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Introduction

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Objectives

  1. Explain why humidification of the inspired gas is necessary during mechanical ventilation.

  2. Compare and passive humidification.

  3. Discuss issues related to the ventilator circuit and gas delivery to the patient.

  4. Describe why circuit compressible volume is an important consideration during mechanical ventilation.

  5. Discuss the appropriate role of alarms during mechanical ventilation.

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Care of mechanically ventilated patients requires attention to both physiologic and technical issues. To deliver an adequate tidal volume, the patient-ventilator circuit and interface must be unobstructed, leak-free, and have minimal compliance and compressible volume. This chapter discusses issues related to humidification and the ventilator circuit.

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Humidification

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Physiologic Principles

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Inspired gases are conditioned in the airway so that they are fully saturated with water at body temperature when they reach the alveoli (37°C, 100% relative humidity, 44 mg/L absolute humidity, 47 mm Hg water vapor pressure). The point in the airway at which the inspired gases reach body temperature and humidity is the isothermic saturation boundary (ISB). Distal to this point, there is no fluctuation of temperature and humidity. The ISB is normally just distal to the carina. Proximal to the ISB, heat and humidity are added to the inspired gases, and heat and humidity are extracted from the expired gases. This portion of the airway acts as a heat and moisture exchanger. Much of this portion of the airway is bypassed in patients with an endotracheal or tracheostomy tube, necessitating the use of an external humidifying apparatus in the breathing circuit. Under normal conditions, there is about 250 mL of insensible water lost from the lungs each day to humidify the inspired gases.

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Inadequate and Excessive Humidity

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Gases delivered from ventilators are typically dry and the upper airways of such patients are functionally bypassed by artificial airways. The physiologic effects of inadequate humidity can be due to heat loss or moisture loss. Heat loss from the respiratory tract occurs due to humidification of the inspired gases. However, heat loss due to mechanisms other than breathing is usually more important for temperature homeostasis. Moisture loss from the respiratory tract, and subsequent dehydration of the respiratory tract, results in epithelial damage, particularly of the trachea and upper bronchi. The result of this is an alteration in pulmonary function such as decreased compliance and decreased surfactant activity. Clinically, drying of secretions, atelectasis, and hypoxemia can occur.

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Overhumidification is possible only if the temperature and humidity of the inspired gases is greater than physiologic conditions. This can occur in the setting of therapeutic hypothermia, as is commonly used in patients following cardiac arrest. In this setting, the inspired gas should be conditioned to the patient's core temperature and 100% relative humidity at that temperature. Although it is difficult to produce excessive humidification with a heated humidifier, complete humidification of the inspired gases (during mechanical ventilation) eliminates the insensible water loss ...

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