Mast Cell Disorders and POTS - Part II

If you have been diagnosed with Postural Orthostatic Tachycardia and/or Orthostatic Intolerance have you ever noticed that your digestive system can get uncomfortable and your body just doesn't like certain foods?  Have you been diagnosed with IBS or do you suspect you have Celiac Disease?  Do you feel better when you eat very simple foods such as proteins and veggies?  Are you sensitive to scents and perfumes? Do you react poorly when taking certain medications?

If you answered yes to any of these questions then investigating whether you have a mast cell disorder may be warranted.

What are mast cells?
According to Theo Theoharides from the Tufts School of Medicine:

Mast cells derive from the bone marrow and migrate into connective and mucosal tissues (Galli, 1993), where they are located at strategic points around capillaries close to nerve endings (Theoharides, 1996). Mast cells are critical for allergic reactions where the stimulus is immunoglobulin E (IgE) and specific antigen; however, there are also other nonimmune mast cell triggers that include anaphylatoxins, kinins, cytokines, as well as various neuropeptides (Baxter & Adamik, 1978; Coffey, 1973) such as somatostatin (Theoharides & Douglas, 1978), neurotensin (NT) (Carraway et al., 1982) and substance P (SP) (Fewtrell et al., 1982). When stimulated, mast cells synthesize and secrete numerous vasoactive, nociceptive and inflammatory mediators (Galli, 1993) that include histamine, kinins, prostaglandins, leukotrienes, cytokines, as well as the proteolytic enzymes chymase and tryptase (Schwartz, 1987). 

Mast cell disorders are still rather poorly understood.  As an Immunologist from Brigham & Women's said, "Our current level understanding of mast cells is similar to how well we understood T-cells in AIDS twenty years ago.  However, there is more research being done now which is very helpful in increasing our knowledge ."

How to find out if you have a Mast Cell Disorder?
The "gold standard" of diagnosing mast cell conditions used to be a serum tryptase test.  This is a blood test that can be requested by your family doctor.  Not many labs actually do tryptase tests so your blood sample could be sent as far away as the Mayo Clinic for investigation.  Tryptase is an enzyme which is produced by mast cells when they degranulate along with histamine and other substances.  An elevated tryptase result could indicate that your mast cells are busy and active.

However, researchers and specialists in mast cells now know a normal tryptase test does NOT rule out a problem with your mast cells.  Here is a link to a article published in the Journal of Hemotology by Molderings, Brettner et al in March of 2011 which not only describes the variety of mast cell misbehaviour but also diagnostic criteria and treatment options.

Does that mean I don't have to exercise anymore if my POTS or OI is related to a mast cell problem?
Whether your POTS is caused by mast cell problems or the cycle of the moon, evidence suggests that maintaining adequate blood volume and increasing your cardiovascular health through a specific program like the Levine protocol will help ameliorate the symptoms of POTS.

Unless you doctor specifically recommends you do not exercise because of some other reason other than you suffer from POTS or OI then you should really consider contacting the Levine team to see if you are eligible to participate in their study (I believe they are still accepting patients).

Mast Cell Disorders and POTS - Part I

Last April as I was happily performing my exercises as per the Dr. Levine protocol my blood pressure crashed.  Up until then I was feeling pretty good: my POTS symptoms were diminished thanks to the exercises and lotsa salt diet and I was looking forward to an awesome summer.

At the time of the crash I was using a rowing machine, I had my Garmin heart rate chest strap on and was tracking my heart rate using iCardio (DigiFit) on my iPad.  Twenty minutes into the exercise the bp crash occurred and I fell off the rowing machine.  The same blood pressure crash occurred the following day and the day after that.   The crash did not seem to occur when my heart rate increased rather it crashed after the increased heart rate was sustained for >5 minutes.This new symptom was perplexing because previous to then I had not had a blood pressure crash while sitting and exercising (rowing).

