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Critical factors influencing Coronary Blood Flow

• Review: critical factors

  •  Heart rate (diastolic filling time)

  •  Coronary perfusion pressure

  •  Coronary vascular tone

  •  Intraluminal obstruction (e.g. plaques)

•  Most vulnerable region to reduce coronary flow (ischemia): subendocardial region--

  • Rationale: The subendocardium has enhanced metabolic requirements due to:

    • relatively increased systolic shortening

    • systolic blood flow restriction

  • Left ventricular subendocardium perfusion -occurs essentially during diastole only

  • Right ventricular subendocardial flow: occurs during systole

    • Rationale differences in left vs. right intracavitary systolic pressures

  • Volume of subendocardial perfusion blood flow-- dependencies

    1.  diastolic pressure

    2.  diastolic duration

• Coronary Perfusion Pressure

  •  Definition  (approximation): aortic diastolic pressure (AoDP) - left ventricular end-diastolic pressure (LVEDP)

  • Factors that reduce flow:

    1.  increased coronary vascular tone

    2.  intraluminal obstruction

  • An increased pressure gradient is favored by a reduced end-diastolic left ventricular pressure (ventricular filling pressure)

  • Reduced (low) ventricular filling pressures would be expected to have a positive consequence in terms of the myocardium because:

    •  improved coronary vascular perfusion

    •  reduced myocardial oxygen  demand (MVO₂) secondary to decrease ventricular volume & wall tension)

      •  recall that wall tension and contractility are the primary determinants of myocardial oxygen  demand.

      •  Ventricular wall tension is directly proportional to:

        • intracavitary pressure & ventricular radius

      •  Ventricular wall tension is inversely proportional to:

        • wall thickness

• Coronary vascular reserve

  • Definition: the difference between supply (autoregulated) - blood supply available with maximal vasodilatation

    •  Coronary Autoregulation-- maintenance of constant blood flow over range of perfusion pressures (50-120 mm Hg) to accommodate a given myocardial oxygen requirement

    •  "Autoregulation" refers to pressure-dependent changes of the level of coronary resistance vessels, principally coronary arterioles (diameter < 150 um) into a lesser extent small coronary arteries

    •  Changes in myocardial oxygen requirements induce changes in the autoregulation state -- principally in response to alter myocardial oxygen tension, PO₂, mediated by agents such as adenosine.

    •  Adenosine -mediated changes in vasodilatory state is related inversely to arterial diameter -- greatest changes (dilation) in the smaller vessels

    •  Coronary autoregulation is associated (coupled) with coronary venous PO₂ (especially with oxygen tension < 25 mm Hg)

  • Autoregulation:

    •  Greater autoregulation in the subendocardium; increased autoregulation in left vs. right coronary arteries (reduced pressure/flow -- O₂ requirement dependencies on the right side may explain this difference)

    •  Regulators:

      •  Metabolic, multiple pathways

      •  Basal vascular tone & normal autoregulation dependent on K_ATP channels which are activated secondary to adenosine receptor activation.

      •  Other possible mediators include: potassium, oxygen itself, prostaglandins, nitric oxide {NO, a.k.a. endothelium-derived relaxing factor}, prostacyclin, histamine and ATP.

        • Prostaglandin E₁: coronary vasodilator (adenosine -mediated)

        • Prostaglandin E₂: coronary artery vasoconstrictor

        • Acetylcholine -- increases flow

        • Histamine promotes coronary arterial vasospasm.

  • Myocardial blood supply requirements (flow) are adjusted by regulation of diastolic vascular resistance in small intramyocardial arterioles

Part of figure 32-1: Reference --Bell, JR, Fox, AC: Pathogenesis those subendocardial ischemia. AM J Med Sci 268:2, 1974, as cited by: Wray Roth, DL, Rothstein, P and Thomas, SJ Anesthesia for Cardiac Surgery, in Clinical Anesthesia, third edition  (Barash, PG, Cullen, BF, Stoelting, R.K, eds), Lippincott-Raven Publishers, Philadelphia, pp. 836, 1997

• With epicardial arterial obstruction,  i.e. more stenosis, flow is maintained by progressive, increasing dilation of the small intramyocardial arterioles -- with attendant reduction in coronary reserve.

• Note that as this pathological condition progresses, reduced coronary reserve makes it more likely that an  imbalance between myocardial oxygen requirements and supply will occur. This condition may lead to ischemia.

