hypoxia meter

I don't think that Texas has any high elevation in which to test it.

–B.G.–

I've owned and played with pulse oximeters over the years. And did it based on clinical observations before that.

At sea level, a non-smoker ought to be 98-99% while a smoker is typically several points lower.

At elevation, everyone drops off, but the less you've acclimitized, the less well you (individually) acclimitize, diseases (cold, flu), disorders (HAPE, AMS), exercise, and greater elevation all reduce it further.

Personally, I can notice the mental symptoms in myself as I do with my core temperature. A lot of people don't/can't – even those who could accurately recite all the signs and symptoms while at sea level. In myself, I notice mental arithmetic gets harder. I'll "carry the 2", do another sub-calculation, focus on the carried digit, and it's just not there in my mind anymore. Multi-step problem solving suffers noticeably and an easy one to check (before everyone got digital watches and smart phones) is reading an analog watch upside down. I also get "sloppy" about things like my foot placement. We're all uber-experienced hikers and subconsciously have learned to place our feet on non-tippy rocks and so as to reduce the vertical rise of each step. So much so, I'd have thought it an "over-learned" behavior that requires no conscious thought. Maybe it is and my subconscious is also de-oxygenated, but I definitely "get sloppy" and plod along plopping one foot down, mindlessly, after another. I'll mentally slap myself, remind myself that a little thought is less draining than poor hiking technique, and then force myself to focus on the task at hand.

With a bit of practice, you can see someone else's experience/skill level by watching them from the ankles down for their foot placement skills. That detriates with hypoxy. As does one's conversational skills and sense of humor. I used to notice hypothermia, heat exhaustion and hypoxy in companions first by changes in their personalities (before skin tone, shivering, etc) but now I keep a close eye on them and correct their clothing or work load sooner.

A fair number of people get cranky, grumpy and argumentative when hypoxic. Some approaches to agree on and practice in advance could include:

Anyone 2-3 points lower than the group average surrenders their pack to the most saturated hiker.

Anyone more than 8% below sea level (at 10-12,000 feet) has to 1) reduce workload, 2) reduce packload, and/or 3) turn around.

In that paradigm, as Bob suggests, yes, having an objective number is useful because it leaves less room for argument and is very quick to check.

I'm most often "at elevation" in an 11-foot-diameter, pressurize aluminum tube – a.k.a. a Boeing 737. Commercial jets maintain a cabin pressure right about 7500 feet density altitude which is, memoriably, 75% of sea level pressure. However, unless joining the Mile High Club, you're not exerting yourself on a flight and a healthy non-smoker might only see a 1-2% drop.

Pilots observe that their night vision and decision making suffers when hypoxic and while supplemental O2 is not required by the FARs below 12,000 feet, increasingly, best practice is to know one's own response and use extra O2 possibly starting at 8,000 feet for some and 10,000 feet for everyone. Pilots' physical workload is less than ours, but their acclimitization time is non-existent, their mental workload higher, and the consequences of a mistake more dire.

"Years ago, a miniature blood oxygen saturation meter cost was about $400, and it slipped onto a fingertip for the optical measurement."

All hospitals use them today. I had one connected a few days ago. Its called pulse Oximetry, See http://en.wikipedia.org/wiki/Pulse_oximetry. they generate a reading within a couple of seconds. Unfortunately by itself it cannot diagnose the cause of the low blood oxygen levels. Low blood oxygen levels can be caused by: Anemia, iregular hart beat, a blood clot (heart or lungs), atrial seeptal defect, lung injury,altutude, just to name a few that I know of. The number a oxygen sensor gives is dependent on the altiude, cause, and individual.

Low blood oxygen levels are often accompanied by rapid heart beat, high blood pressure, and many be headache, dizziness, or chest pain. The more symptoms you have the more rapidly you need to take action which typically means getting the victim down to a lower altitude and then to a hospital.

