Abstract

Oxygen-Hemoglobin Dissociation Curve. There is a nonlinear relationship between the percentage of total hemoglobin saturated with oxygen (SaO₂) and PaO₂, as demonstrated by the oxygen-hemoglobin dissociation curve illustrated in Figure 1. Hemoglobin saturations of 95% and higher sustain PaO₂ at or above 80 mm Hg, preventing hypoxemia. At 90% saturation, the curve becomes steep, and within a relatively narrow period, the percent hemoglobin saturation and PaO₂ decline dramatically. The relationship between SaO₂ and PaO₂, although in different units, approximates a value of 30 during this rapid decline. Intracellular oxygen tension or that of mixed venous blood is normally 40 mm Hg, so that hemoglobin saturation below 70% would indicate that normal cellular function is compromised.

Approximate Lung Volumes and Capacities.

Hemoglobin Desaturation Following Apnea. All patients were preoxygenated prior to apnea following neuromuscular blockade. Normal adults remain well oxygenated for 8–9 minutes despite absence of ventilation. Obese patients and children have reduced functional residual capacity, so despite preoxygenation, commence significant desaturation within 3–4 minutes. In all cases, desaturation would have occurred far more rapidly had the functional residual capacity not been concentrated with oxygen by preoxygenation (adapted from Benumof JL ⁷ ).

Amplified Precordial Stethoscope. Continuous auscultation of breath sounds can be accomplished using a stethoscope head and amplifier (A) connected to either conventional ear plugs (B) or a Blue Tooth listening device (C).

Capnographic Waveform. A capnographic waveform reflects carbon dioxide content of gas during a ventilatory cycle. In Figure 5 expiration occurs along points A to D as follows: A–B, exhalation of dead space; B–C, exhalation of lower airway; and C–D, exhalation of alveoli. Inspiration occurs from points D to E. End-tidal CO₂ is recorded at point D.

Supplemental Oxygen and Respiratory Monitoring. Capnography (ETCO₂) is the purest measure of hypoventilation. The lower tracing in this graph shows a simultaneous elevation in ETCO₂ as hypoventilation commences. The top 2 tracings show pulse oximeter readings for patients supplemented with oxygen and those breathing room air. Note the top tracing shows no warning of hypoventilation, but the tracing for patients breathing room air declines in concert with the capnographic reading (adapted from Fu ES et al ⁸ ).