• Generally, intraoperative hypotension is more likely to cause myocardial ischemic states than hypertension

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Myocardial Oxygen Supply & Blood Oxygen Content

• Blood oxygen content = hemoglobin concentration X O₂ saturation X 1.34

• Since blood volume & oxygenation is typically well maintained during anesthesia, management of oxygen supply with changing oxygen demand depends on management of coronary blood flow.

Supply Ischemia

• An example of  "Supply" ischemia would be transient, possibly sudden, changes in cross-sectional luminal vessel area within attendant reduction in blood flow to the myocardium.  These dimensional changes may be sufficient to induce ischemia.

• Angina as well as myocardial infarction may occur even in patients with apparently normal arteriograms as a result of vasospastic constriction of coronary vessels.

•  The likelihood of supply ischemia is increased with preexisting coronary vascular disease, reflected by the presence of atheromas/plaques within the vessel.

• Intraoperative factors that may induce "supply" ischemia may include:

  • an increase in circulating catecholamines

  • a possible enhancement of interaction between platelets and atherosclerotic regions

• Intraoperative ischemia, as judged by ECG changes for example, may occur without changes in:

  1. heart rate

  2. blood pressure

  3. ventricular filling pressure

• Most intraoperative ischemic episodes are not associated with hemodynamic changes

•  Intraoperative coronary vasospasm may be treated/prevented by:

  • nitroglycerin

  • calcium channel blockers

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Hemodynamic Goals in Anesthesia

• Primary goal: prevent myocardial ischemia
• Secondary goal:
  • rapid identification of intraoperative ischemic episodes
  • rapid corrective measures

"The "P wave presents atrial activation; the P-R interval is the time from onset of atrial activation to onset of ventricular activation.  The QRS complex represents ventricular activation; the QRS duration is the duration of ventricular activation.  The ST-T wave represents ventricular repolarization.  The QT interval is the duration of ventricular activation and recovery.  The U wave probably represents 'afterdepolarizations' of the ventricles"-courtesy of  Frank G.Yanowitz, M.D. &  The Alan E. Lindsey  ECG Learning Center, used with permission; http://medstat.med.utah.edu/kw/ecg/index.html

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• Ischemic Heart Disease: ECG manifestations: ST segmental changes

ST Segment Depression

• courtesy of  Frank G.Yanowitz, M.D. &  The Alan E. Lindsey  ECG Learning Center, used with permission, http://medstat.med.utah.edu/kw/ecg/index.html
• note that "upsloping" ST depression is not considered an ischemic abnormality

• Subendocardial ischemia (above) -- exercise induced or during anginal episode (courtesy of  Frank G.Yanowitz, M.D. &  The Alan E. Lindsey  ECG Learning Center,used with permission, http://medstat.med.utah.edu/kw/ecg/index.html)

• Methods for detecting intraoperative ischemia
  • In the awake patient:Pain-especially if the pain in his been previously identified with angina
    • Problematic: similar pain may be associated with biliary spasm which may occur following opioid administration {biliary pain is naloxone (Narcan)-sensitive; angina isn't {
    • Chest pain ( in just apresenting symptoms are similar to previously
  • Current standard, primary method = monitoring ST segment in multiple ECG leads, typically leads II and V₅.

  

  "ST segment depression is a nonspecific abnormality that must be evaluated in the clinical context in which it occurs. In a patient with angina pectoris S/P depression usually means subendocardial ischemia and, unlike ST elevation, is not localized to a particular coronary artery lesion" -- courtesy of Frank G. Yanowitz, M.D. and The Alan E. Lindsay ECG Learning Center, http://medstat.med.utah.edu/kw/ecg/index.html used with permission

  • ⁴Nature of ECG changes:
    • ST segment depression (usually > 1 mm): suggestive of "injury pattern", may indicates an actual infarct
      • Factors which may make ST segment analysis problematic (because of baseline changes in ST segment, secondary to repolarization)
        1. left bundle branch block
        2. ventricular hypertrophy
        3. "filtering" on the ECG monitor system {filtering reduces baseline change & limits interference due to use of electrocautery}, which the limits detection ST segment changes
        4. ST segmental changes observed on a "filtered" ECG system might not be apparent on a standard 12-lead ECG.  Suggestion: obtain a preoperative &  preinduction baseline ECG strip {using the intraoperative monitoring system} in the diagnostic mode, if available, instead of the filtered monitoring mode.
        5. The preinduction filtered monitoring mode ECG should be compared for other intraoperative recorded strips -- particularly important for all patients who would appear to be at risk for ischemia --see reference 4
      • Lead V₅: most likely to detect lateral left ventricle ischemia
        • 75% of ischemic episodes will be shown on lead V₅ (manifestation is by ECG ST segment changes on 12-lead recording)
        • if  both leads V₅ and II are monitored, 80% of ST-segmental changes will be identified^4--- (London, MJ, Hollenberg, M, Wong, MG, et al. Intraoperative myocardial ischemia localization by continuous 12 lead electrocardiography. Anesthesiology 1998: 69:237, primary reference)
      • Lead II: most likely to detect inferior wall ischemia

