Cardiovascular Disease and Monitoring
Mayur Narayan
Thomas M. Scalea
I. Introduction
Cardiovascular monitoring can help guide optimal therapy by ensuring adequate oxygen delivery to tissues while allowing for assessment and manipulation of cardiac contractility.
Hemorrhage is the most common etiology of shock after injury. Patients with blood loss or those undergoing surgery may also develop cardiac dysfunction. Critically ill patients at risk for development of systemic inflammation from trauma or sepsis will require monitoring to guide supportive measures and limit organ dysfunction. Shock and hypoperfusion can be occult or compensated, yet just as deadly and require more aggressive monitoring. Specific injuries require targeting specific endpoints, such as maintenance of cerebral or spinal cord perfusion pressures.
II. Electrocardiography
Electrocardiogram (ECG) is a static depiction of the electrical events of the heart. The basic tools are a 12-lead ECG coupled with continuous monitoring of one or two leads allows early detection of heart rate (HR) or ischemic changes, transient abnormal beats, and dysrhythmias. Inadequate electrode–skin contact, electrical interference within the equipment, patient motion or shivering, may produce artifacts and an inaccurate HR.
Heart rate monitoring
The most practical clinical estimate of HR is the pulse rate palpated in an extremity.
More accurate determination of HR can be assessed by:
Observing the arterial pressure waveform of an invasive arterial catheter
Observing ventricular contractions during echocardiography
Observing changes in pressures using a pulmonary artery catheter (PAC)
Auscultation for cardiac sounds alone is often unreliable in the ED and ICU, especially when HR is rapid or with excessive ambient noise.
Note: The pulse rate typically equals the mechanical HR. However, do not assume this to be true. Exceptions to this rule include irregular ventricular contractions, states of poor peripheral perfusion, or cardiac pump dysfunction. It is important to recognize the patient with an electrical heart rate on the monitor but without corresponding cardiac activity and perfusion (electromechanical dissociation).
Ischemia monitoring
ECG changes suggestive of myocardial ischemia may occur without associated symptoms or signs. Early detection allows management of myocardial ischemia. The precordial leads are the most sensitive for detection of myocardial ischemia, particularly V1, aVf, and V5. V5 is the most sensitive lead, detecting 75% of ischemic events. Lead II is most sensitive for P wave evaluation and inferior wall ischemia. Simultaneous monitoring of leads II and V5 is recommended.
ST changes
Elevation of the ST segment 1 mm or more in the clinical setting of acute ischemic chest pain represents acute myocardial injury before it evolves into irreversible infarction; it is an indication for thrombolysis or percutaneous revascularization. A down sloping depression more often represents ischemia than a horizontal or up sloping segment. In patients with Q waves, ST segment elevation may be related to wall motion abnormalities. In patients without Q waves, leads with ST segment
elevations are specific indicators of the location of ischemia. T wave inversions and new Q waves are suggestive of either recent or evolving MI. Other causes of ST elevation include pericarditis and coronary vasospasm.
Table 36-1 Commo n Dysrhythmias
Atrioventricular
Atrioventricular junctional rhythm
Atrioventricular nodal re-entrant tachycardia
Accelerated atrioventricular junctional rhythm
Wolff–Parkinson–White syndrome
Ventricular
Ventricular tachycardia
Ventricular flutter
Ventricular fibrillation
Supraventricular
Sinus bradycardia
Sinus tachycardia
Sinus arrest
Sick sinus syndrome
Sinus node re-entrant tachycardia
Atrial flutter
Atrial fibrillation
Multifocal atrial tachycardia
Ectopic atrial tachycardia
Atrial re-entrant tachycardia
ST segment depression occurs when coronary perfusion drops below a critical value but before injury (latter causes ST elevation).
Dysrhythmia monitoring
Continuous ECG monitoring identifies changes in cardiac rhythm. Dysrhythmias are divided into tachydysrhythmia (ventricular rate >100 bpm) or bradydysrhythmia (ventricular rate <60 bpm). The origin of the electrical impulse can be sinoatrial (SA), atrial, atrioventricular (AV), or ventricular. The presence of P waves suggests SA node origin and is best observed in lead II. Lack of P waves or abnormal P wave morphology indicates an origin other than the SA node, and an associated QRS complex widening may help localize the source to the AV junction or the ventricles (QRS >0.14 msec is usually from a ventricular source).
A full rhythm strip can determine the rhythm and reveal ectopic or multifocal patterns exist. Conduction sequence analysis determines whether the conduction pattern is normal, delayed, blocked, or aberrant. These identifying points facilitate appropriate treatment selection. Table 36-1 outlines common dysrhythmias.
Arterial pressure monitoring
Measurement of arterial blood pressure quantifies the hydraulic pressure head supplying the cardiovascular system. Depending on the method used, there can be great measurement variability. The mean arterial pressure (MAP) is the mean blood pressure during the cardiac cycle and estimates end organ perfusion. It is calculated by MAP ≈ DP + 1/3(SP − DP), where DP is diastolic pressure and SP is systolic pressure. MAP is highest in the ascending aorta, and drops until the peripheral arteriolar bed is reached.
