How to Think About Shock?
4 HOW TO THINK ABOUT SHOCK
Alden H. Harken M.D.
1. Define shock. Show answer
Shock is:
* Not just low blood pressure
* Not just decreased peripheral perfusion
* Not just limited systemic oxygen delivery
Ultimately, shock is decreased tissue respiration. Shock is suboptimal consumption of oxygen and excretion of CO2 at the cellular level.
2. Is shock related to cardiac output? Show answer
Yes. A healthy medical student can redistribute blood flow preferentially to vital organs. After a 3-4-U bleed, your typical young gunslinger can still think: “four dudes jumped me.”
3. Is organ perfusion democratic? Show answer
No. Limited blood flow always is redirected toward the carotid and coronary arteries. Peripheral vasoconstriction steals blood initially from the mesentery, then skeletal muscle, then kidneys and liver.
4. Is this vascular autoregulatory capacity uniform in all patients? Show answer
No. With age and atherosclerosis, patients lose their ability to redistribute limited blood flow. A 20% decrease in cardiac output (or a fall in blood pressure to 90 mmHg) can be life-threatening to a Supreme Court justice, whereas it may be undetectable in a triathlete.
5. For diagnostic and practical therapeutic purposes, can shock be classified? Show answer
Yes.
1. Hypovolemic shock mandates volume resuscitation.
2. Cardiogenic shock mandates cardiac stimulation (pharmacologic and eventually mechanical).
3. Peripheral vascular collapse shock mandates pharmacologic manipulation of the peripheral vascular tone (and direct attention to the cause of the vasodilation-typically sepsis).
6. Is it advisable to treat all shock in the same sequential fashion?
Ultimately, yes. Whether a cigar-chomping banker presents with a big gastrointestinal bleed (hypovolemic shock) or crushing substernal chest pain (cardiogenic shock), the surgeon should take, the following steps in order:
1. Optimize volume status; give volume until further increase in right-sided (central venous pressure [CVP]) and left-sided (pulmonary capillary wedge pressure[PCWP]) preload confers no additional benefit for cardiac output or blood pressure. (This step is Starling’s law-place the patient’s heart at the top of the Starling curve.)
2. If cardiac output, blood pressure, and tissue perfusion remain inadequate despite adequate preload, the patient has a pump (cardiogenic shock) problem. Infuse cardiac inotropic drugs (β-agonist) to the point of toxicity (typically cardiac ectopy)-lots of frightening premature ventricular contractions. For pharmacologically refractory cardiogenic shock, insert an intra-aortic balloon pump (IABP).
3. If the patient exhibits a surprisingly high cardiac output and a paradoxically low blood pressure (such unusual loss of vascular autoregulatory control is associated typically, but not always, with sepsis), infuse a peripheral vasoconstrictor drug (α-agonist).
7. What is the preferred access route for volume infusion? Show answer
Flow depends on catheter length and radius. Volume may be infused at twice the rate through a 5-cm, 14-gauge peripheral catheter as through a 20-cm, 16-gauge central line (see Chapter 2). Assessment of central venous pressure (and left-sided filling pressure) is necessary if the patient fails to respond to initial volume resuscitation.
8. Should one infuse crystalloid, colloid, or blood? Show answer
If the goal is only to improve preload and to repair cardiac output and blood pressure, crystalloid solution should be sufficient. It is controversial whether infused colloid remains in the vascular compartment. If the goal is to augment systemic oxygen delivery, red blood cells bind much more oxygen than plasma (see Chapter 6). Crystalloid should enhance flow, and blood should augment oxygen delivery.
9. When cardiac preload is adequate, which inotropic agents are useful? Show answer
Dobutamine, epinephrine, and norepinephrine are the chocolate, vanilla, and strawberry of the 32 flavors of cardiogenic drugs. These three drugs are all that the surgeon really needs.
10. Is dopamine the same as dobutamine? Show answer
No. Dopamine stimulates renal dopaminergic receptors and may be useful in low doses (2 mg/kg/min) to counteract the renal arteriolar vasoconstriction that accompanies shock. Dopamine has no place as a primary cardiac inotropic agent.
11. Discuss the use of dobutamine, epinephrine, and norepinephrine. Show answer
See Table 4-1.
12. When is an IABP indicated? Show answer
Mechanical circulatory support is indicated when the preload to both ventricles (CVP and PCWP) has been optimized and further cardiac stimulatory drugs are limited by frightening runs of premature ventricular contractions. Do not be afraid to resort to mechanical support.
KEY POINTS: SUMMARY OF ADRENERGIC AGENTS
1. Dobutamine: β1 agonist (cardiac inotrope) with mild-to-moderate β2 effects (peripheral vasodilation).
2. Epinephrine: combined β- and α-adrenergic agent, with the β effects predominating at lower doses and progressive vasoconstriction accompanying increased doses.
3. Norepinephrine: combined β- and α-adrenergic agonist, with the α effects predominating at all doses.
13. What does an IABP do? Show answer
Diastolic augmentation and systolic unloading.
Table 4-1. USE OF DOBUTAMINE, EPINEPHRINE, AND NOREPINEPHRINE
14. What is diastolic augmentation? Show answer
A soft 40-mL balloon is inserted percutaneously through the common femoral artery into the descending thoracic aorta. The balloon is not occlusive (it should not touch the aortic walls). When it is inflated, it displaces 40 mL of blood and is exactly like acutely transfusing 40 mL of blood into the aorta, augmenting each left ventricular stroke volume by 40 mL. Balloon infusion is triggered off the QRS complex from a surface ECG (any lead). The balloon always is inflated during diastole to increase diastolic blood pressure and augment coronary blood flow (CBF). Eighty percent of CBF occurs during diastole.
KEY POINTS: INTRA-AORTIC BALLOON PUMP
1. Indicated for cardiogenic shock refractory to pharmacologic manipulation.
2. Triggered by QRS complex of surface ECG; inflates during diastole (T wave) and deflates on systole (R wave or at dicrotic notch on aortic pressure curve).
3. 80% of coronary blood flow occurs during diastole.
4. Mechanistically results in diastolic augmentation and systolic unloading (afterload reduction).
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15. What is systolic unloading? Show answer
Balloon deflation is an active (not a passive) process. Helium abruptly is sucked out of the balloon, leaving a 40-mL empty space in the aorta. The left ventricle can eject the first 40 mL of its stroke volume into this empty space-at dramatically reduced workload. An intra-aortic balloon increases coronary oxygen delivery (CBF) during diastole, while decreasing cardiac oxygen consumption just presystole.
16. Name the contraindications to IABP. Show answer
Aortic insufficiency: diastolic augmentation distends and injures the left ventricle.
Atrial fibrillation: balloon inflation and deflation cannot be appropriately timed.
References
WEB SITE
http://www.aic.cuhk.edu.hk/web8/IABP.htm
BIBLIOGRAPHY
1. Harken AH: Cardiac dysrhythmias. In Wilmore DW, Cheung L, Harken AH, et al (eds): Scientific American Surgery. New York, Scientific American, 1999.
2. Holcroft JW: Shock. In Wilmore DW, Cheung L, Harken AH, et al (eds): American College of Surgeons Surgery. New York, WebMD Corporation, 2002.
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Abernathy’s Surgical Secrets, Updated Edition (Book w / Student Consult)
Author / s: Harken Alden H., Abernathy Charles, Moore Ernest Eugene
Year: 2004
Pages: 473
Publishers: Elsevier Mosby; 5th Bk & Acc edition
ISBN: 0323034160
