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KEY POINTS

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KEY POINTS

  1. Arterial blood gases are used to assess oxygenation, ventilation, and acid–base balance.

  2. The right-to-left shunt fraction is the gold standard index of oxygenation efficiency in the lungs.

  3. Dead space is that portion of the minute ventilation that does not participate in gas exchange.

  4. Acid–base balance is explained by the Henderson-Hasselbalch equation or the strong ion difference.

  5. Whereas arterial blood gases primarily reflect lung function, venous blood gases reflect the adequacy of tissue oxygenation and tissue carbon dioxide clearance.

  6. Pulse oximeters pass two or more wavelengths of light through a pulsating vascular bed to measure arterial oxygen saturation, carboxyhemoglobin, and methemoglobin.

  7. The role of monitoring cerebral oxygenation in patients with traumatic brain injury is unclear.

  8. Capnometry is the measurement of CO2 at the airway opening during the ventilatory cycle.

  9. The use of capnometry to monitor procedural sedation is increasingly common.

  10. Current standards recommend the use of capnography to assess the quality of cardiopulmonary resuscitation.

  11. Transcutaneous Po2 is measured with a Clark electrode and transcutaneous Pco2 is measured using a Severinghaus electrode.

  12. Pulmonary mechanics is the expression of lung function through measurements of pressure and flow; from these measurements, a variety of derived indices can be determined, such as volume, compliance, resistance, and work of breathing.

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INTRODUCTION

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Monitoring is the continuous, or nearly continuous, evaluation of the physiologic function of a patient in real time to guide diagnosis and management decisions, including when to make therapeutic interventions and assessment of those interventions.1 Many physiologic parameters can be monitored during mechanical ventilation, including both invasive and noninvasive monitoring. Respiratory monitoring is an integral part of the care of mechanically ventilated patients in operating rooms and intensive care units (ICUs). Arterial blood gases are also commonly used to assess respiratory function. This chapter reviews the use of blood gases and monitors for assessment of respiratory function.

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ARTERIAL BLOOD GASES

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Arterial blood gases refer to measurements of Pco2 and Po2. The measurement of pH is also included with blood gases. Measured hemoglobin saturation with oxygen (HbO2), carboxyhemoglobin (HbCO), and methemoglobin (Hbmet) may be included. Many laboratories also report calculated values of oxygen saturation, bicarbonate concentration, and base excess (BE). These measurements assess oxygenation, ventilation, and acid–base status.

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OXYGENATION

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Hypoxemia results from decreased delivery of oxygen from the atmosphere to the arterial blood, and hypoxia refers to decreased delivery of oxygen to the tissues (Table 28-1). Oxygen content is a combination of dissolved oxygen and that bound to hemoglobin. The amount dissolved in plasma is small and directly related to Po2. The normal range of arterial Po2 (Pao2) is 80 to 100 mm Hg in healthy young adults breathing air at sea level. Pao2 decreases with increasing age and ...

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