 "Ischemia of the inferior myocardial wall is generally caused by occlusion of the posterior descending coronary artery (often the distal portion of the right coronary) or a distal part of the left circumflex coronary artery branch.  Long axis views show the posterior wall while the apical views two-chamber and short axis views are best at defining the inferior myocardial wall segments. One of the most useful views for regional ischemia is the short axis of the left ventricular myocardium, where multiple segments with distinct coronary supplies can be simultaneously compared."-- Yale center for Advanced Instructional Media, Yale Tech University School of Medicine, Medical Editor: C. Carl Jaffe, MD; Site Producer: Patrick J. Lynch (http://info.med.yale.edu/intmed/cardio/imaging/contents.html)-used with permission, copyright 2000,  Yale University School of Medicine

• Rationale for monitoring lead choice: 
  1. lead demonstrating previous changes, e.g. during preoperative stress test
  2. knowledge of location coronary artery lesion

• Posterior wall ischemia best appreciated using:
  • atrial lead
  • esophageal lead
• Three-lead intraoperative ECG monitor: modified V₅ lead:
  • left arm lead in the V₅ position while monitoring lead I

Reference 4 (Figure 6-19:McGough, EK, in: Manual of Complications During Anesthesia, (Gravenstein, N, ed), J. B. Lippincott Co., Philadelphia, p 221 1991 (Three-lead system modification which retains lead II and makes a modified V5, by placing the left arm electro-over the V₅ position.  "Modified V₅ is monitored by using the lead selector on lead I" RA = right arm; LA = left arm; LL = left leg

• Heart rate & Blood Pressure  Monitoring: rate-pressure product (heart rate x peak systolic pressure) or pressure-rate ratio-- probably NOT sensitive or specific indicators of intraoperative ischemia (lack of correlation with ECG changes; lack of correlation with direct monitoring of ventricular wall motion changes by using transesophageal echocardiography)

 

• Pulmonary Artery Catheter: usefulness in detecting/identifying myocardial ischemia: "Little value" in monitoring myocardial ischemia³.
  • Value of pulmonary artery catheter data -- identification of systolic +/- diastolic dysfunction by sudden pulmonary arterial or capillary wedge pressure elevations
  • Cardiac surgical patients (even high-risk) may be safely interoperability managed without routine pulmonary artery catherization -- the pulmonary artery catherization is required during the operative procedure, contemporary placement does not change surgical outcome
    • Pulmonary artery catheter data provide important information concerning patient's volume status and cardiac output, is not considered a reliable/sensitive index for identification of ischemic episodes

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"Transesophageal echocardiography is performed by using a miniature high frequency ultrasound transducer mounted on the tip of a directable gastroscope-like tube about 12 mm in diameter. Using topical mouth anesthesia and a little sedative, most individuals can swallow the probe without difficulty.  Because the transducer lies in the lower esophagus enclose direct fluid contact with the posterior of the heart, the images are superb since there is no interference by lung tissue" -- Yale center for Advanced Instructional Media, Yale Tech University School of Medicine, Medical Editor: C. Carl Jaffe, MD; Site Producer: Patrick J. Lynch (http://info.med.yale.edu/intmed/cardio/imaging/contents.html)-used with permission, copyright 2000,  Yale University School of Medicine