I emailed Dr. Levine's team describing these events.  The response was as follows: 1) these events are not consistent with typical POTS, and 2) they are probably not due to a structural defect with the heart (I was thoroughly checked for this).  Intriguingly I was asked if developed hives or shortness of breath while exercising.  I didn't but the question led me to investigate exercise anaphylaxis.

Exercise Anaphylaxis: Def - A form of allergy manifest by a sensation of skin warmth, pruritis and erythema, urticaria, hypotension, upper airway obstruction DiffDx Cholinergic urticaria, anaphylaxis.  Websters Dictionary

Well, it just so happens that a group at Brigham & Women's Hospital is conducting a clinical study on exercise anaphylaxis.  This group specializes in immunology, and more specifically, mast cell disorders.  I called one of the researchers and gave her my abbreviated medical history (the long form of the history takes several hours) which she kindly listened to.  She suggested that based on my history of POTS and anaphylaxis as a child I be investigated for a mast cell disorder.

Can mast cell disorders cause POTS?  The answer to that question appears to be yes.  In an article entitled Understanding the Mechanisms of Anaphylaxis by Richard D. Peavy and Dean D. Metcalfe from the Laboratory of Allergic Diseases, NIH they talk about when mast cells are activated producing histamine.

 Histamine stimulates vasodilation, and increases vascular permeability, heart rate, cardiac contraction, and glandular secretion.

Sudden vasodilation causes blood pressure to crash and people to fall off rowing machines.  Mast cell disorders and POTS...a worthy line of investigation for someone looking to get at the root cause of their POTS!!  

OI, POTS and Exercising - Procrastinating...?

I was planning on writing a pots on the exciting Midrodine story: the drama of the FDA vs Patients but elected to write about the POTS and exercise debate instead.  I personally believe this matter to be so important to the health of so many POTS sufferers I thought I would weigh in again.

POTS or OI can be caused by a number of different conditions but can ALSO BE CAUSED or MADE WORSE by deconditioning.  This is a fact.  By embarking upon an exercise program which helps overcome exercise intolerance it can (not in all cases) greatly assist with managing symptoms.  In some people it can be a CURE for POTS. (cure meaning the patients no longer met the criteria for POTS)  This is also a fact.

People who have OI may also have received a diagnosis for POTS (and vice versa).  Regardless of whether you have been diagnosed with POTS and/or OI you may still benefit from a managed exercise routine.

What is the difference between Orthostatic Intolerance and Postural Orthostatic Tachycardia?
Here is how Vanderbillt describes Orthostatic Intolerance and Postural Orthostatic Tachycardia:

When orthostatic symptoms occur in patients, but blood pressure does not fall as much as 20/10 mmHg on assumption of upright posture, the patient has orthostatic intolerance (OI). Additional criteria used for the diagnosis of OI at Vanderbilt’s Autonomic Dysfunction Center include an increase in heart rate of at least 30 beats per minute with standing, and a standing plasma norepinephrine level of at least 600 pg/ml. Because upright heart rate is usually greatly increased, the term Postural Tachycardia Syndrome (POTS) is also used.

Did you know Orthostatic Intolerance can be induced in otherwise healthy subjects?
Yes, that is correct.  Evil scientists can inflict this dreadful condition on subjects by enforcing bed rest, particularly if the bed rest includes a head down tilt posture.  There is a report by Waters et al which compiles the results from 16 studies from 10 different researchers on enforced bed rest and its consequences.  Each of these studies look at the MECHANISMS of orthostatic hypotension such as cardiac atrophy (aka "Grinch Heart)", sympathetic dysfunction, arterial and venous alteration.  It appears that a decrease in stroke volume is the nastiest culprit in promoting symtpoms of OI.

If I tie my husband to the bed for 2 weeks will he get POTS?

It seems that some otherwise healthy people are pre-disposed to getting POTS under certain circumstances and others are not.  In studies I have read up to 80% of healthy subjects can exhibit symptoms of OI after 2 weeks of bed rest.  My suggestion is to tie your husband down for two weeks if he appears unsympathetic to your symptoms.