I mentioned that a few days ago I had one connected. Last friday I hiked from Tualoman medow in Yosemite to Glen Allen high sierra camp. It was the first day of week long hike. my pluse was about 120 after we stopped for breaks. It probably was much higher when I was moving. At Glen allen a Yosemite ERT volunteer measured by blood pressure at 160 over 90 with a pulse of 90 with a blood oxygen level of 92. I was carried out by hourse and at the hospital I was diagnosed with Anemia and low blood iron levels. I didn't have any headaches, tunnel vision or vomiting. Doctor says I won't be doing any hiking a couple of months.

three years ago I was diagnosed with an Atrial Septal Defect which is surprisingly common (about 5% of the population has it}. Often it is not diaganosed until after it causes a serious problem (in my case a stroke). With it I often had problems with altitude and when I stopped to take a break my pulse would increase and I would vomit. sometimes I had tunnel vision. I suffered with this for 30 years but doctors couldn't figure it out since it is not listed as a symptom of ASD.

'I think we would all like to have a warning _before_ the point of pulmonary edema. For some strange reason, I don't vomit or get a severe headache. If I am pushing really hard, I will get tunnel vision." Bob tunnel vision is not listed as a symptom of Pulmany edema. it could easily be a heart problem instead and you should have your doctor look into it.

"Usually when my pulmonary edema sets in I'm vomiting and have a severe headache, so no tester needed for me."

That's usually what happens to me when I get close to the altar….." Christopher and Douglas, again I would suggest you get checked out by a doctor. Since I have had my heart fixed I have not been able to push myself hard enough to vomit, even at 11,000 feet. I always completely run out of gas before I get to that point. The only stomach problem I have had is a lack of apatite which was likely due to my recent diagnosis of Anemia.

Don't aussume your symptoms are pulminary edema. It could easily be something else which might be more serious or something simple and easy to fix. You should always have your doctor check it out.

"Bob tunnel vision is not listed as a symptom of Pulmany edema."

No, Silly, tunnel vision is a symptom of hypoxia.

No, I've never had any symptoms of pulmonary edema, although I have made a non-professional diagnosis of it on a patient.

–B.G.–

"Lots of old or sick people with decreased pulmonary function use them to let them know when they need to breathe oxygen."

A coworker told me about his father with emphysema who used it to let him know when he was ready for his next cigarette.

Yes, what The Tick (Ben) said re: making a ratio. Although, Lord knows that some sort of newfangled one might work a different way. I could think of other ways- maybe just reading an "average" color, or something?

And- yes- it will still measure saturation in an anemic patient. It doesn't matter how many red cells you have- it just tells you what percent of them are fully saturated with 4 molecules of O2 per hemoglobin molecule. 14/14=100% and 27345/27345=100%.

When you shine a light through anything as dense/opaque as human dissue it will diffuse so, no, you generally don't find the laser just hitting one vessel and thus skewing the reading. Hold a mini-maglight up to your finger in a darkened room to see what I mean.

EDIT– just to be sure I wasn't hallucinating I googled this:

"A source of light originates from the probe at two wavelengths (650nm and 805nm). The light is partly absorbed by haemoglobin, 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 haemoglobin which is oxygenated."

And, heck, the Wikipedia page for Pulse Oximetry gives a good description.

they appear to use them to monitor for hypoxia and to monitor the response when they administer 02. i've seen it in action. once where they put the climber on O2 and the level went from the low 90's to 99% almost instantaneously and the other when the level started in the low 90's and didn't move much. needless to say they expedited the second climbers exit from altitude.

John Shannon thinks it is a waste of money.

–B.G.–

I used to carry a pulse oximeter (Nonin G02) all the time on group hikes, mainly because my ex-GF used to get altitude sickness pretty bad above 12k. We would always pass the pulse oximeter around the group and find out if anyone was having AMS symptoms. Here are some random observations that I made in using it.

There is no surefire way to diagnose AMS with a pulse oximeter. Some people feel AMS effects with an 85% oxygen level, others feel just fine at 80%. It depends very much person to person. There is, however, a very strong correlation between feeling altitude's effects and oxygen %. Almost always the strongest hikers in the group would have the highest blood oxygen level, while those feeling the effects of low oxygen would have the lowest %, as you'd expect.