• Summary:
  • Most common method for monitoring possible myocardial ischemia: ECG-ST analysis
    • Sensitivity of specific leads:
      •  V₆: 82% detection frequency
      •  V₅: 72% detection frequency
      •  V₄:: about 50% detection frequency
      •  lead II: about 33% detection frequency
    • ST segment changes
      •  ST segment depression  --most likely associated with ischemia
      • ST segment elevation-less specificity, but suggestive if it is known that the patient has coronary vascular disease
      •  "Trend" analysis is not available for patients with left bundle branch block or in the presence of ventricular pacing
  • Hemodynamic changes-with impairment of overall left ventricular function
    • Elevated pulmonary artery occlusion pressure (PAOP): manifestation-large V waves in the PAOP tracing secondary to (a) changed left ventricular compliance, (b) papillary muscle dysfunction causing mitral valve regurgitation
    •  Cardiac output reduction-least sensitive index of ischemia
  • Transesophageal ultrasound (TEE ultrasound) -- indications
    • new ventricular wall motion anomaly in a patient with coronary artery disease: a good indication of ischemia
    • TEE more sensitive than ECG in detecting ischemia; however, the methods may be complementary in that one method will detect ischemia in some circumstances in which the other method will not -- also transesophageal ultrasound may not be routinely available
  •  Ischemia occurring in the post-bypass time frame tend to be more common and more related to negative clinical outcomes such as perioperative myocardial infarction, serious, life-threatening ventricular tachyarrhythmias & death.

Intraoperative Transesophageal Echocardiography (TEE): Utility and Assessment of Myocardial Ischemia

Source: Practice Guidelines for Perioperative Transesophageal Echocardiography, A Report by the American Society of Anesthesiologists and the Society of Cardiovascular Anesthesiologists Task Force on Transesophageal Echocardiography,Anesthesiology 1996: 84:986-1006 (http://www.medana.unibas.ch/eng/educ/Tee.htm)

• TEE Overview
  • Regional ventricular dysfunction during surgery probably increases the patient's risk of perioperative myocardial infarction and sudden death
  • Transesophageal echocardiography is one method that allows detection of regional ventricular wall motion dysfunction
  • There is a high incidence of intraoperative ventricular dysfunction which may be correlated with myocardial ischemia; there is an important concern about the frequency of false-positive results.
• Analysis of animal experiments and angioplasty research studies indicates that wall motion anomalies precede ECG changes during myocardial ischemia.
  • Transesophageal echocardiography may provide a "more meaningful" reference standard for ischemia than ECG.
  • Limitations of TEE:
    1. TEE changes are often more subjective compared to quantifiable ECG changes (e.g. ST segment  depression)
    2. Interpretation is influenced by translational cardiac motion, bundle branch block, and ventricular pacing
    3. A clear worsening no segmental wall motion & thickening is required ("in the absence of similar global changes") to suggest strongly a diagnosis of ischemia
    4. Segmental wall motion may be caused by other factors, non-ischemia, e.g.:
       • previous myocardial infarction
       • myocarditis
       • myocardial stunning following cardiopulmonary bypass
  • Conclusions: 
    • Use of TEE may allow early identification of myocardial ischemia, permitting early treatment which will improve survival
    • Intraoperative TEE detection of ischemia allows corrective action including common
      1. changes in anesthetic management
      2. alterations in surgical procedure
      3. postoperative triage -- preventing perioperative complications
    • ⁵Increasing availability and the usefulness of TEE suggest that the cardiac anesthesiologist should acquire skills required for TEE applications.

• ¹Primary Reference:  Ross, AF, Gomez, MN. and Tinker, JH Anesthesia for Adult Cardiac Procedures in  Principles and Practice of Anesthesiology (Longnecker, D.E., Tinker, J.H. Morgan, Jr., G. E., eds)  Mosby, St. Louis, Mo., pp. 1659-1698, 1998.
• ²Primary Reference: Shanewise, JS and Hug, Jr., CC, Anesthesia for Adult Cardiac Surgery, in Anesthesia, 5th edition,vol 2, (Miller, R.D, editor; consulting editors, Cucchiara, RF, Miller, Jr.,ED, Reves, JG, Roizen, MF and Savarese, JJ) Churchill Livingston, a Division of Harcourt Brace & Company, Philadelphia, pp. 1753-1799, 2000.
• ³Primary Reference: Wray Roth, DL, Rothstein, P and Thomas, SJ Anesthesia for Cardiac Surgery, in Clinical Anesthesia, third edition  (Barash, PG, Cullen, BF, Stoelting, R.K, eds), Lippincott-Raven Publishers, Philadelphia, pp. 835-865, 1997
• ⁴McGough, EK, in: Manual of Complications During Anesthesia, (Gravenstein, N, ed), J. B. Lippincott Co., Philadelphia, pp. 219-223, 1991
• ⁵Practice Guidelines for Perioperative Transesophageal Echocardiography, A Report by the American Society of Anesthesiologists and the Society of Cardiovascular Anesthesiologists Task Force on Transesophageal Echocardiography,Anesthesiology 1996: 84:986-1006 (http://www.medana.unibas.ch/eng/educ/Tee.htm)

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