CV Monitoring Flashcards
Why do we need to monitor ECG?
- 1/3 of all patients undergoing non-cardiac surgery have risk factors for CAD
- Postoperative MI is 3x as frequent in patients with ischemia
- Prolonged stress-induced ischemia is a major cause of cardiac morbidity after major vascular surgery
- Incidence of perioperative ischemia in patients with CAD ranges from 20-80%
- ECG monitoring remains a non-invasive and low cost diagnostic tool that can reduce morbidity
What is the Cardiac Conduction System?
Heart muscle conduction system automatically initiates and coordinates muscle contraction
Heart is able to generate rhythmic electrical impulses
The conductive system consists of:
SA node: has the most rapid rate of spontaneous depolarization, is the dominant cardiac pacemaker
Inter-nodal tracts
AV node
AV bundle
Purkinje system
What are the functions of the SA node, Internodal tracts, AV node, Bundle of His and Purkinje system in the cardiac conduction system?
-SA node: – pacemaker; junction of SVC and RA
- Internodal tracts: atria; conduction pathways b/w SA and AV nodes
- Bachmann’s- anterior
- Wenckebach- middle
- Thorel’s- posterior
- AV node: right side of atrial septum, anterior to coronary sinus
- Bundle of His: conduction pathway from atria to ventricles
- Purkinje system: right and left bundle branch system
What are the two types of ECG leads that are used in Standard Recording Electrodes and Leads?
- Bipolar: consistws of 2 electrodes placed at 2 different sites to measure the difference in potentials between these 2 electrodes
- Unipolar: measure the absolute electrical potential at one site in relation to a reference, or remote site where the potential is zero
What is special about a 12-lead EKG?
A 12 lead EKG includes recordings from 12 leads, including:
3 bipolar leads (I, II, III)
6 unipolar precordial leads (V1-V6)
3 modified unipolar limb leads (augmented limb leads AVR, AVL, AVF)
What do each of the blocks on an ECG strip indicate?
Time 1 small block = 0.04 sec 5 small blocks = 0.20 sec Amplitude/Voltage 1 small block = 0.1 mV
What is indicated by: P-wave PR interval QRS QT interval T-wave
P-wave: atrial depolarization PR interval: atrial systole and AV nodal delay QRS: ventricular depolarization QT interval: ventricular systole T-wave: ventricular repolarization
What occurs during normal electrical activity of the P wave?
Sinoatrial (SA) node has the most rapid spontaneous depolarization rate and is the dominant cardiac pacemaker
The impulse normally spreads to the atrioventricular (AV) node, through 1-sided and 2 sided pathways
The P wave is the result of normal depolarization of the atria
The anterior right atrium is activated first, then the signal shifts posteriorly as activation proceeds over the left atrium
The P wave in the right precordial leads (V1 and V2) is commonly biphasic, a positive deflection, followed by a negative deflection
In L
What occurs during normal electrical activity of the PR Interval?
Is the temporal bridge between atrial and ventricular activation
Conduction systems are activated
AV node
Bundle of His
Bundle branches
Intraventricular conduction systems
Most of the conduction delay during this segment is due to slow conduction within the AV node
Normal 0.12-0.20 secs
Good estimate of AV node function
The PR segment is an isoelectric line, therefore, it is used as the reference point for measuring ST elevation and depression
What occurs during myocardial activation of the QRS complex?
Is the manifestation of left and right ventricular muscle depolarization
Ventricular excitation spreads from the bundle branches to the Purkinje fibers, which are broadly dispersed throughout the entire endocardial surfaces of both ventricles
Excitation of the Purkinje-ventricular muscle junctions in the endocardium then proceeds by conduction from muscle cell to cell to activate the entire ventricle thickness toward the epicardium
The normal pattern of activation of the ventricles starts in the intraventricular septum
Produces an initial small positive R wave in AVR and V1 and small negative waves in I, AVL, V5, V6
Normally less than 0.12 secs
What occurs during normal activation of the ST segment and T wave?
ST segment starts at the end of the QRS to beginning of the T wave
Represents the period when the ventricles are repolarized
Normally are deflecting in the same direction
The junction of the QRS complex and the ST segment is called the J Junction
J-point and the ST segment are compared to the PR interval to detect elevation or depression
What is the QT interval
Duration from the Q wave to the end of the T wave
Is highly HR dependent
Time interval from beginning of depolarization to end of repolarization
Should not exceed ½ the length of the R-R interval
Long QT is a risk factor for tachyarrhythmias and sudden death
U wave: hypokalemia or hypomagnesemia
What are the normal measurements for PR, QRS, QT, J point and ST segment?
PR interval: 0.12 to 0.20 seconds (3-5 blocks)
QRS interval: < 0.12 seconds (<3 blocks)
QT interval: 0.36-0.44 sec (9-11 blocks)
J point: a transition point measured between end of QRS and start of ST
ST segment: normally level with isoelectric line
What is the difference between single vs multiple lead monitoring?
Misplaced leads can lead to delayed diagnosis of ischemia
Surgical procedures can interfere with proper lead placement
Single lead monitoring is inadequate to monitor patients with CAD
Monitoring with multiple leads enhances patient safety
If a preoperative 12 lead ECG has been done, it should serve as the primary guide for lead selection in the perioperative period
Which leads are best for detecting ST segment changes?
Research studies all agree that lead II is recommended for assessment of arrhythmias
1988 recommendations for V5 and lead II as preferred leads to monitor for ST segment changes has been challenged by 2002 Landesburg study
Nagelhout: Lead V3 was found to detect ischemia earliest and most frequent (87%).
V4 was second most diagnostic lead (79%) and V5 was 3rd (66%)
With those patients sustaining an MI, V4 was the most sensitive lead (83%) with V3 and V5 being the second most sensitive (75%)
V3, V4, V5, III, and AVF (in this order of preference) should be selected for continuous monitoring For ST segment changes
Ehrenwerth: V4 was the most sensitive to ischemia (83%), followed by V3 and V5 (75%)
V4 was closest to the isoelectric level on the baseline ECG, rendering it most suitable for ischemia monitoring
Miller: Leads V3 and V4 were more sensitive than V5 in detecting perioperative ischemia
Among patients who experienced an MI, V4 was most sensitive for ischemia (83%), followed by V3 and V5 (75%)
Ehrenwerth and Miller: Monitoring both V3 and V5 increased the sensitivity to 97%