The only moderately accurate way to use the pulse oximeter to predict AMS is to take measurements in the exact same way every time. Basically, you need to rest for 5 minutes and then take a seated measurement with breathing normally. Standing, walking, being out of breath, breathing abnormally all very obviously effect the measurement, so u really need to take measurements the exact same way every time for it to be of any use. Then over time you can start to get a general idea of when you're gonna start having AMS issues, but even then just because you are at some oxidation level does not guarantee you will have problems. Its just significantly more likely.

The classic mountaineering technique of pressure breathing (Whittaker wheeze) really really works. You can quickly get your blood oxygen level from 90% to 100% with about 5 seconds of pressure breathing. When you do this all day, your resting oxidation level at the end of the day is higher.

Overall, I found carrying a pulse oximeter to be entertaining for groups and moderately useful, but not a surefire way to predict AMS symptoms and severity with any degree of accuracy. I think it gives you a general idea of whether AMS symptoms are more likely, but you probably already new that from past experiences at that altitude. I've never had a group where someone came down with HAPE or HACE, just the usual headache nausea and such, so I don't have any experience trying to predict anything like that. For me, I think its worth the 1.3oz, but then again I'm not an ultralighter =]

lol..that Shannon guy needs to keep his trap shut. Bob, did you buy one? I figure the only ones that really can use them are medical personnel on mountaineering expeditions way above 14,000 ft (Everest and the like).

Greg, I tend to agree with your observations. I've been around hikers getting pulmonary edema and one getting cerebral edema, but most people getting problems at 12,000 or 14,000 feet are more at the AMS stage or better.

In a week or two I will have a couple of newbies with me above 14,000 feet, so the little gizmo might get used. Of course, considering my age, they might need to use it on me.

Some years ago, I was on a trip going above 19,000 feet. In advance, I passed out litle record forms to the other trip members, and I asked them to record their rest pulse each morning before they got up, and to record the elevation at that camp, and to note whether or not they were taking Diamox.

Want to know the result? Nothing. Despite my printed instructions, nobody could remember to do what I asked. I attribute that to failing brainpower at high elevations.

–B.G.–

"I figure the only ones that really can use them are medical personnel on mountaineering expeditions way above 14,000 ft (Everest and the like)."

+1 Will our fascination with shiny, new gadgets never end? ;0)

"Bob, did you buy one?"

I ordered one, but it may be a few days before delivery.

Some years ago the thinking was like yours, that trained medical personnel were needed to effectively use the gizmo. However, I believe that thinking has changed now. Some of my friends who work in health care tell me how easy they are to use, and most of the models give consistent results once you've practiced a bit.

I studied biomedical engineering almost forty years ago, and we had to design our own instruments like this. So, I am not a complete fool on this sort of thing. Still, I intend to practice with the new gizmo at various elevations between sea level and 14,000 feet. It is necessary for operation to be so automatic that I can do the right thing even if my own brainpower is fading.

I tend to have more trouble with newbies letting their blood sugar sink down too low. A blood glucose meter might help with that. I'll have dextrose tablets for them.

You see, in Texas you don't have the high elevation problem, so you don't get concerned. In California, we have lots of peaks above 14,000 feet, so it is an issue.

–B.G.–

Hypoxia meters, blood glucose meters, dextrose tablets…Sounds like you might not be too far from throwing in an O2 tank and AED for good measure.

Who are you dragging into the mountains Bob?!

"Who are you dragging into the mountains Bob?!

Well put.

"Who are you dragging into the mountains Bob?!"

Each year I drag some newbies up a big peak. For sea level, they are adequate hikers, but they have zero high elevation hiking experience. On some years, a newbie will have one minor problem or another, and I just like to be able to shepherd them through it without a big problem.

As you might guess, we will be hiking up high for a few days before the big peak. So, if they really have some hidden problem, it's likely to show up early.

Often the newbie will bring the wrong kinds of food. Then their appetite goes south. Then their blood sugar goes south. Then they tend to fall over a lot. I like to have some sugary snack that will push their blood sugar back up in a hurry.

I once had a newbie with a hidden case of asthma. They got so dizzy that they could barely stand up. It would be nice to be able to distinguish an oxygen problem from a sugar problem, so I might try the new gizmo.

