Cardiovascular Monitoring during Anesthesia 台大醫院麻醉部 詹偉弘
Electrocardiogram (EKG) One of the standardized monitors during any form of anesthesia. For detection and diagnosis of • dysrhythmias • conduction defects • cardiac ischemia • electrolyte disturbance
Types of Monitoring Three-electrode system: right arm, left arm, left leg; white is always right, black is on the left, red is even lower. (Racial discrimination!!)
Types of Monitoring • Five-electrode system: one on each lead and one precordial lead (V5, along the anterior axillary line in the fifth intercostal space for detection of anterior ischemia).
Electrocardiogram (EKG) Kaplan et al. (Anesthesiology, 1976): 90% of intraoperative cardiac ischemia will be detected by multiple EKG, especially V5. At least two leads should be simultaneously showed on the monitor.
Position of the Leads • The four limb leads should be placed on the back of shoulders and hips, where they will disturb the operative field the least. • 顏色:白綠黑棕紅 • Every lead should be fixed and protected with tape to prevent dislodgement of leads during operation.
Artifact Source of EKG • loss of insulation • motion artifacts • crossing cables (especially the pulse oximeter cable, which transmits an amplified signal.) • Electrocauterization
Electronic Filtering System 1. Monitoring mode: 0.5~40 Hz, eliminates high and low frequency artifacts, but distorts the height of QRS and the degree of ST depression. 2. Diagnostic mode: 0.05~100 Hz, does not filter high frequency artifacts.
Non-invasive Blood Pressure (NIBP) Monitoring • Drawbacks: inaccuracy, intermittent data, requirement of a pulsatile flow. • Common methods of detection of change in flow: 1) Auscultation of Korotkoff sounds 2) Microprocessor-assisted interpretation of the oscillations.
(from Essential Noninvasive Monitoring in Anesthesia. New York: Grune and Stratton, 1980)
Non-invasive Blood Pressure (NIBP) Monitoring • Familiarity with various type of the automated monitors is of prime importance.
Cuff Size • Too small cuff will result in high blood pressure reading. • A loosely applied cuff will also produce a reading higher than it should be. • Too large cuff will severely distorted the reading obtained.
Cuff Size from Barbara Bates: A Guide to Physical Examination
Intravascular Pressure Measurement • Components include: intravascular catheter, fluid-filled connector tubing, transducer, electronic analyzer, display system. • Rapid establishment of pressure-transducer system is essential in emergent conditions.
Definitions of Terms • Frequency Response (Amplitude ratio): Amplitude of input vs. output. Ideally 1. • Natural frequency (fn, or Resonant frequency): the frequency at which the monitor system itself resonates and amplifies the signal. • Common natural frequency of transducer system is 10~20 Hz.
Intravascular Monitoring • The natural frequency of the measuring system should be at least 10 times of the fundamental frequency (HR). • When heart rate is fast (more close to natural frequency of the system), wave can be abnormally amplified Damping is necessary.
D: diameter; L: length of tubing : density of the fluid; P/V: compliance
Damping Coefficient • Damping coefficient: the rate of dissipation of the energy of a pressure wave. It can correct the erroneous amplification of under-damped system.
Flush Test • The damping coefficient can be evaluated by “flush test”(pop test). • After a high-pressure flush, an under-damping system will continue to oscillate for a long period of time. Overestimation of SBP and under-estimation of DBP.
Flush Test • An over-damped system will not oscillate after a flush but will return to baseline slowly. • An optimally (or critically) damped system will oscillate one or two times only and will reproduce BP accurately.
Sources of Errors 1)Low-frequency responses: Air in line or failure in flush device with formation of partial clot in catheter. Over-damping. 2)Catheter whip: Motion of catheter tip itself produces a noticeable pressure swing; not common in A-line but common in PA catheter.
Source of Errors 3)Resonance in peripheral vessels: The systolic pressure measured in a radial artery may be up to 20~50 mmHg higher than in the central aorta. 4)Change in electronic balance: electronic zeroing should be done periodically to preclude baseline drift (for example: due to change in room temperature).
Source of Errors 5)Transducer position error: ideally at right atrium. (Midaxillary line in supine patients.) Changes may be less significant in blood pressure but significant in CVP or PA pressure/wedge pressure.
