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INTRODUCTION

Critical care medicine is the discipline of caring for patients with acute life-threatening conditions or conditions likely to cause serious harm if not rapidly addressed. This work requires a detailed understanding of human physiology, the pathophysiology of severe illness and injury, the intricate interactions between organ systems and therapies, as well as an understanding of and experience with the rapidly changing technologies available in a modern pediatric intensive care unit (PICU). The science of caring for the critically ill patient continues to advance rapidly as the molecular mediators of illness have become better defined and new therapies are brought into clinical use. Critical care, then, is a highly complex, multidisciplinary field in which optimal patient outcomes require a team-oriented approach, including critical care physicians and nurses; respiratory therapists; pharmacists; consulting specialists; physical, occupational, and recreational therapists; and social services specialists.

MONITORING AND TECHNOLOGY

MONITORING

Respiratory Monitoring

Pulse oximetry measures arterial oxygen saturation (SaO2) continuously and noninvasively. However, pulse oximetry readings can be much less accurate in patients with saturations below 80%, poor skin perfusion, or significant movement. In addition, pulse oximetry can be dangerously inaccurate in certain clinical settings such as carbon monoxide poisoning or methemoglobinemia. To directly measure arterial oxygen content, direct arterial blood gas sampling must be performed.

Knowing the partial pressure of oxygen (PaO2) and inspired oxygen content (FiO2), gas exchange impairment may be estimated through measures such as the PaO2/FiO2 ratio (used in diagnosing acute respiratory distress syndrome, ARDS) or the alveolar-arterial oxygen difference (A–aDO2, or A–a gradient). (Table 14–1). The A–a gradient is less than 15 mm Hg under normal conditions, widening with diffusion impairment, shunts and V/Q mismatch; gradients over 400 mm Hg are strongly associated with mortality. Assessment of intrapulmonary shunting (the percentage of pulmonary blood flow that passes through nonventilated areas of the lung) may also be helpful. Normal individuals have less than a 5% physiologic shunt from bronchial, coronary, and Thebesian (cardiac intramural) circulations. Shunt fractions greater than 15% usually indicate the need for aggressive respiratory support. However, calculation of the shunt fraction requires a pulmonary arterial catheter, the use of which has decline significantly over the past decade.

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Table 14–1. Respiratory and hemodynamic parameters.
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Parameter Calculation Normal Values
Alveolar partial pressure of oxygen PAO2 = (Barometric pressure – 47) × % inspired oxygen concentration  
Alveolar-arterial oxygen difference (mm Hg) A–aDO2 = [PAO2 – (PACO2/R)] – PaO2 5–15
Cardiac output (L/min) CO = HR × SV  
Cardiac index (L/min/m2) CI = CO/BSA 3.0–4.5
Oxygen content of arterial blood (mL/dL) CaO2 = (1.34 × hemoglobin × SaO2) + (0.003 × PaO2) 17–24

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