Sometimes the newbie will get weak from simple dehydration. That's easy to fix.

If we need an AED, it's more likely to be for me.

–B.G.–

Pulse oximetry is a great tool, however they are inherently inaccurate… their most important role is to detect TRENDS in the progression/deterioration of a person. To answer some of the questions (which on the most part, have been answered correctly), I've simply copied and pasted one of my learning packages (a collaborative learning package) that I put together for my students (My day job sees me lecturing in pre-hospital medicine). The document is long, but thought it useful for those interested in the subject.

History
The use of oximetry was first reported by German researchers in 1939 using an ear meter
with red and infra-red light, but it was not until the Second World War that interest in
oximetry was established due to the need to evaluate oxygenation of pilots at high altitude
(Treacher and Leach 1998).

It was the British researcher Glenn Allan Millikan who between 1940 and 1942 produced a
lightweight ear oxygen meter he called an ‘oximeter’. Over the years many modifications
were made and the oximeter was used in various studies in physiology and aviation,
however the expense of the machines and their bulky nature prevented widespread clinical
use. In the 1970s the Japanese bioengineer Takuo Aoyagi recognised the importance of
oximetry while looking for a non-invasive way to measure cardiac output. Following this a
period of corporate interest and development led to significant reductions in size and cost,
resulting in the hand-held machines encountered today (Treacher and Leach 1998).

How it works
The principle of pulse oximetry is based on the absorption of red and infrared light by
oxygenated and deoxygenated haemoglobin (Limon 2008). Oxygenated haemoglobin
absorbs more infrared light and allows more red light to pass through. Deoxygenated (or
reduced) hemoglobin absorbs more red light and allows more infrared light to pass through
(Bilan et al 2010).

Pulse oximetry uses a light emitter with red and infrared LEDs that shine through a
reasonably translucent location with good blood flow. Typical adult/paediatric locations are
the finger, toe and the pinna (top) or lobe of the ear. Infant sites are the foot or palm of the hand and the big toe or thumb. Opposite the emitter is a photo detector that receives the
light that passes through the measuring site (Sedaghat-Yazdi et al 2010; Vestbo et al 2009).

After the transmitted red (R) and infrared (IR) signals pass through the measuring site and
are received at the photo-detector, the R/IR ratio is calculated and a saturation reading given
(Kamaras et al 2010).

At the measuring site there are constant light absorbers which are always present: skin,
tissue, venous blood and the arterial blood (Bishop and Nolan 1991). However, with each
heart beat the heart contracts and there is a surge of arterial blood which momentarily
increases arterial blood volume across the measuring site. This results in more light
absorption during the surge. If light signals received at the photo detector are looked at as a waveform (Bishop and Nolan 1991), there should be peaks with each heartbeat and troughs
between heartbeats. Hence, pulse oximetry is the measurement of the saturation of
peripheral oxygen (SpO2) of haemoglobin in the arterial blood. It also serves as an indirect
indicator of cardiac output (Bishop and Nolan 1991).

What is Oxygen Saturation?
Oxygen saturation (SpO2) is a measure of how much oxygen the blood is carrying as a
percentage of the maximum it could carry. Oxygen is carried in the red blood cells attached
to haemoglobin molecules (Simmonds and Simmonds 2004).

One haemoglobin molecule can carry a maximum of four molecules of oxygen (Simmonds
and Simmonds 2004). If a haemoglobin molecule is carrying three molecules of oxygen then
it is carrying 3/4 or 75% of the maximum amount of oxygen it could carry.
One hundred haemoglobin molecules could together carry a maximum of 400 (100 x 4)
oxygen molecules. If these 100 haemoglobin molecules were carrying 380 oxygen
molecules, they would be carrying (380 / 400) x 100 = 95% of the maximum number of
oxygen molecules they could carry and combined would be 95% saturated.

