Surgical Critical Care and Emergency Surgery

Clinical Questions and Answers

Forrest "Dell" Moore, Peter M. Rhee, Samuel A. Tisherman, Gerard J. Fulda, Forrest "Dell" Moore, Peter Rhee, Samuel A. Tisherman, Gerard J. Fulda, Peter M. Rhee, Forrest "Dell" Moore, Peter Rhee, Samuel A. Tisherman, Gerard J. Fulda

• Publisher: Wiley-Blackwell
• Year: 2012
• Print ISBN: 9780470654613
• eBook ISBN: 9781118274170
• Edition: 1
• Topic: Medicine
• Subtopic: Nursing

This is a unique question-and-answer book for surgical residents and trainees, concentrating on the growing subspecialty of surgery in critical care and emergency surgery. This book covers all surgical aspects of critical care and acute or emergency surgery, making it an ideal learning and review  text for surgical trainees and those professionals specializing in these fields.

PART ONE

Surgical Critical Care

Chapter 1

Respiratory and Cardiovascular Physiology

Marcin A. Jankowski, DO and Frederick Giberson, MD, FACS

1. All of the following are mechanisms by which vasodilators improve cardiac function in acute congestive heart failure except:

A. Increase stroke volume

B. Decrease ventricular filling pressure

C. Increase ventricular preload

D. Decrease end-diastolic volume

E. Decrease afterload

Most patients with acute heart failure present with increased left-ventricular filling pressure, high systemic vascular resistance, high or normal blood pressure and low cardiac output. These physiologic changes increase myocardial oxygen demand and decrease the pressure gradient for myocardial perfusion resulting in ischemia. Therapy with vasodilators in the acute setting can often improve hemodynamics and symptoms.

Nitroglycerine is a powerful venodilator with mild vasodilitory effects. It relieves pulmonary congestion through direct venodilation, reducing left and right ventricular filling pressures, systemic vascular resistance, wall stress, and myocardial oxygen consumption. Cardiac output usually increases due to decreased LV wall stress, decreased afterload, and improvement in myocardial ischemia. The development of tolerance within 16 to 24 hours of starting the infusion is a potential drawback of nitroglycerine.

Nitroprusside is an equal arteriolar and venous tone reducer, lowering both systemic and vascular resistance and left and right filling pressures. Its effects on reducing afterload increase stroke volume in heart failure. Potential complications of nitroprusside include cyanide toxicity and the risk of “coronary steal syndrome.”

In patients with acute heart failure, therapeutic reduction of left-ventricular filling pressure with any of the above agents correlates with improved outcome.

Increased ventricular preload would increase the filling pressure, causing further increases in wall stress and myocardial oxygen consumption, leading to ischemia.

Answer: C

Hollenberg, MS (2007) Vasodilators in acute heart failure. Heart Failure Review 12, 143–7.

Marino P (2007) The ICU Book, 3rd edn, Lippincott Williams & Wilkins, Philadelphia, PA, Chapter 14.

Nohria A, Lewis E, Stevenson, LW (2002) Medical management of advanced heart failure. Journal of the American Medical Association 287 (5), 628–40.

2. Which is the most important factor in determining the rate of peripheral blood flow?

A. Laminar flow

B. Length

C. Viscosity

D. Radius

E. Pressure gradient

The forces that determine peripheral blood flow are derived from observations on ideal hydraulic circuits that are rigid and the flow is steady and laminar. This is quite different from the human circulatory system which is compressible and flow is pulsatile and turbulent. The Hagen-Poiseuille equation states that flow is determined by the fourth power of the inner radius of the tube (Q =

), where P is pressure, μ is viscosity, L is length, and r is radius. This means that a twofold increase in the radius will result in a sixteenfold increase in flow. As the equation states, the remaining components of resistance, such as pressure difference along the length of the tube and fluid viscosity, are inversely related and exert a much smaller influence on flow. Although this equation may not accurately describe the flow state in our circulatory system, it has useful applications in describing flow through catheters, flow characteristics of different resuscitative fluids and the hemodynamic effects of anemia and blood transfusions on flow. With turbulent flow (Fanning equation), the impact of the radius is raised to the fifth power (r5) as opposed to the fourth power in the Poiseuille equation.

It is important to realize that flow through compressible tubes (blood vessels) is greatly influenced by external pressure surrounding the tubes. Therefore, if a tube is compressed by an external force, the flow will be independent of the pressure gradient along the tube.

Answer: D

Brown SP, Miller WC, Eason JM (2006) Exercise Physiology; Basis of Human Movement in Health and Disease, Lippincott Williams & Wilkins, Philadelphia.

Marino P (2007) The ICU Book, 3rd edn, Lippincott Williams & Wilkins, Philadelphia, PA, Chapter 1.

3. Choose the correct physiologic process represented by each of the cardiac pressure-volume loops below.

A. (1) Increased preload, increased stroke volume,
(2) Increased afterload, decreased stroke volume

B. (1) Decreased preload, increased stroke volume,
(2) Decreased afterload, increased stroke volume

C. (1) Increased preload, decreased stroke volume,
(2) Decreased afterload, increased stroke volume

D. (1) Decreased preload, decreased stroke volume,
(2) Increased afterload, decreased stroke volume

E. (1) Decreased preload, increased stroke volume,
(2) Increased afterload, decreased stroke volume

One of the most important factors in determining stroke volume is the extent of cardiac filling during diastole or the end-diastolic volume. This concept is known as the Frank–Starling law of the heart. This law states that, with all other factors equal, the stroke volume will increase as the end-diastolic volume increases. In Figure 1, the ventricular preload or end-diastolic volume (LV volume) is increased, which ultimately increases stroke volume defined by the area under the curve. Notice the LV pressure is not affected. Increased afterload, at constant preload, will have a negative impact on stroke volume. In Figure 2, the ventricular afterload (LV pressure) is increased, which results in a decreased stroke volume, again defined by the area under the curve.

Answer: A

Mohrman D, Heller L (2010) Cardiovascular Physiology, 7 edn, McGraw-Hill, New York, Chapter 3.

Shiels HA, White E (2008) The Frank–Starling mechanism in vertebrate cardiac myocytes. Journal of Experimental Biology 211 (13), 2005–13.

4. An 18-year-old patient is admitted to the ICU following a prolonged exploratory laparotomy and lysis of adhesions for a small bowel obstruction. The patient has had minimal urine output throughout the case and is currently hypotensive. Identify the most effective way of promoting end-organ perfusion in this patient.

A. Increase arterial pressure (total peripheral resistance) with vasoactive agents

B. Decrease sympathetic drive with heavy sedation

C. Increase end-diastolic volume with controlled volume resuscitation

D. Increase contractility with a positive inotropic agent

E. Increase end-systolic volume

This patient is presumed to be in hypovolemic shock as a result of a prolonged operative procedure with inadequate perioperative fluid resuscitation. The insensible losses of an open abdomen for several hours in addition to significant fluid shifts due to the small bowel obstruction can significantly lower intravascular volume. The low urine output is another clue that this patient would b...

Table of contents

• Cover
• Title Page
• Copyright
• Contributors
• Preface
• PART ONE: Surgical Critical Care
• PART TWO: Emergency Surgery
• Plates
• Index