concepts

• 1 mm Hg


  1.36 cm H2O
• 1 kPa


  7.5 mm Hg
• 1 Torr


  1 mm Hg
• 1 atm


  760 mm Hg = 1034 cm H2O
• Accessory Muscles of Breathing

  • Scalene (anterior, medial, and posterior)
  • Sternocleidomastoids
  • Pectoralis (major and minor)
  • Trapezius
  • Rectus abdominus
  • External oblique
  • Internal oblique
  • Transverse abdominal
  • Serratus (anterior, posterior)
  • Latissimus dorsi

  
• Carbon dioxide, which is a major by-product of aerobic metabolism, is then exchanged between the cells of the body and the systemic capillaries
• Gas Flow and Pressure Gradients During Ventilation

  For air to flow through a tube or airway, a pressure gradient must exist (i.e., pressure at one end of the tube must be higher than pressure at the other end of the tube). Air will always flow from the high-pressure point to the low-pressure point.
• During a spontaneous inspiration, the pressure in the alveoli becomes less than the pressure at the airway opening (i.e., the mouth and nose) and gas flows into the lungs

  
• During exhalation, gas flows out of the lungs because the pressure in the alveoli is higher than the pressure at the airway opening
• When the pressure at the airway opening and the pressure in the alveoli are the same, as occurs at the end of expiration, no gas flow occurs because the pressures across the conductive airways are equal (i.e., there is no pressure gradient)
• Airway opening pressure (Pawo)


  Most often called mouth pressure (PM) or airway pressure (Paw)
• Pressure at the body surface (Pbs)

  Equal to zero (atmospheric pressure) unless the person is placed in a pressurized chamber (e.g., hyperbaric chamber) or a negative pressure ventilator (e.g., iron lung)
• Alveolar pressure (PA or Palv)


  Also called intrapulmonary pressure or lung pressure
• Pressure Gradients


  • Transairway pressure (PTA)
  • Transthoracic pressure (PW)
  • Transpulmonary pressure (PL or PTP)
  • Transrespiratory pressure (PTR)
• Transairway pressure (PTA)


  Pressure difference between the airway opening and the alveolus: PTA = Paw - Palv
• Transthoracic pressure (PW)


  Pressure difference between the alveolar space or lung and the body's surface (Pbs): PW = Palv - Pbs
• Transpulmonary pressure (PL or PTP)


  Pressure difference between the alveolar space and the pleural space (Ppl): PL = Palv - Ppl
• Transrespiratory pressure (PTR)


  Pressure difference between the airway opening and the body surface: PTR = Pawo - Pbs
• During negative pressure ventilation, the pressure at the body surface (Pbs) becomes negative, and this pressure is transmitted to the pleural space, resulting in an increase in transpulmonary pressure (PL)
• During positive pressure ventilation, the Pbs remains atmospheric, but the pressures at the upper airways (Pawo) and in the conductive airways (airway pressure, or Paw) become positive. Alveolar pressure (PA) then becomes positive, and transpulmonary pressure (PL) increases
• Transpulmonary pressure (PL)


  The pressure difference between the airway opening and the body surface: PL = Pawo - Pbs
• Transrespiratory pressure (PTR)


  The pressure difference between the airway opening and the body surface: PTR = Pawo - Pbs
• Transrespiratory pressure


  • Has two components: transthoracic pressure (the pressure required to overcome elastic recoil of the lungs and chest wall) and transairway pressure (the pressure required to overcome airway resistance)
• The definition of transpulmonary pressure varies in research articles and textbooks. Some authors define it as the difference between airway pressure and pleural pressure. This definition implies that airway pressure is the pressure applied to the lungs during a breath-hold maneuver, that is, under static (no flow) conditions.

  
• Compliance (C)


  The relative ease with which a structure distends, the opposite of elastance (e)
• Compliance of the respiratory system is the sum of the compliances of both the lung parenchyma and the surrounding thoracic structures

  
• Normal compliance in spontaneously breathing patients

  • 0.05 to 0.17 L/cm H2O or 50 to 170 mL/cm H2O
• Normal compliance in intubated patients

  • Males: 40 to 50 mL/cm H2O, up to 100 mL/cm H2O; Females: 35 to 45 mL/cm H2O, up to 100 mL/cm H2O
• Calculating pressure needed to attain a tidal volume of 0.5 L (500 mL) for a patient with a normal respiratory system compliance of 0.1 L/cm H2O

  Pressure = Tidal Volume / Compliance = 500 mL / 0.1 L/cm H2O = 5 cm H2O
• Changes in the condition of the lungs or chest wall (or both) affect total respiratory system compliance and the pressure required to inflate the lungs
• Diseases that reduce the compliance of the lungs or chest wall increase the pressure required to inflate the lungs
• Acute respiratory distress syndrome and kyphoscoliosis are examples of pathologic conditions that are associated with reductions in lung compliance and thoracic compliance, respectively
• Emphysema is an example of a pulmonary condition where pulmonary compliance is increased due to a loss of lung elasticity
• Static compliance (CS)


  Compliance measured during static or no-flow conditions, using the plateau pressure
• Airway resistance (Raw)


  The frictional forces that must be overcome during breathing, described by the equation Raw = PTA/flow
• In normal, conscious individuals with a gas flow of 0.5 L/s, resistance is about 0.6 to 2.4 cm H2O/(L/s)
• Airway resistance is increased when an artificial airway is inserted, due to the smaller internal diameter of the tube
• In conscious, unintubated subjects with emphysema and asthma, resistance may range from 13 to 18 cm H2O/(L/s)
• Airway resistance


  • Increased when an artificial airway is inserted
  • Smaller internal diameter of the tube creates greater resistance to flow
  • Pathologic conditions can also increase airway resistance by decreasing the diameter of the airways
• Still higher values of airway resistance can occur with other severe types of obstructive disorders