Evaluation of SpO2 Measurements
SpO2 of greater than 95% is generally considered normal. A fit, healthy person should have
oxygen saturation on room air in excess of 95%. With deep or rapid breathing, this can be
increased to 98-99%. While breathing oxygen enriched air (40% – 100%), the oxygen
saturation can be pushed to 100% (Oh 2003).
· SpO2 of 92% or less (at sea level) suggests hypoxemia (Oh 2003)
· In a patient with acute respiratory illness (e.g. influenza) or breathing difficulty, a SpO2
of 92% or less indicates a need for oxygen supplementation (Oh 2003)
· In a patient with stable chronic disease (e.g. COPD) a SpO2 of 92% or less should
prompt referral for further investigation of the need for long term oxygen therapy (Oh
2003)

Oxygen saturation will fall if:
· Inspired oxygen levels are diminished, such as at increased altitudes
· Upper or middle airway obstruction exists (e.g. asthma)
· Significant alveolar lung disease exists, interfering with the free flow of oxygen across
the alveolar membrane e.g. COPD ( Limon 2008)

Oxygen saturation will rise if:
· Deep or rapid breathing occurs
· Inspired oxygen levels are increased, with supplementary oxygen, such as a Hudson
mask (Hampson, 1998)

What is Partial Pressure?
Partial pressure is the individual pressure exerted by a particular gas within a mixture of
gases. The air we breathe is a mixture of gases: primarily nitrogen, oxygen and carbon
dioxide (Hampson 1998). That part of the total pressure generated by oxygen is the 'partial
pressure' of oxygen, while that generated by carbon dioxide is the 'partial pressure' of
carbon dioxide (Limon 2008; Hampson 1998).

Given that total atmospheric pressure (at sea level) is about 760 mmHg and air is about 21%
oxygen, (Hampson 1998) then the partial pressure of oxygen in the air is 0.21 times 760 mm
Hg or 160 mmHg (Milton 2009).

With external respiration (exchange of oxygen and carbon dioxide between the atmosphere
and body cells) a pressure gradient exists, therefore partial pressure of oxygen in arterial
blood under normal resting circumstances is approximately 104 mmHg. This equates to a
fully saturated haemoglobin molecule.

PO2 (Partial Pressure of Oxygen) reflects the amount of oxygen gas dissolved in the
blood. It is determined by inspired oxygen concentration, barometric pressure, alveolar
ventilation, oxygen diffusion and distribution (Hampson 1998). It primarily measures the
effectiveness of the lungs in pulling oxygen into the blood stream from the atmosphere.

Elevated pO2 levels are associated with:
· Increased oxygen levels in the inhaled air
· Polycythemia
· Oxygen therapy and ventilation (Hampson 1998)
Decreased PO2 levels are associated with:
· Decreased oxygen levels in the inhaled air (such as at high altitude)
· Anaemia
· Heart failure
· Chronic obstructive pulmonary disease
· Hypoventilation (Limon 2008; Hampson 1998)

Physiology of Oxygen Transport
Movement of air in and out of the lungs is referred to as ventilation. Air reaches the alveoli and gas exchange then occurs across the thinly walled capillaries. Once oxygen has
crossed into the cardiovascular system, it is transported in the blood in two forms,
approximately 97% is bound to haemoglobin and the remaining 3% is unbound in the
plasma. Arterial oxygen saturation is referred to as SaO2, or when measured non-invasively
as SpO2, with peripheral monitoring providing a high degree of accuracy for saturations
between 70%-100% (Bishop and Nolan 1991).

The cardiovascular system may deliver oxygen to the tissues, but as oxygen has an affinity
(attraction) to haemoglobin, it may not readily release the oxygen molecule. When the
percentage of saturation of arterial haemoglobin with oxygen is plotted against the partial
pressure of oxygen (PO2), which is a measure of the oxygen concentration in the
surrounding medium, the oxyhaemoglobin dissociation curve is created, as shown in the
diagram below.

Google the oxyhaemoglobin dissociation curve and pull up a picture.

As illustrated, the thick black line shows the usual relationship between oxygen saturation
and the affinity it has for haemoglobin.

SpO2 levels above 95%, or when haemoglobin is nearly fully saturated, correlates to a PO2
value in the normal range of 80 to 100 mmHg (Simmonds and Simmonds 2004).

