
    • Goal ≤40 mm Hg
      

DIAGNOSIS

• Indications for mechanical ventilation:
  
  • Failure to oxygenate:
    
    • Diffusion defect (i.e., pulmonary edema, pneumonitis, pneumonia)
      
    • Severe ventilation/perfusion mismatch (i.e., PE, severe hypoventilation)
      
    • Severe shock:
      
      • Shock = oxygen supply does not meet oxygen demand by tissues
        
      • Mechanical ventilation can help improve shock states in 2 ways:
        
      • Increased oxygen delivery
        
      • Reducing overall oxygen demand by replacing organ system with high oxygen requirement
        
• Failure to ventilate:
  
  • Obtundation/sedation
    
  • Loss of ability to control diaphragm or intercostals (i.e., high spinal cord injury)
    
  • Severe myopathy
    
  • Dysfunctional chest wall (i.e., flail chest, increased abdominal pressures leading to decreased chest wall excursion, obesity)
    
  • Increased dead space (large PE, airway obstruction)
    
  • Metabolic acidosis (creates need for higher MV to compensate)
    
• Other:
  
  • Patient safety/need for evaluation
    
  • Predicted deterioration in clinical course
    
• Ventilation strategy should specifically address the indication for mechanical ventilation!
  
  • Example: In the setting of severe acidosis a preferred mode would be one where you could control MV closely
    
  • Example: In severe pulmonary edema controlling the MV is not as important as ensuring oxygen delivery
    

• Focus on underlying etiology for respiratory failure
  
• Exam on mechanical ventilation should include assessing oxygen saturation, evaluating end-tidal CO
  ₂
  (ETCO
  ₂
  ) with capnometry and capnography (see below), auscultating lung sounds/air movement, observing chest wall rise, palpating for abdominal distension
  
• ETCO
  ₂
  :
  
  • Capnometry is the quantitative partial pressure of ETCO
    ₂
    .
    
  • Capnography is the graphic representation of the changes in ETCO
    ₂
    with respiratory cycle
    
  • Normal lungs have a small degree of ventilation/perfusion mismatch as well as anatomic dead space. As a result, ETCO
    ₂
    is usually around 2–5 mm Hg lower than PaCO
    ₂
    
  • Capnometry will be affected by: Amount of dead space or ventilation/perfusion mismatch; changes in metabolic CO
    ₂
    production (although ratio between PaCO
    ₂
    and ETCO
    ₂
    will not change); venous return (also will not affect ratio)
    
  • Evaluation of the ETCO
    ₂
    waveform can be very useful:
    
    • Can help assess response to bronchodilator therapy as waveform in airway obstruction has a steeper upslope instead of a plateau given the prolonged expiratory phase
      
    • Can help assess adequacy of CPR (will see return of waveform with good compressions)
      
    • Can help assess cause of tachypnea or dyssynchrony
      
• Monitor hemodynamic status closely with mechanical ventilation
  

• Arterial blood gas (ABG):
  
  • Should be checked within 15–30 min of initiation of mechanical ventilation and repeated with any change in clinical status
    
  • pH and PaCO
    ₂
    will help assess ventilation
    
  • PaO
    ₂
    will assess oxygenation
    
  • With ability to assess continuous oxygen saturation and ETCO
    ₂
    need for frequent ABGs, even after change in ventilator settings, may be eliminated or reduced
    
• Serum chemistries including basic electrolytes with bicarbonate, liver function, renal function may help assess acid/base status which will affect ventilation strategy
  
• Hemoglobin/hematocrit will help describe state of oxygen delivery
  

Imaging may include beside US, chest x-ray, and chest CT to assess for endotracheal tube placement and pathophysiology of the lung and chest wall

See indications for mechanical ventilation above

TREATMENT

Respiratory support per local EMS protocol

• Cardiac monitor
  
• BP monitoring
  
• Pulse oximetry
  
• End-tidal CO
  ₂
  monitoring when available
  

Critical actions include: Choosing appropriate ventilatory mode; assessing and adjusting ventilator settings; standard postintubation care; treatment of the underlying process.

• Postintubation care is of utmost importance. Includes: Sedation and/or analgesia; confirmation of tube placement; adjustment of ventilator settings based on clinical condition and ABG; establishing ETCO
  ₂
  gradient if using capnometry; elevating head of bed; placement of NG or OG tube.
  
• Settings common to most modes include:
  
  • RR:
    
    • In all modes, but will be set by patient in more spontaneous modes
      
    • Normal starting rates can vary from 12–20
      
    • Consider underlying pathophysiology before arbitrarily setting rate (i.e., elevated ICP, severe asthma)
      
  • Fraction of inspired oxygen (FiO
    ₂
    ):
    
    • Oxygen concentration in gas mixture
      
    • Usually start out with FiO
      ₂
      of 1 (100%) but wean down quickly after confirmation of stable oxygenation with prompt ABG
      
  • PEEP:
    
    • Pressure that is applied to end expiration to maintain alveolar recruitment
      
    • Significant increase in work of breathing is required to open up collapsed alveoli
      
    • Collapsed alveoli do not participate in gas exchange, creating ventilation/perfusion (V/Q) mismatch and difficulty oxygenating and ventilating
      
    • By stenting open more alveoli, increased PEEP can improve oxygenation, especially at lower TVs (although be careful of high PEEP and overdistension which can lead to significant alveolar injury)
      
    • With normal chest wall compliance, basic starting PEEP will be 5–10 mm Hg
      
    • In setting of low chest wall compliance (obesity, anasarca, abdominal distension) may need to start with higher PEEP, around 10–15 mm Hg
      
  • Inspiratory:expiratory ratio (I:E): Will alter flow rates. Allows for optimal mechanics in disease specific situations: E.g., increase E fraction in obstructive airway disease to prevent “breath stacking.” Normal ratio ∼1:2.