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Superman, Strength and POTS

Living with a chronic condition can be a total drag for you, your family, friends. Postural Orthostatic Tachycardia is a particularly nasty variety of chronic illness because it can affect both the cognitive and physical functions of the body.

Working wirh only half a brain while struggling with pain and fatigue is a real challenge. In addition to coping with the physical realities there are also life changes that must be dealt with: the financial impact of leaving a job, the disruption to daily responsibilities such as grocery shopping, paying bills, even taking the dog out for a walk. These changes can create feelings of guilt over the necessity to rely on others to take over ordinary duties, of failure over loss of control over one's life, and of frustration that more cannot be done.

I have experienced all of these feelings at various times. Sometimes it seems very easy to cope and at other times it is hard to cope. And in the very hardest times I try to cling to my three mantras:
1) be hopeful 2) be forgiving and 3) recognize my inner strength.

Be Hopeful

Hope and faith are mixed together and like peanut butter and jelly in a sandwich; they compliment each other beautifully. We can draw upon faith to recover lost hope and in the presence of hope we are reminded of our faith.

Be Forgiving
Taking responsibility for our actions is logical and honorable. Feeling guilty for what is beyond our control is not. When I start feeling guilty about being sick and what effect my illness may be having on others I ask myself "have I done everything I reasonably can to help myself"? If the answer is no (for example not drinking enough fluids so triggering symptoms or binging on potato chips ;-) then I accept responsibility for my choice and try harder. Guilt is a negative feeling and not conducive to recovery... get rid of it.

Recognize inner strength
One doesn't have to go far to stumble over stories of great courage, endurance and perseverance.  But even the most heroic characters can have moments of weakness. Recognize all the things we do which can be considered heroic: To get out of bed and face another day when we are hurting and sick is heroic.  Doing exercises when it is hard to stay upright is heroic. Staying positive when we want to give in to sadness takes strength.

I congratulate myself on the small stuff.
Let's give ourselves credit for facing tough challenges!

Soldier On!

Is the Levine Protocol a Cure for POTS?

Excellent question if I do say so and I believe there is a very simple answer: If a patient's POTS or OI is caused by deconditioning then the answer could be "yes".  If the POTS is caused by something else then it probably will not cure POTS because in order to do that you would need to cure the underlying condition. 


If a patient's POTS is caused by something other than deconditioning then will they benefit from the protocol?

Here is my rather long answer to the above.  POTS does not seem to be a disease rather is a cluster of physical symptoms which present when certain conditions are met.

For example, here are some conditions which may cause and/or make POTS symptoms worse:

• Ehlers Danlos Syndrome
• Parkinson's
• Multiple Sclerosis
• Mastocytosis
• Mast Cell Activation Disorder
• Hypthyroidism
• Menstruation

But regardless of which set of conditions are present in a patient certain factors appear to be key to ameliorating the nastier symptoms of POTS.  Namely:

1. Ensuring adequate blood volume
2. Promoting and maintaining healthy cardiovascular and circulatory systems

The protocol is designed to help with both 1 and 2 above while at the same time dealing with the inconvenient matter of exercise intolerance which POTS patients can suffer from.

So to answer the question above as "If a patient's POTS is caused by something other than deconditioning then will they benefit from the protocol?"  the answer is most probably yes!

Wall Street Journal article POTS, Grinches and Astronauts

Check out this article published in the Wall Street Journal about Dr. Levine's research with astronauts and its application to treating symptoms of Postural Orthostatic Tachycardia or Orthostatic Intolerance:

One Giant Step For Better Heart Research?
By AVERY JOHNSON
Researchers in the midst of a several years' study of how astronauts' hearts react during long space voyages reported initial findings that they say could help improve cardiac care on Earth.


They found that astronauts benefitted from certain types of exercise, something that could help patients with heart failure or abnormal heartbeats, or those who are bedridden after surgery, a stroke, or during pregnancy, whose hearts atrophy much like an astronaut's in space.


Heart muscles don't have to work as hard to circulate blood in space because gravity doesn't exert the same force. Astronauts returning from missions frequently feel lightheaded and sometimes pass out. After weeks or months in space, the heart appears to shrink, cardiologists say, blood volumes decrease, and the astronaut can experience hypotension, or abnormally low blood pressure.