Functionally, this corresponds to the physiological situation in the lungs, where we want the
haemoglobin to have a high affinity to O2, pick it up, hold onto it and carry it to the tissues.

An SpO2 of 90% or below correlates to a PO2 of 60 mmHg or below. It is important to note
the shape of the curve, and recognise that as PO2 falls there is a precipitous drop in SpO2.
This corresponds to the situation in the peripheral tissues and capillaries, where we want thehaemoglobin to release the bound O2 to the tissues. Therefore, the PO2 of the blood
determines the amount of O2 that binds to haemoglobin in the red blood cells (or the affinity
of haemoglobin for O2).

This relationship can change, dependent on the conditions within the body, such as
temperature, pH (as reflected by H+) and amount of 2-3 DPG (a by-product of metabolism
that helps release O2 from haemoglobin). If the curve shifts to the left, it means that oxygen is more tightly bound to the haemoglobin, and can be caused by increased pH, decreased
temperature or decreased PaCO2. If the curve shifts to the right, it means that oxygen is not
as tightly bound to haemoglobin, and can be caused by decreased pH, increased
temperature and increased PaCO2 (Simmonds and Simmonds 2004). This makes it harder
for haemoglobin to bind to oxygen (requiring a higher partial pressure to achieve the same
oxygen saturation), but it makes it easier for the haemoglobin to release bound oxygen to
the tissues. Conversely, a leftward shift increases the affinity of haemoglobin to oxygen
resulting in reduced tissue oxygenation (Limon 2008).

The key points of the oxyhaemoglobin dissociation curve include:
· Oxygen saturation indicates the amount of oxygen bound to haemoglobin
· Oxygen saturation does not reflect oxygen delivery to the tissues
· As PO2 drops, oxygen saturation falls off precipitously
· The relationship between oxygen and haemoglobin can be affected by a number of
physiological factors, which then impact oxygen delivery to the tissues

What is Oxygen Delivery?
Oxygen delivery is a result of the respiratory system working in conjunction with the
cardiovascular system to supply oxygen to all cells of the body and to eliminate waste,
primarily carbon dioxide (Davis, Hwang and Dunford 2008).
Effective oxygen delivery requires the following:
· Ventilation moving air into your lungs
· External respiration, moving the oxygen from alveoli into your blood (Davis et al
2008)
· Enough normal haemoglobin to carry the oxygen
· An efficiently working cardiovascular system to transport the blood around your body
However the oxygen has still not reached the body’s cells.

The last stage of the journey is for the oxygen to get from the haemoglobin in the red blood
cells to all the other cells within the body, a process known as internal respiration (Davis et al 2008). In the capillaries oxygen detaches from the haemoglobin and diffuses through the
capillary wall to the cells around the capillary (Simmonds and Simmonds 2004).
If any of these factors are compromised, patients may present short of breath due to
underlying hypoxia. Hypoxia is defined as a state of decreased oxygen content at the tissue
level, while hypoxaemia is a state of decreased oxygen content of arterial blood (Simmonds
and Simmonds 2004)

Types of Hypoxia:
Hypoxaemic hypoxia: the decreased oxygen level in the blood results in decreased oxygen
diffusion to the tissues. It may be caused by hypoventilation, high altitude, pulmonary
embolism, atelectasis or other pulmonary diffusion defects (Davis et al 2008).

Anaemic hypoxia: occurs when either the total amount of haemoglobin is too small to
supply the body's oxygen needs (as in anaemia or severe haemhorrage), or haemoglobin
that is present is rendered non-functional. A transient example of the latter case is carbon
monoxide poisoning, in which carbon monoxide preferentially binds to haemoglobin
(displacing oxygen) resulting in diminished oxygen carrying capacity (Oh 2003; Davis et al
2008).

Stagnant hypoxia: blood flow through the capillaries is insufficient to supply the tissues.It
may result from heart disease that impairs the circulation, impairment of venous return or low flow states such as shock or cardiac arrest. Local stagnant hypoxia may be due to any
condition that reduces or prevents the circulation of blood in any area of the body. Examples
include thrombosis, Reynaud’s disease and Buerger's disease (which restrict circulation in
the extremities) and the application of a tourniquet to control bleeding (Davis et al 2008).