Air Bubbles in Line • Air bubbles can result in a lower frequency response and greater resonance response. • Small amount may augment systolic pressure reading; while large amount cause an over-damped system.
Site for Arterial Cannulation • Radial artery: Modified Allen’s test: Apply pressure over both radial and ulnar arteries simultaneously and ask the patient to squeeze his or her hand several times to promote exsanguination. Then release the pressure on the ulnar side and measure the time for refill of the nailbed of the capillary. If refill time is greater than 15 sec, it is a positive test, indicating inadequate collateral flow.
Site for Arterial Cannulation • Femoral artery: excellent access to central arterial system. Insertion with a Seldinger method. • Brachial artery: medial in antecubital fossa, but since it is an end artery, its cannulation is relatively contraindicated. However the incidence of thromboembolism is low.
Site for Arterial Cannulation • Dorsalis pedis and posterior tibialis artery: distortion of the artery wave. • External temporary artery: rarely used
Choice of Cannulation SiteSpecial Considerations • Blalock-Taussig shunt: contralateral to the shunt • Coarctation of Aorta: right radial/brachial is preferred • Thoracic aortic aneurysm: right radial • Ascending aorta surgery: left radial • Liver transplantation: two cannulations are required
Complications of Arterial Catheterization • Ischemia: incidence is reportedly low. • Thrombosis: incidence may be high but no adverse sequelae were reported. • Infection: incidence is higher in femoral cannulation. • Bleeding
Complications of Arterial Catheterization • False lowering of radial artery pressure immediately after cardio-pulmonary bypass: up to 72% immediately after cardiopulmonary bypass the radial artery pressure is significantly lower than the aortic pressure. Mechanism: Vasodilatation? Hypovolemia? AV shunting?? If an arterial trace is damped after cardiopulmonary bypass, a direct pressure measurement should be obtained from central site.
Central Venous Pressure (CVP) • measures right atrial pressure • A wave: with P wave, atrial contraction • C wave: QRS complex, tricuspid bulging into RA • X descent: when tricuspid valve is pulling down during the latter stage of ventricular systole; indicating emptying of blood from ventricle.
Central Venous Pressure (CVP) • V wave: after T wave, RA filling before tricuspid opening. • Y descent: when tricuspid valve opens and atrium begins to empty. • TR: giant V wave replacing C wave • af: absence of A wave
Central Venous Pressure (CVP) • AV dissociation: frequent cannon A wave (atrial contraction over a closed tricuspid valve) • Rapid and exaggerated x, y descent in constrictive pericarditis.
CVP values • a direct measurement of RV filling pressure, CVP is good measurement of LV filing pressure only in the absence of pulmonary hypertension or mitral disease.
Site of Insertion • Internal jugular vein, external jugular vein, subclavian vein, femoral vein • PICC: peripheral inserted central catheter, inserted from antecubital vein (or basilic, cephalic vein).
CVP and PEEP • The effect of PEEP is rarely significant when PEEP is less than 7.5 cmH2O. • PEEP may cause increased CVP value while decreases cardiac output. • Discontinuation of PEEP for measurement of CVP is not recommended.
Pulmonary Artery Catheter It can measure: • PA pressure • Pulmonary capillary wedged pressure (PCWP): a balloon at catheter tip (volume 1.5 ml), when the balloon is inflated and the vessel is wedged, a valveless hydrostatic column exists between the distal port and LA.
Pulmonary Artery Catheter • CVP: a port for CVP measurement is located at 30 cm from the tip • Cardiac output: measurement of RV output • Blood temperature • Derived hemodynamic data • Mixed Venous O2 saturation (SvO2)
Pulmonary Artery Catheter • PA Port: YELLOW • CVP Port: BLUE • PA balloon Port: PINK
Length of Insertion Usual conditions (just for ease to memorize) • 35 cm: RV • 45cm: PA • 55 cm: wedge • But the actual length may vary greatly between patients!
Hemodynamic Measurements--Normal Range • RA pressure: 2~6 mmHg • RV pressure: systolic 15~25 mmHg; diastolic 0~4 mmHg • PA pressure: systolic 15~25 mmHg; diastolic 8~16 mmHg • Mean PAP: 10~20 mmHg • Pulmonary Capillary Wedge Pressure (PCWP): 6~12 mmHg