Noninvasive arterial pressure monitoring
Noninvasive techniques utilize a cuff that is connected to a sphygmomanometer that occludes a peripheral artery to the point of no-flow. Measurements that occur at the time of arterial occlusion and return of flow correlate with arterial blood pressure.
Palpation
Systolic arterial blood pressure can be estimated, based on measuring the pressure required to compress the brachial artery. This is often used when a stethoscope is not readily available or when background noise precludes auscultation. The cuff should be inflated to a level about 30 mm Hg above the pressure at which the pulse disappears. Next, release the pressure slowly until the return of a regular palpable pulse. This pressure is a rough estimate of systolic arterial pressure.
Auscultation
Using a stethoscope for auscultation gives the clinician the ability to measure diastolic pressure in addition to a more accurate systolic pressure. Releasing
the cuff pressure allows return of pulsatile flow producing Korotkoff sounds. These sounds can be grouped into phases ranging from 1 to 5 according to AHA guidelines. Phase 1 is when sounds are first heard and corresponds with systolic arterial blood pressure. Phase 5 is when the sounds disappear corresponding to diastolic pressure.
Automated intermittent devices: Oscillometry
Automated noninvasive blood pressure provides regular and repeated pressure measurements using oscillometry. As the occluding cuff is slowly deflated, the return of arterial pulsations results in a counter pressure onto the cuff. With continued cuff deflation and progressive increase in arterial pulsations, the oscillation amplitude increases. The pressure is the point of rapid increase in oscillation amplitude, while the mean pressure is the point of maximum oscillation amplitude. With some devices, diastolic pressure is taken to be the point of rapid decrease in oscillation amplitude while with other manufacturers, the diastolic pressure is a derived value based on the systolic and mean pressure measurements. In the hypotensive patient (SBP <80 mm Hg), the automated intermittent devices may falsely overestimate actual blood pressure. In these situations, manual techniques or an invasive arterial line are better.
Future of continuous automated technology
Photoplethysmography
Photoplethysmography relies on infrared light transmission to monitor the volume in a finger. An inflatable cuff maintains the finger at a constant volume, and continuous arterial pressure is measured as the counter pressure required to maintain constant finger volume. Measurements of pressure with this technique are sensitive to misapplication of the finger cuff, contributing to errors and limiting the reliability of the device.
Arterial tonometry
This technique utilizes a surface pressure transducer applied onto the skin directly over an artery, to measure transmitted arterial pressure directly generating a continuous waveform and pressure measurement. In normotensive and hypertensive patients, tonometric pressures and waveforms usually correlate with the intra-arterial measurements. The reliability of the measurements is limited during rapid or large changes in blood pressure.
Invasive arterial pressure monitoring
Intra-arterial cannulation with continuous blood pressure transduction is the common method for arterial blood pressure monitoring in critically ill patients (Table 36-2).
Table 36-2 Indic ations for Invasive Intra-Arterial Blood Pressure Monitoring
Patient-related indications
- Shock states
- Significant coronary artery disease
- Myocardial pump dysfunction
- Significant cerebrovascular disease
- Significant pulmonary disease, COPD, PE, pulmonary hypertension, ARDS, pneumonia
- Severe renal, acid–base, electrolyte, or metabolic disorders
- Severe burns
- Major procedures involving large fluid shifts or blood loss
- Anticipated deliberate hypotension, hypothermia, or hemodilution
- Procedures with high risk for spinal cord ischemia
- Liver, heart, or lung transplantation
- Aortic cross clamp or other major vascular procedures
Intra-arterial site selection
Sites commonly used include the radial, femoral, and axillary arteries. Typically, the radial artery of the non-dominant hand is utilized.
Alternate sites can be chosen based upon several criteria:
The artery should be large enough to accurately reflect systemic blood pressure.
The chosen site should be free of infection.
There should be sufficient collateral flow to prevent distal ischemia.
The limb should be free of injury (e.g., proximal or distal fracture, crush, etc.).
Allen’s test is often done prior to cannulation of the radial artery to assess adequacy of perfusion and collateral flow. The radial and ulnar arteries are compressed. The patient’s fist is clenched until it blanches. The patient’s hand then relaxed and the pressure on the ulnar artery is released. If the hand becomes hyperemic, collateral ulnar flow is considered adequate for radial artery cannulation. Cold, ischemic digits are an absolute contraindication to radial arterial cannulation. Large pre-operative trials note that this test has limited utility, though avoiding cannulation when collateral flow is absent is wise. If short term cannulation (hours to day) is planned, clinically important occlusion is rare.
Complications of intra-arterial pressure monitoring
Mechanical
Thrombosis is the most common mechanical complication. Partial or complete radial artery occlusion occurs in more than 25% of patients after prolonged (>1 day) radial artery cannulation. Necrosis requiring amputation is rare. Remove any arterial cannula as soon as evidence of ischemia, infection, or embolic events is seen.Related posts:
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