Hystotoxic hypoxia: the cells of the body are unable to use the oxygen, although the
amount in the blood may be normal and under normal tension. Although characteristically
produced by cyanide, it may be caused by any agent that decreases cellular respiration.
Some of these agents are narcotics, alcohol, formaldehyde, acetone, and certain
anaesthetic agents (Davis et al 2008).

Signs of Hypoxia
Symptoms of hypoxia vary greatly depending on the severity and may include:
· Headache
· Fatigue
· Nausea
· Vertigo
· Visual impairment
· Shortness of breath
· Altered conscious state
· Seizure activity
· Unconsciousness
(Davis et al 2008)
Infants Children
• Lethargy • Restlessness
• Bradycardia • Tachypnoea
• Hypotension • Tachycardia (bradycardia late sign)
• Apnoea • Cyanosis
• Pallor

Limitations of Pulse Oximetry
Pulse oximetry has limitations in its ability to provide a useful clinical measurement, and it is important for paramedics to have an appreciation of these in order to use it as an effective tool:

Motion artefact: due to oximetry measuring the difference between pulsatile (arterial) and
non-pulsatile (veins, bone, muscle) tissue, motion of the patient can cause a false reading by inducing pulsations in usually non-pulsatile tissue. Patient movement may lead to disrupted signals and artefacts (Pruitt and Jacobs 2004; Bilan et al 2010).
Poor perfusion: it can be difficult to get a reading on a poorly perfused or vasoconstricted
patient, due to the lack of pulsatile tissue in the periphery where the SpO2 sensor is reading.

Pulse oximetery is not reliable in conditions of severe hypotension and in such conditions an
ear probe may be more reliable than a finger probe (Bishop and Nolan 1991).
Similarly, avoid compromising blood flow to the limb (e.g. by inflating a BP cuff) to which the probe is attached to prevent a false low reading.

Dyshaemoglobins: pulse oximeters only differentiate between two haemoglobin species.
The presence of alternative haemoglobin types, such as COHb and MetHb can influence the
oximeter readings and give false SpO2 readings.

Pigmentation/dyes: skin pigmentation, dyes or other colourings such as nail polish can
provide interference resulting in false readings (Bishop and Nolan 1991). Dark pigmentation
may have a small effect, usually overestimating SpO2. If nail polish is thought to be
interfering in readings the probe can be placed sideways on the finger, or an ear probe used.

Irregular Pulsations: arrhythmias may not provide uniform pulsations, which can interfere
with oximeter readings (Bishop and Nolan, 1991). Venous congestion/tricuspid
incompetence may also cause inaccurate readings due to substantial venous pulsations.
Improper fit: if the probe does not fit properly, the light can be shunted from the LEDs
directly to the photo-detector affecting the accuracy of the measurement (Bishop and Nolan
1991).

Oxygen saturation only: the pulse oximeter measures oxygenation, not ventilation, that is
not CO2 levels, and not work of breathing. This is particularly important to remember, given
that there can be a time lag from a change in patient condition to the reading on the oximeter
(Bishop and Nolan 1991).

Alarm fatigue: as with any machine that has an alarm, if it goes off too often, paramedics
will tend to ignore it, or fail to explore why it has alarmed (Limon 2008).
Low readings: oximeter readings can be inaccurate below 70%. This is because the
original trials to establish oximetry readings done on human subjects were deliberately

Advantages of Pulse Oximetry
· Indicates early hypoxia more effectively than vital signs
· Identifies hypoxia before the onset of cyanosis
· Measures effective treatment
· Useful in assessing trends
· Non-invasive method of continuous SpO2 monitoring
· No calibration required
· 2% accurate between SpO2 70%-100%
(Bishop and Nolan 1991; Mardirossian and Schneider 1992)

