Foundations: oxygen, gas exchange, and the first look

Look before you touch

From the doorway you already have most of what you need: position, effort, color, and whether the patient can speak in full sentences. A patient sitting forward on the edge of the bed with a tripod posture has told you more about their work of breathing than the monitor will.

Count respirations yourself for a full fifteen seconds. Monitor-derived rates are the single most commonly wrong number on the flowsheet, and the boards write questions where the correct action depends on a rate you are expected to have verified.

Oxygenation and ventilation are different problems

A low SpO2 is an oxygenation problem. A rising PaCO2, a falling rate, and a patient who is getting quiet are a ventilation problem. Patients die from the second one while the first one still looks acceptable on the pulse oximeter.

When you see a normal saturation on a patient with obvious distress, ask what is being spent to keep that number normal. The saturation is the outcome; the accessory muscles are the price.

Trend beats snapshot

One vital sign is data. Three in a row is a direction. Before you report anything, look at the prior two sets — a rate that has climbed from 18 to 24 to 32 is a deteriorating patient even if 32 alone would not alarm anyone.

Low flow means the patient dilutes it

A nasal cannula delivers a fixed flow into a variable demand. A calm adult drawing 20 L/min of inspiratory flow gets a very different FiO2 from a distressed adult drawing 60 L/min at the same 4 L/min setting. That is why every low-flow FiO2 in a textbook is prefaced with 'approximately'.

The working estimate is room air plus roughly 4% per liter: 1 L is about 24%, 4 L is about 36%, 6 L is about 44%. Beyond 6 L/min a cannula mostly dries out the nose without meaningfully raising FiO2.

High flow means the device wins

An air-entrainment (Venturi) mask meets or exceeds the patient's inspiratory flow, so the delivered FiO2 stays where you set it. This is the device you want in the patient whose FiO2 must be precise — most classically the COPD patient with a hypoxic drive concern who still needs a known, controlled oxygen concentration.

A nonrebreather with a properly inflated reservoir and a tight seal is the emergency answer, not the maintenance answer. If the bag collapses on inspiration, flow is too low and the patient is entraining room air around the mask.

Step 1 — read the pH first, never the CO2

The pH tells you which direction the body has actually landed, and it is the only value that defines the primary disorder when two abnormalities coexist. Normal pH is 7.35–7.45; below is acidemia, above is alkalemia.

If the pH is abnormal, apply ROME — Respiratory Opposite, Metabolic Equal. Metabolic disorders move in the same direction as the pH derangement (low pH with a low HCO3). Respiratory disorders move in the opposite direction (low pH with a high PaCO2).

A normal pH with an abnormal PaCO2 and HCO3 means full compensation or a mixed disorder — it never means a normal patient.

Step 2 — oxygenation is a separate question

Acid-base and oxygenation are two independent readings from one sample. Finish the acid-base statement completely, then turn to the PaO2 and the FiO2 that produced it. A PaO2 of 70 on room air and a PaO2 of 70 on 100% are entirely different patients.