What's Lost in Space

Studying how astronauts' bodies change in space may help patients on Earth.

• Heart mass decreases up to 25%
• Blood volume decreases up to 20%
• Bone density decreases about 3%
• Blood pressure stays relatively the same. (Back on Earth, blood pressure can decrease by 20% to 25% or even more.)

Source: Benjamin Levine; Michael Bungo

The research is in its early stages, but some of it offers fresh interventions for heart patients. Benjamin Levine, one of the leaders of the NASA project, is studying astronauts exercising in space to help patients with what he calls "Grinch Syndrome," characterized by low blood pressure and the inability to stand up without losing consciousness. (The name refers to how it causes the heart to be two sizes too small like that of the Dr. Seuss character.) Dr. Levine found that seated exercise, such as using a rowing machine, can aid those patients.

The cardiologists presented the research from the NASA-funded project last week at a symposium held every other year called "Humans in Space," and earlier this month at the annual meeting of the American College of Cardiology.

"Cardiovascular research in space gives us the unique opportunity to study the effect of gravity on the heart and has led to novel understandings and therapies," said Dr. Levine, a cardiologist at University of Texas Southwestern Medical Center.

Twelve astronauts are expected to undergo detailed ultrasounds while in space and MRIs before and after flight for half a year. The project has been collecting data for two years and so far five astronauts have completed the testing. In addition to changes in blood volumes and heart size, researchers are also investigating reports of heart palpitations, known as arrhythmias, in space. To Full Article...

Love and POTS

I received this press release via email the other and thought I should share the news about this happy couple!  (I have made one or two edits to the original release but only to remove the gritty details on the contest and to calm down the sales pitch)  I think it is really great that these two have found joy and comfort in one another.   May their swoons be swoons of happiness and not fainting episodes from low blood volume!

In February of this year, Steuben, a crystal glass manufacturer in the US, held a sweepstakes contest where the entry winner with the most heartfelt sentiment would receive a fiery red crystal heart- Internal Flame heart, engraved with the sentiment personalized as an engraving.

Kyli Wolfson’s entry of “Our love is a song I want to hear every day,” won the vote and when the folks from Steuben asked for more of her story they learned she and her fiancé, Dustin, suffered from Postural Orthostatic Tachycardia Syndrome. Dustin had said the words of comfort to Kyli during a night when she was particularly sick.

A condensed description of the disorder in the couple’s words is: excessive heart rate increments upon upright posture – meaning you feel faint when standing up – but that’s just part of it. Your heart races as if you’re running in place all the time. Research shows that a POTS patient’s quality of life is similar to that of a congestive heart failure or chronic obstructive pulmonary disease patient. Coping with this disorder requires some serious lifestyle adjustment, and still results in many hospital stays.

Remarking that they’d seen each other at their very worst, Kyli pointed out that they see every day as a date. She continued, “Whether we’re on a picnic or at the hospital, there’s always love between us and a reason to smile.”

Interestingly, although Dustin’s declaration to Kyli describes how many couples feel regardless of age or health, one of the symptoms of POTS this couple shares is an increased sensitivity to sound. When they met by chance, each traveling with separate groups of friends, they were often retreating for quiet time at the same intervals.

In Dustin’s words, “It’s not so much about music…the love is like the melody and the lyrics are how you show your love.”  The tune of his statement was fresh in Kyli’s mind when she made the winning entry to the crystal heart sweepstakes and in Dustin’s mind when he proposed soon thereafter. As the couple plans their April, 2012 ceremony, they will have the hand-engraved Internal Flame as a lasting symbol of their commitment.

Kyli summarized their struggle and triumph with POTS, “Life with chronic illness gets terrifying at times. It’s almost always hard, always a challenge and often very, very lonely. There are times when I feel like I’m stuck inside a house that’s crumbling to pieces…times when the rest of the world seems like it’s moving, moving, moving...and I’m stuck in the same place. But on nights like this one, sick and scared, I remember the blessings…big and small…that have come out of this illness of mine. And I’m grateful.”