Summary
· Pulse oximetry is a noninvasive method that enables rapid measurement of the oxygen
saturation of hemoglobin in arterial blood, thereby providing an early warning of
dangerous hypoxemia, prior to symptoms being obvious
· Pulse oximetry can be a useful aid to clinical decision-making, but it is not a substitute
for clinical assessment. Understanding that patient presentations vary considerably and
that pulse oximetry has limitations, pulse oximetry readings may not truly reflect the
patient’s condition
· Remember to treat the patient and not necessarily the numbers
· The PO2 of the blood determines the amount of O2 that binds to haemoglobin, as
represented by the oxyhaemoglobin dissociation curve
· Most of the oxygen in the blood is carried by haemoglobin so in severe anaemia the
blood will carry less total oxygen, despite the haemoglobin being 100% saturated
(Sedaghat-Yazdi et al 2008). As such, pulse oximetry must be used as an adjunct to
CPG A0103: Respiratory Status Assessment
· Pulse oximetry has limitations in its ability to provide a useful clinical measurement, and
it is important for paramedics to have an appreciation of these in order to use it as an
effective tool

So there you have it. The smaller units produced today are as accurate, and should be treated with the same caution, speculation and respect as the larger ones.

Ps. Pulse oximeters may or may not reflect accurate oxygen saturation, depending on the type of anaemia that is present. A person may be anaemic yet have 100% oxygen saturation, and vice versa.

Jase

"You see, in Texas you don't have the high elevation problem, so you don't get concerned. In California, we have lots of peaks above 14,000 feet, so it is an issue."

Guess a guy can't leave his state now?

Aside from that flippant response it is an interesting discussion. Report back your experiences please bob.

"Each year I drag some newbies up a big peak. For sea level, they are adequate hikers…"

I rescind my comment.

Hiker number two just had a cardiac incident of some type near sea level and was hauled off to the hospital. I hope he recovers.

–B.G.–

Egad, yes, let's hope he's all right.

I certainly see how the "pressure-breathing" would work. But I imagine that it gets tiring. And is difficult to do while sleeping. :)

Hmm. Another thing that might work is intentionally auto-PEEPing by exhaling against pursed lips, like asthmatics do instinctively when they get a little tight. Might cause other problems, though.

PS Jason- Really? Do you think many people here will follow the bit about oxyhemoglobin dissociation curves?

Wait, what am I saying? I guess this is BPL, isn't it? Not many other communities would be so critical of the experimental method used when measuring guyline tension or CO accumulation while cooking in a tent, eh?

"Egad, yes, let's hope he's all right."

After spending a night at the hospital, they are still waiting on test results. It seems like it was not a full-blown heart attack, and more like an episode of tachycardia. Still, that isn't good.

I don't believe that he will be going on the peak with us, so my oximeter gizmo may not see so much use.

–B.G.–

"PS Jason- Really? Do you think many people here will follow the bit about oxyhemoglobin dissociation curves?

Wait, what am I saying? I guess this is BPL, isn't it? Not many other communities would be so critical of the experimental method used when measuring guyline tension or CO accumulation while cooking in a tent, eh?"

…..they were my exact thoughts. What I dig about this forum is that people love to learn. :-)

The gizmo seems to function OK. For a while, I kept wondering why the results kept showing strange alphanumeric characters instead of four digits in seven-segment displays. Then I realized that I was looking at it upside down. Oh, well.

It almost seems like the pulse rate is a more interesting result, but when I get up to high elevation, the oximeter may get interesting.

At least it is under two ounces with batteries onboard, and they are supposed to be good for 24 hours of continuous operation.

–B.G.–

"For a while, I kept wondering why the results kept showing strange alphanumeric characters instead of four digits in seven-segment displays. Then I realized that I was looking at it upside down."

That's a good example of multi purpose gear, I'd say. If you hold it upside down it's a self administered a mental impairment test.

Alphanumeric characters are quite puzzling to decipher when you look at them upside down.

If you keep your fingers outstretched, the gizmo's display is facing away from you, so it looks funny. If you fold your test finger at the knuckle, you have the gizmo in the palm of your hand, and the display looks right.

Now, if I can't remember to push the ON button, that would be a good mental impairment test.

The other mental impairment test I have is to try to change some of the second-level operations in the menu on my digital camera. That's hard enough to remember at sea level.

–B.G.–