Oxygen Delivery to the Body’s Tissues is a Two Step Process

Step one: The Respiratory System functions to Oxygenate the Arterial Blood

This first step is easy to visualize. Oxygen saturation is easily measured with probes on the skin. The visual appearance of cyanosis relates only to this first step. We cannot see cyanosis of internal organs or muscle tissue.

Step two: The Arterial Blood has to deliver the Oxygenated Blood to the Tissue

This second step is much more difficult to measure or appreciate and frequently misinterpreted. It is almost universally thought that if the blood pressure is adequate then this second step will occur. In essence this equates shock to blood pressure. Blood flow or perfusion can fall as a result of a rise in the resistance to blood flow which will often be accompanied by a rise in blood pressure. Almost all clinicians proceed as if blood pressure is related directly to blood flow. A rise in blood pressure is accepted as always indicating improvement. This is a practical method of assessing perfusion but more than inaccurate it is incorrect. A great deal of effort is frequently employed to measure what does not tell us what we need to know. Frequent, urgent and continuous blood pressure monitoring are relied upon. Invasive arterial lines are a hazardous method for perhaps being mislead. What we really need to know is perfusion not pressure.

Arterial blood sampling provides information relating to perfusion. The presence of metabolic acidosis is missed if pulse oximetry replaces arterial blood gas sampling. This is an important indication that perfusion is inadequate. If the blood pressure is combined with a saturation by pulse oximetry two inadequate parameters are integrated into the usual clinical assessment.

An assessment of the first step arterial oxygenation may be fairly evaluated but the second step, organ perfusion is frequently not.

It is difficult to appreciate that this erroneous view is occurring. The only thing may be that the patient is doing poorly usually interpreted as the illness is progressing. We may believe that the infection causing the septic picture is worsening. We may believe that bleeding inside spaces that are concealed, such as intestinal or intraperitoneal bleeding is continuing. This may be because of our interventions rather than in spite of our interventions. Typically we have no way of knowing other that to believe what we want to believe.

The only parameter that relates directly to perfusion is metabolic acidosis, that is the level of bicarbonate. This requires blood sampling and cannot be done in real time.