About Steuben
Steuben is the premier crystal glass manufacturer in the United States, and the world’s finest maker of pure crystal (according to Steuben). Since the 1930s, Steuben has collaborated with some of the world’s most important artists, including Henri Matisse, Salvador Dali, and most recently Ross Bleckner.

Salt Intake and Managing Blood Volume

There has been some really interesting posts on the new Facebook Group about sodium intake.  When I first started the Levine protocol I gradually built up to 10,000mg per day with the help of my nutritionist.  Working with my local physician I had my blood tested weekly to ensure my potassium levels were ok.

After about a month such high levels of salt I no longer needed such a high dose and gradually reduced the salt to a point that "felt" right.  I found that too much could trigger what felt like heart palpitations and other unpleasant feelings.

Since the purpose of the high sodium is to help with blood volume, it makes sense to combine the effort with the "bed wedge", carefully avoiding diuretics like caffeine and alcohol (I must admit the occasional glass of wine will sneak by my lips) and ensuring plenty of fluids.

The blood volume challenge is, for me, one of the less onerous lifestyle changes to make in helping managing POTS symptoms. Love that salt! ;-)

Day 44 of Dr. Levine's Exercise Protocol for POTS (A.S.*)

Tracking my Heart Rate using Digifit and an iPad
For the past 4 workouts I have being using my iPad, DigiFit software, and an ANT+ chest strap to view and record my heart rate during exercise.  I have found that my heart rate is much easier view on the iPad than on the wrist watch where I have to take my hand off the rowing machine bar to see the read-out.  A difficult maneuver when you're really going at it.

I can input the various workouts prescribed in Dr. Levines exercise protocol into the Digifit software on the iPad.  I can include the duration of the exercises and the specific heart rates zones they are to be performed within.  When I am ready to do a workout I simply go to the appropriate date in the calendar and click on the exercise.  Digifit automatically connects to the heart rate strap and away I go.  Once I am done I can upload the heart rate record to Training Peaks or even New Leaf (if I decide to start a metabolic exercise program).  The only thing Digifit does not record are the gasps and groans coming out of me while performing the exercise.

Today was a day when the protocol dictates I do my exercise on my rowing machine at the highest heart rate zone.  I had to keep my heart rate above 140 for 30 minutes.  This is, by far, one of the more difficult exercises in the protocol but I did it and I did not expire.  This being my second time around for Dr. Levine's exercise protocol for POTS I know that although I FEEL like I am going to expire I actually won't.  (If, on the other hand, I unexpectedly do expire then at least Digifit will be recording the event for posterity.)

My Golden Retriever has POTS...?

You want me to do what?

We know that physical deconditioning can cause or contribute to POTS- like symptoms.  We know that sleeping with your head down and your feet up can induce orthostatic intolerance in otherwise healthy people.  So, if the same can be said for dogs then my golden retriever MUST have Orthostatic  Intolerance or even Postural Orthostatic Tachycardia!

Over the winter months when the snow is on the ground and squirrels are snoozing in their nests, our dog Jack is a master at chilling on his bed.  We have known him to sleep for days, stirring himself only for the most serious situations such as eating, receiving ear scratches and tummy rubs or going outside to do "his business",  If the weather is inclement then he simply refuses to go out of doors, choosing instead to go back to sleep until such time as the weather is conducive for a short walk.  The winter for Jack is couch potato time.

Now that spring has arrived (whoopee!!!) and the little red squirrels are out in force, Jack will be resuming sentry duties guarding against 4 legged trespassers.  Will Jack experience a racing heart rate, chest pain, dizziness, exercise fatigue?  Will the winter-long deconditioning affect his squirrel chasing performance?

Obviously the danger of Orthostatic Intolerance from too much bed rest is of greater concern to working professionals such as circus dogs who must remain upright for prolonged periods.  As for Jack, we will keep him on all four paws and gradually get him back into shape in time for summer.

