The purpose of this article is to enrich the clinician's understanding of Oxygenation and evaluative abilities of technical O2 data related to patient care. However, the most important part of analyzing lab values is to remember to treat the patient, not the numbers. Lab results should always be correlated with good clinical data. Accurate history and physical examinations are a clinician's best resources.
Definition of Oxygenation: To add or infuse oxygen into blood or other medium.
This may be OK from the dictionary for the layman, but as clinicians this is not quite sufficient. A better clinical definition is the infusion of oxygen into body tissue via the cardiopulmonary system.
I like this definition better because it is at this point of oxygen transfer that a clinician can gather clinical data to correlate with the lab values we are about to discuss.
FIO2: Fraction of Inspired Oxygen: Ambient air is naturally 21% oxygen 78% nitrogen, 1% other. When we add O2 via mask/vent, the fraction increases in correlation to flow (LPM). 2 LPM of O2 will increase FIO2 to approximately 24%. An oxygen analyzer is needed for accurate determination.
Ventilation: The flow of oxygen in and out of the lungs.
Ventilation is, however, not synonymous with oxygenation. Ventilation refers only to air moving in and out of the lungs. If everything else is working the way it should, then oxygenation happens.
Perfusion: The flow of blood into blood vessels in order to reach an organ or tissues to supply nutrients and oxygen.
Normal Q (perfusion) is 5L of blood per minute (for the average adult). Normal V (ventilation) is 4 L of air per minute. Normal V/Q ratio is 4/5 or 0.8. When this fraction is out of balance, hypoxia occurs.
To help us assess our patients' O2 Saturation, we check the Pulse Ox. Its reading is a SpO2.
SaO2: Saturation of Arterial oxygen via Arterial Blood Gas sample
SpO2: Saturation of Peripheral oxygen. Peripheral, as it is taken via a lead placed on fingers, toes, earlobes, or, if the infant is small enough, on the foot above the pedal pulse. The light and lead must be 180 degrees facing each other for accuracy.
SpO2 is the same reading as SaO2 so for definition purposes we will discuss them as the same. However, we will highlight the few differences later.
What exactly do we mean by saturation? SaO2 measures the degree to which oxygen is bound to hemoglobin and is expressed as a percentage. Basically, each hemoglobin molecule has four oxygen binding sites. When the average sites are full, the saturation is 100%.
Only four binding sites sounds easy to fill. However, Hemoglobin binding sites can hold molecules other than oxygen.
For example, smokers and people exposed to smoke, automobile exhaust, or other chemicals can have hemoglobin saturated with carbon monoxide (CO). The hemoglobin molecule in these conditions is unusable. If enough hemoglobin is inactivated like this, it can cause tissue hypoxia.
Hypoxia, reduced oxygen supply to the tissues, can also occur even in the presence of 100% oxygen. This can be a life-threatening condition. This can happen for several reasons:
Circulatory problems: vascular constrictions due to cold ambient temp, edema, alkalosis and sluggish flow due to cardiac conditions. Blood PH: Acidosis inhibits Hgb from picking up O2 in the lungs. Alkalosis inhibits Hgb from releasing O2 to tissue. Fever causes acidosis, and acidosis causes fever.
The difference in SaO2 and SpO2 is in the way it works. A source of light originates from the probe at two wavelengths (650nm and 805nm). The light is partly absorbed by hemoglobin, by amounts which differ depending on whether it is saturated or desaturated with oxygen. By calculating the absorption at the two wavelengths, the processor can compute the proportion of hemoglobin which is oxygenated. The oximeter is dependent on a pulsatile flow and produces a graph (wave form) of the quality of flow. Where flow is sluggish (i.e dehydration or vasoconstriction), the pulse oximeter may be unable to function accurately or at all. Both technologies measure oxygen saturation of hemoglobin. However, beware that SpO2 is an indirect measurement where the light must pass tissue, dirt on the skin and obstructions that create opacities of the nail bed (i.e. nail polish). Dark skin will not affect accuracy. Peripheral arteries constrict much more readily. For these reasons SpO2 has an accuracy rate of 75%-99%.
This brings us back to "Treat the Patient not the monitor." Technology is wonderful, but the best tools you have are your assessment skills. In light of all the wonderful technology we have, this is what to look for when assessing oxygenation in your patient (in conjunction with your technological data):
Good ventilation: Normal (equal) rise and fall of chest, breath sounds free of stridor, crackles, coughs or wheezing. Ease of respirations free from nasal flaring, retractions (sterna, intercostals or abdominal), moaning. Sputum free of abnormal color or malodorous. Normal rate for patient's age and condition.
Good Perfusion: distal capillary refill less than 3 seconds, normal color of skin and mucus membranes. Regular heart rate with cardiac tones free of Murmur (clicks or swishes) Pink nail beds, lack of edema, dizziness or anxiety.
Good Support Equipment: Check proper fit of masks. Ensure valves or regulators are open and tubing is attached. Look for proper fluid levels when indicated. On a pulse ox look for regular rhythmic wave forms. If waveform is irregular, the reading is inaccurate.
Shawn A. Pickett, BSN, RN, TNS
Clinical Manager/Supervisor, AHHC