The   heart  location  and the relationship with the 12 eletrocardiographic leads, is responsible for diferents R/S ratio in normal or abnormal ECG.  The electric forces between LV/RV change in normal 12 leads and also in LV  or RV hypertrophy  and also in myocardial infarction

QRS may also change as a function of chest position which changes the position of the heart in the chest. QRS being the resultant of many vectors, any change in position or body composition will change the QRS height. The morphology is more a function of the progression of the electrical activity within the heart.

If QRS amplitude changes every other complex, it's electric alternance, which is often a sign of heart failure.

In every cardiac beat there is systole and diastole and vectors variation in morphlogy and in cavity diameter size are both responsible for such amplitude variation of the QRS in the same position.

This is a simple question with a simple explanation:

The electric vector of the heart, i.e. the summation of all single cellular dipoles active at the same time, forms an electric vector loop in space throughout the cardiac cycle. This vector loop changes its orientation  in a high temporal resolution during systole and diastole. The electrical leads present on the ECG are nothing but representations of this geometric movement according to their own spatial orientations.

Using Cabrera's Circle the electric main axis of the heart at the peak QRS complex can be calculated. Therefore, it is not to be expected that the electrical activity of the heart results in similar R vector sizes on different ECG leads. The rather rare case of this entity (R vector in I, II, and III positive and similar in size) is called "sagittal type".

Hi Richard -

To try to put it very simply, the morphology of the waves being similar tells you that the electrical activity is following the same pathway through the heart with each beat. When a beat arises in an unusual (ectopic) location or follows an unusual pathway through the heart, the wave morphology will change. Do an internet search for a "PVC" to see what this looks like.

The height of each complex is determined by the electrical axis of the heart. As described above, this axis is the "net sum" of all of the tiny little vectors that occur as the depolarization wave sweeps through the heart. Under usual circumstances, the vector in a human generally points from the right atrium to the tip of the left ventricle.

The relationship of the axis to individual ECG leads is this: if the vector points toward a lead, it causes a positive deflection. If it goes away from a lead, it causes a negative deflection. The lead that the axis points most directly toward (in many cases, II) will be the tallest (most positive).

Also as suggested above, mechanical things happening in the heart can cause subtle beat to beat variability in the position of the heart. If it occurs with every other beat, it is more suggestive of problems (search for "electrical alternans" as suggested above). 4mm of variability is a pretty significant difference - it is had to speculate on what might cause a change of that magnitude without knowing more.

Dear Lynch, Richard M

Please, attach this ECG.

Voltage changes of 0.4mV or more ( I assume you have been referring to a 10mm/mV calibration) on one strip within one lead are unusual. If they are slowly periodic they might be due intrathoracic changes like pneumothorax, beat to beat changes can be indicative of a pericardial effusion. Of course badly placed leads or other artifacts can cause similar changes. As Adail mentioned you  should upload the ECG for clarification

Are the changes random or periodic? I would suppose a periodic change would be due to variyng distance/position between the leads and the myocardium - e.g. respiratory excursions, swinging heart in pericardial effusion. Could you upload the image? Any clinical or echocardiographic hints? 

Thank you to everyone who has submitted an answer

Didn't know I could upload pictures

The ones attached are a good example of what my question relates to. Not all the QRS complexes in some leads are the same height as the other QRS complexes in the same lead. Can anyone explain?  Richard

Looks related to breathing - it is clearly a change in impedance, the voltage change is more than usually expected so I would suspect some airway disease/ increased space between pericardium and chest wall.

ELETRIC ALTERNACE(V1) IS COMPATIBLE WITH PERICARDIUM EFFUSION, IN THIS OLD PATIENT WE OBSERVED "Q" WAVES  IN AVR AND V1 AND THIS IS COMPATIBLE WITH MYOCARDIAL INFARCTION

The first ECG is a typical example of what we use to call a "respiratory sinus arrhythmia". Completely physiological, and no single ectopic beat visible; just a little bit high heart rate (in case this comes from an appropriately taken resting ECG). For taking a resting ECG appropriately, one should ensure that the person has been at rest for at least past 5 minutes before ECG, and neither sleeping nor aroused nor subject to posture changes.

If you would measure the RR interval as well as the peak amplitude (R to S' difference), you`d get an approximate sinus function for each. Inbreath causes a restriction for the operating volume of the heart, decreasing the heart beat interval and increase of the electric power (amplitude) measured, while outbreath does the opposite.

The second ECG is nut normal, a little puzzling, as there is a high heart rate (~120 beats per minute, I suppose) and no clear p-waves visible, not even in the precordial leads. Unfortunately, the rhythm strip is missing. In seems that we see a P-wave in superposition to the T-wave. As I can't see a stimulus from a pacemaker, this is probably not a pacemaker ECG (but I'm not completely sure as the print only gives limited details. There is no notable variation in RR intervals, which would be typical for atrial fibrillation. But, from the missing P-waves I would guess that there may be a sick sinus syndrome. Usually, with sinus function missing, people fall into an escape mode of stimulation. On atrial escape rhythm, I would expect a heart rate of 60 to a maximum of 90, but not 120. On junctional escape rhythm from tissues adjunct to the AV node, I would expect a heart rate between 30 and 60. On ventricular escape rate from conductive tissue like Purkinje fiber we would expect extremely low rates of 25 to 50 beats/minute. Help us solve the puzzle of the second ECG....

The qrs amplitude in the black and white ecg seems to vary with respiratory cycle.  Looking at the v5 lead rhythm strip, you can see that not only is the amplitude changing but the rate changes some as well.  This may suggest that the is a change in impedence ( and hence waveform amplitude) with inspiration.  It is interesting however that this is a much more subtle change in heart rate than often seen with pronounced sinus arrhythmia.  Any more details about that patient? The colored ecg has some clear abnormalities but lacks a rhythm strip

Because the heart does not beat always the same way (it does not get filled the same way either) and also because it moves within the chest (mainly due to respiration). Variability is actually good for you. :-)

Thank you to everyone for your comments

The pink tracing I think is atrial flutter with 2:1 block

I understand that if RR interval varies so too will the SV due to differences in filling time. However this is not the case here.

Movement of the heart within the chest =/- sinus arrhythmia (but that is not present here either)

Am I missing something obvious / important. Height of first complex in V3/4 different to the others.

The black and white ECG - I need to review the presenting complaints - I think that this is sinus arrhythmia with multifocal atrial rhythm

I welcome your comments

Richard

Now looking at the black and white ECG you can clearly see that the variation in QRS voltage is periodical, the frequency of this change is 10/min, which would nicely corresponmd to breathing - as the patient inhales the insulation layer of air between the heart and the chest wall increases so the voltage decreases. You can draw a line over the tips of the QRS complexes and see that it is a wave.