ECG Changes in Electrolytes Imbalance (Theory)
ECG is known as the recording of activity of heart. Electrolytes are responsible for different body mechanisms. Before we understand the effects of Electrolyte Imbalance on ECG, we need to understand the basis of ECG. Let’s start.
Normal ECG wave :
P Wave – Atrial Depolarization
QRS Complex – Ventricular Depolarisation
T Wave – Ventricular Repolarization
Pr Interval (Normal) = 0.12 - 0.20
Qt Interval (Normal) = Men (420 – 440 Ms) & Women (440 – 460 Ms )
Qt Interval Shows Ventricular Depolarization + Ventricular Replarisation
Basics Of Physiology
1. Scenario 1 (Resting Phase) -
Sodium Potassium Pump Is Present Which Throws 3 Na+ Out And Puts 2K+ In. As This Ratio Is Not Equal, So Outside Becomes More Positive, And Inside Becomes More Negative. This Difference Is Known As Resting Membrane Potential Which Is -90mv.
2. Scenario 2 (Depolarization) –
Na+ Channel Opens, Na+ Moves From Outside To Inside. Membrane Potential Starts Becoming Positive. It Is Known As Depolarization. So It Means Sodium Influx Results In Depolarization.
Special Point : Calcium Ion Is Like Sodium Ions. When Calcium Channel Opens And Calcium Moves Inside Cells. It Results In Increase Of Positive Ions Inside Cell Membrane So The Positive Charge Inside The Cell Increases In The Cell. Hence It Is Understood That It Will Also Cause Depolarization.
3. Scenario 3 (Repolarization) –
When K+ Channel Opens, K+ Moves Out From The Cell. Due To This No. Of Positive Ions Inside The Cell Decreases So It Reduces The Positive Charge Inside The Cell Produced By Na+ And Ca2+ Ions. This Process Is Known As Repolarization. So It Is Clear That K+ Efflux Results In Repolarization.
CORRELATION OF CARDIAC ACTION POTENTIAL TO ECG
At Resting Phase The Resting Membrane Potential (Rmp) Is -90mv. Cardiac Action Potential Is Divided In 4 Phases -
1. PHASE 0 : In This Phase Na+ Channel Opens Up So Na+ Moves Inside The Cell, Which Increases No. Of Positive Ions Available Inside The Cell. Membrane Potential Starts Becoming Positive. This Increases The Cardiac Muscle Contractility And Causes Ventricular Systole. This Is Known As Depolarization. Voltage Increases From -90mv To +30mv. So In Phase 0 Depolarization Starts.
2. Phase 1 : In Phase 1, Na+ Channel Closes And K+ Channel Opens. K+ Channel Opens And K+ Starts To Move Out , So Repolarization Starts. You Will Notice That There Is A Decline Of Voltage In The Graph.
3. Phase 2 : K+ Channels Are Still Open And Additionally Ca2+ Channels Also Open . As We Have Discussed That The Ca2+ Ions Are Like Na+ Ions Which Causes Depolarization. So Now It Is Clear That These Both Ions Will Counter Each Other’s Function. Hence Repolarization Could Not Be Completed. You Can See In The Graph That In Phase 2 There Is Straight Line. SO THIS Phase Is Also Known As Plateau Phase.
4. Phase 3 : In Phase 3 , Calcium Channels Close And Potassium Channels Are Still Open. So Now Potassium Starts To Repolarize The Cell. This Results In Decreased Mucle Contractility. So There Is Complete Decline In The Graph. K+ Completes Repolarization And Rmp Of 90mv Is Restored.
Now The Basics Of Ecg Is Clear. Now We Will Discuss About
The Ecg Changes In Electrolyte Imbalance
1.Calcium : Whenever You Try To Think About The Ecg Changes
In Electrolyte Imbalance, You Need To Think About The Action Potential Curve.
A. Hypercalcemia : Short ST Segment, Short QT Interval
In Hypercalcemia There Is More Calcium In Blood, When
Calcium Channel Opens Then More Calcium Ions Will Enter In The Cardiac Cell
This Increases The Positive Charge Inside The Cell And Results In
Depolarization. So Muscle Contractility Will Be More. Due To This Phase 2
Becomes Shorter In Duration. That Leads To Short St Segment. As St Segment Is
The Part Of The QT Interval So QT Interval Also Reduces.
B. Hypocalcemia : Long ST Interval , Long QT Interval
In Hypocalcemia There Is Less Calcium In Blood, When Calcium
Channel Opens Then Less Calcium Ions Will Enter In The Cardiac Cell This
Decreases The Positive Charge Inside The Cell. So Muscle Contractility Will Be
Less. Due To This Phase 2 Becomes Longer In Duration. That Long To Short ST Segment.
As ST Segment Is The Part Of The QT Interval So QT Interval Also Increases.
2. Potassium : Potassium Is The Most Important Electrolyte
As It Causes Maximium Ecg Changes. So It Is Necessary To Understand This
Phenomenon. Maximum Questions Are Being Asked On This Topic.
So As I Have Said First We Should Think About Cardiac Action
Potential Curve.
A. Hyperkalemia :
Potassium Correlates With Phase 3 Of Cardiac Action
Potential. So In Hyperkalemia More Potassium Ions Will Come Out Of The Cardiac
Cell Through K+ Channels. This Will Cause Increased Repolarization So The T
Wave Will Be Peak And Tall.
>5.5 = Tall And Peak T Wave
>6.5 = Wide P Wave , Prolonged PR Interval, Eventually P
Wave Disappears
>7.5 = Affects Phase 1, Widening Of QRS Complex And Also
Causes Narrow ST Segment
>8.5 = Fusion Of QRS Complex & T Wave , This Leads To
Appearance Of Sine Wave Pattern Of The ECG.
B. Hypokalemia : In Hypokalemia Less Potassium Ions Will
Come Out Of The Cardiac Cell Through K+ Channels. This Will Cause Decreased
Repolarization And Muscles Will Not Be Relaxed, So The T Wave Will Be Flat And
Inverted. Repolarization Reduces It Leads To Prolonged Qt Interval. T Wave Becomes Flat, This Leads To ST Depression.
Important Note : In Hypokalemia The Cardiac Muscles Will Not
Be Relaxed This Leads To Chances Of Arrythmia (ATRIAL FIBRILLATION). Earliest Change
Will Be Presence Of A Prominent ‘U’ Wave Which Develops Just After T Wave.
Hypokalemia Also Causes Prolonged PR Interval Because It
Reduces The Conduction Through AV Node And Bundle Of His.
QRS Complex Is Normal In Hypokalemia
3. Magnesium: Magnesium Abnormalities Are Same As Potassium
Abnormalities.
A. Hypermagnesemia :
·
Wide QRS Complex
·
Prolonged PR Interval
·
Tall And Peak T Wave
B. Hypomagnesemia :
·
ST Depression
·
Prolonged PR And QT Interval
·
Flat T Wave Or Inversion Of T Wave