Background:  I am 46 now and 225-230 lbs depending on what day of the week I weigh myself.  This time last year i was 270-275...By changing my diet and exercising more (mostly crossfit/circuit training classes) I was able to achieve my current weight.  Plan is to get to 180-190 lbs by this time next year.

One thing I am struggling greatly with though is my heart rate.  It simply gets too high too quickly and I can't catch my breath, get faint, dizzy...etc.  

Example, running on the treadmill at a 5.5 mph pace, my HR can easily get to 160 in 3-4 minutes and than I need to slow it down to a walking pace for 2 minutes or so to bring it down.

Will this get better as I do more and more cardio??

I would see a Dr. and get a stress test.  That will give you some of what you need to know.

I believe also your body needs time to often adjust to weight changes.

You have to build your aerobic base.
You have to work in the easy zone and stay there.   Long slow distance, even walking is fine will build you aerobic system base.

There's two rules out there.   one is  Maffletones, which is stay below 180-your age = HR.   This is really low, but it does work.

the other one is find your VT1, it's the HR where talking becomes uncomfortable.  Below this HR you can talk pretty comfortably, above this, you can talk, but in much shorter pieces.  Get on a treadmill with a HR monitor (the hands work) and go a little faster every 2 minutes and say the pledge of allegiance.  You'll know when you reach it.  Record your VT1, stay below that marker.  It should (and probably will) be higher than the Maffletone HR.  

But part of it is that your body is doing a LOT of work, running a big body is a lot of work.  

Fat people shouldn't run, find something else to do as cardio that isn't so taxing on your joints.

A high heart rate by itself isn't necessarily a bad thing.  For any given age, the higher your heart rate can go in strenuous exercise, the better,

as the higher rate is associated with a younger cardiovascular system.



But don't confuse a steady rapid beat with a racing heart.  
What you should look for is how quickly your heart rate returns to normal after you've completed the exercise.



The faster it comes down, the better.





Hit the elliptical trainer, rowing machine, or other aerobic/cardio machine and put in a full hour at a level you can manage.

Do this three times a week.  Or twice, anyway.



Yes, an hour.  Even if that's an hour at a slow walk.  



You WILL build cardio endurance.



I try to maintain a heart rate of between 140 and 145 for my hour on the elliptical machine.  I'm 48.  



When I'm done my heart rate is back to normal in two minutes or less.  

See a doc. If the doc says nothing is wrong...

If you get winded after 3-4 minutes or a 5.5 mph pace, then 5.5 mph is too fast for your current level of fitness

Your HR will decrease at a given pace as a result of some combination of:  losing weight so your power output is less for a given pace and  your heart getting larger in response to increased demands put upon it during exercise (which means an increase stroke volume.  Cardiac output = stroke volume * heart rate)..

HR is an output, a dependent variable.  How much work you are doing is one of many inputs to your HR.  Just go easier and find a sustainable pace for yourself.

I don't know enough about your situation and how you exercise to offer medical advice (and I'm not a doc) but if you aren't warming up before you run (raising your heart rate through warmup exercises) it is pretty common for your heart rate to peak 3-5 minutes into a run and then drop back down to a steady state rate after your body has adapted to your pace. You are a bit on the heavy side so if you are just running with no warmup you are asking your body to go from rest to full speed ahead in a very short period of time. A warm up essentially does that for you in a more gradual and controlled setting before you start to run and generally your pulse won't peak as high and will settle into steady state more quickly. If I run with no warmup my pulse will go from 55-60BPM to 160-165 before dropping back to my steady state running rate of about 130 at an 8 minute pace. The phenomenon of falling back to that steady state is called your second wind. A high pulse is not necessarily a signal of danger but as most everyone has said, if this is all new to you a general medical checkup isn't a bad idea. And warm ups do more than get your pulse up in advance, they also warm the muscles and can result in fewer injuries.

Getting checked out by a doctor and then finding your max heart rate (hint: it's not 220-age) would be a good idea.

My max HR is around 215. 160 is a slow run to me, and I can't even maintain a HR lower than 150 when running.

I run a lot on treadmills in the winter. My first run in January after taking a couple months off my heart rate soars over 180 and doesn't recover as fast. Now, it takes a lot of effort to get it up there and keep it.

I hate to say it, but it just sounds like you are  a little out of shape, and it should get noticeably better as you stick to running and push yourself to get better.

Actually,  the true "second wind" is when your body switches over from aerobic metabolism to anaerobic metabolism.  Intake oxygen not being

sufficient to fuel the sustained exertion level,  your body seeks alternative energy sources and actually reduces oxygen demand.   But at a price.

Your body will burn fatty acids and even protein to sustain energy demands.



It's really a total emergency reflex.  "Out of breath, must escape the bear at any cost.  Burn the muscles for fuel!"
 

Also, you could try interval training, which it sounds like you are doing. You might not quite be ready for HIIT yet.

Example:
1Warm up: 10 minutes of walking at 3.8 mph
2Run: 3 minutes at 6 mph
3Walk: 2 minutes at 3.8 mph
Repeat 2 and 3 for an increasing amount of time. At first, go for 25 minutes. Eventually you can work yourself up to 45 minutes, and start increasing your speed on the intervals as well. This forces your body into work/recovery cycles that should help.

This is no more correct than the last time you posted it.

It's my understanding that your body metabolizes both fat and muscle glycogen simultaneously. However eventually your muscle glycogen supply runs out and your are mostly metabolizing fat as your exert yourself.

The body is always using fat (aerobically) and glycogen (aerobically and anaerobic).  At low intensity levels, the percentage of ATP coming from aerobic metabolism of fats is high.  As intensity increases, the percentage of ATP coming from metabolism of fatty acids decreases and ATP from glycolysis increases.  At very high intensity levels, the preponderace of ATP come from non-aerobic metabolism.

Non aerobic metabolism can produce ATP faster, but the body cannot sustain the process so the tradeoff is an increased work rate (power output) but over a shorter duration.  That's why an increasing reliance on anaerobic metabolism is not a second wind.  It's a last ditch effort to sustain a work rate and the more ATP that are produced that way, the less sustainable the effort.

If there is such a thing as a second wind beyond a mental breakthrough, it probably happens when you voluntarily or involuntarily reduce power output back to a level that is sustainable primarily by aerobic metabolism.

thanks for the great replies....

bottom line is i just need to do cardio more often....

As far as fat burning goes, based on the post prior to this one low intensity workouts burn more fat than high intensity?

Not necessarily.

Some studies show greater fat loss from HIIT than steady state.  Do your homework to check the study definitions of each.

Low intensity exercise burns a larger percentage of calories from fat, but moderate intensity burns more total fat.  The relevant analogy is "would you rather have 50% of what's in my wallet or 10% of what's in my bank account"

Don't want to put all your eggs in one basket?  Alternate HIIT and moderate intensity endurance work.