Chest Pain and Postural Orthostatic Tachycardia Syndrome

Have you ever wondered why people with POTS or OI get chest pain?  According to Dr. Raj in his paper The Postural Tachycardia Syndrome (POTS): Pathophysiology, Diagnosis & Management, "chest discomfort" is one of the common clinical features of the condition.

I get chest pain frequently when I am in Stages IV or V and only occasionally when I am in Stages II or III.  It can be a very frightening aspect of POTS and OI because although you may have experienced the same pain many times in the past, there can be a nagging doubt in the back of one's mind that says "OK this time it's really a heart attack"

Over the years I have asked a number of cardiologists and general practitioner MDs for an answer as to what causes the pain and I always receive the same response.  (See picture on right.)

That is until I asked two experts; our very own Dr. B. Levine and a Dr. Klabunde, Associate Professor of Physiology at Ohio University who has a cool website called cvphysiology.com which has lots of interesting and colourful pictures describing the cardiovascular system.

Why do POTS patients have chest pain?
Answer #1 from Dr. Levine
I find this a rather comforting response without being a definitive answer to the question. The response refers to the fascinating theory of "hyper-vigilance" which makes POTS patients sound like people who weep over stubbing their toe or getting splinters in their fingers....

"This is a very difficult question and I must admit that I am not certain of the answer.  I am quite confident that it is not heart pain though, at least in the vast majority of POTS patients.  I do have to be cautious however since every person is an individual and there may be unusual patients with coronary anomalies (when the coronary arteries are in the wrong place; associated with ~ 15% of deaths during sports in young people), genetic abnormalities of cholesterol metabolism (such as familial hypercholesterolemia), or even true coronary spasm (constriction of the heart arteries, either in the large or small blood vessels; this latter is also called "syndrome X" and in some women is associated with a reduced ability to increase blood flow in the microcirculation of the heart). However most POTS patients are previously healthy young women with normal coronary arteries, and ECGs, even during chest pain virtually always fail to show any changes of myocardial ischemia/injury, such as might be seen with coronary spasm. 

It is important to remember that there are lots of things in the chest other than the heart that can cause pain -- the esophagus, joints connecting the ribs to the sternum, chest muscles, lining of the lungs and heart, etc and these can be quite painful.   I think that most patients have reflux with esophageal spasm, or occasionally inflammation of the chest wall joints or structures. Most POTS patients have normal heart function on echocardiograms (all in my experience) and therefore no evidence that the heart is being injured or damaged. 

The Mayo clinic group have written about the "hypervigilance" associated with POTS and I believe this plays some role too.  Let me emphasize that this does not mean the patients are making it up -- rather that sensations that fail to rise to our awareness in most individuals, seem to be more acutely sensed by POTS patients.  To keep in line with our model, as a counterpoint to the "runners high" (associated with endorphin release and downregulation of endorphin receptors), we jokingly refer to this problem as the "couch potatoe's low" (reduced endorphin levels and dowregulation of receptors) and first noticed the phenomenon in our bed rest studies.  Procedures like pulling off of electrodes, or putting in IVs which were almost painless before bed rest, became nearly intolerable after bed rest, with one volunteer literally jumping off the bed after a blood draw."

Answer #2 from Dr. Klabunde
The Klabunde answer has an interesting theory but please also read Dr. Levine's reply following this answer.

"Chest pain can occur in patients with tachycardia because increased heart rate increases myocardial oxygen demand (seehttp://www.cvphysiology.com/CAD/CAD004.htm). Furthermore, increased heart rate reduces the length of diastole (cardiac relaxation and filling) relative to the length of systole (cardiac contraction and ejection). Because most of the coronary blood flow occurs during diastole (see http://www.cvphysiology.com/Blood%20Flow/BF001.htm), tachycardia can impair coronary blood flow and therefore oxygen delivery. The bottom-line is that tachycardia can reduce the oxygen supply-demand ratio (see http://www.cvphysiology.com/CAD/CAD006.htm).  This leads to myocardia hypoxia and anginal symptoms in some patients, particularly those who already have underlying coronary or heart disease."

Response from Dr. Levine to Dr. Klabunde's answer:

"POTS patients have normal coronary arteries and young, healthy people often get very high heart rates during periods of increased myocardial oxygen demand, LIKE DURING EXERCISE! This does not cause angina. In my opinion, as both a cardiovascular physiologist and clinical cardiologist, the chances of this pain being due to the heart's being starved of oxygen is virtually zero."

Creepy Heart Cartoon

So although many people with POTS and OI experience chest pain or discomfort, there could be variety of explanations as to why.  However, I believe that the two answers above serve to highlight the importance of a full diagnostic work up to rule out other possible conditions that may be causing POTS and OI symptoms such as chest pain.

They also serve to remind me that I prefer NOT to encourage any other conditions like heart disease to be troublesome along with POTS and OI...hooray for carrots (and other vegetables)

Choosing a Heart Rate Monitor for Orthostatic Intolerance and Postural Orthostatic Tachycardia Syndrome (POTS)

I had received some emails asking which heart rate monitor I would recommend to use for POTS exercise training.  When performing Dr. Levine's exercise protocol for POTS or for other programs for POTS or OI, it is critical that all cardio exercises are performed within specific heart rate ranges with the objective to strengthen the heart muscle and improve cardiovascular performance.  Obviously a reliable heart rate monitor is essential to keeping a close eye on what your heart rate is doing.

Heart rate monitors are made up of two different components; the sensor and the receiver.  Sensors can be built into the receiver like the Timex T5G971 sport watch or they can be built into a separate chest strap sensor.

There are a few different types of wireless signals used by monitors which you should be aware of: the POLAR wireless signal is proprietary to POLAR and one of the first ever to be used for the purpose.  At a 5.3 Khz frequency it has a very short range and can be confusing at the gym when your heart rate signal gets mixed up with the guy next to you doing a marathon "spinning" session.   However, many gym equipment manufacturers have built POLAR receivers into their machines; a consideration if you work out regularly at a gym.

The newer standard is a digital 2.4 Ghz frequency.  Used by Nike+ and POLAR Wind in a proprietary format, it is also available in an open standard called ANT+ that allows for interoperability between devices such as the iPhone or other smartphones.  The 2.4Ghz ANT+ also is a low-power consumer so you don't have to change batteries nearly as often as the POLAR 5.3Khz models.

What I look for in a heart rate monitor
#1  Clarity of the Display
When I am on the rowing machine gasping and panting, I prefer not to have to squint at a tiny display to see what my heart rate is doing.  A large visible screen that can be easily read when eyes are full of perspiration is important.

#2  Will work when wet
In the summertime, I love to do my cardio in the pool. Finding a monitor that works reliably when swimming is a challenge. Water can get in between the sensor and skin to interrupt the connection which is a nuisance when you're mid-workout.  In my experience, a tightly fitted chest strap sensor works better than an all-in-one for pool use.

#3  Compatibility

ANT+ HRM

If you work out in a gym then the cardio equipment you are using could be helpfully capable of receiving a heart rate signal.  Sometimes you can see  a "Polar" logo on the equipment's control panel or you can ask the gym management if their equipment is capable of receiving a heart rate signal and if yes, then from which manufacturer.  Watching your heart rate on the machine you are using can be very convenient.

For Apple Junkies you can get an ANT+ transceiver for the iPhone or iPad and with an ANT+ chest strap monitor you can not only display the heart rate on your phone or iPad but also can input your heart rate data into the cool "DigiFit" software available on iTunes.

Next month I will be purchasing the Apple transceiver to go with my iPad and iCardio app from DigitFit.  According to the developer, iCardio can:

"turn your iPhone, iPod touch or iPad into a heart rate monitor and fitness computer. Using Digifit Connect and a heart belt, iCardio tracks BPM, calories, zones and much more."

Exciting! Hopefully with DigiFit I will be able to closely track my incredible improvement in cardio performance over time.