Physiology of Shock: Beyond Hinshaw -Cox

1. Physiology of Shock:Beyond Hinshaw-Cox Matthew Boland MD, FCCP Pulmonary/CCM

2. A definition of SHOCK • Global tissue hypoxia • “global” implying systemically while • “tissue hypoxia” implies inadequate oxygen delivery/utilization • May be independent of, or even inversely proportional to “perfusion” • Hypoxia ≠ Hypoxemia

3. Hinshaw-Cox Approach • Hypovolemic • Cardiogenic • Obstructive • Distributive

4. A Physiologic Approach • Shock ≈ ↓ ḊO2 • (inappropriate to V̇O2) • ḊO2 = CO x CaO2 • CO= HR x SV • SV ∫ Afterload, Preload and Contractility • CaO2 = Hgb x SaO2 x 1.34 x (0.003 x PaO2)

5. So… • ḊO2 = HR x (∫ Afterload, Preload and Contractility) x Hgb x SaO2 x 1.34 x (0.003 x PaO2)

6. So…4 types of Shock • Circulatory Hypoxia • This is where Hinshaw-Cox categories really fit… • Anemic Hypoxia (low hgb) • Hypoxemic Hypoxia (low SaO2) • And…

7. Cellular Hypoxia • AKA cytopathic hypoxia or cytotoxic hypoxia • Disutilization of oxygen at the cellular level (usually mitochondrial) prompts anaerobic metabolism and lactate production independent of O2 delivery. • Examples: cyanide poisoning, sepsis or anything that uncouples oxidative phosphorylation

8. Recognition of ShockPhysical Exam • Shock Index= SBP/HR; the lower the quotient, the “shockier” the patient • Decreased Cap refill or pulses • Skin exam • “warm shock” vs “cold shock”

9. Recognition of ShockBasic Labs • Chem 7 • Low bicarb, high anion gap • ABG • Metabolic acidosis ± respiratory alkalosis • VBG • Low SvO2 (though may be high, especially in cellular hypoxia) • Lactate- elevated (though can be normal if shock is well compensated)

10. Recognition of Shock:PAC • PAC, though rarely used in today’s Critical Care environment, can be used to determine/narrow the underlying pattern/cause of shock

11. Treatment of Shock • ID and treat underlying cause WHILE • Optimizing ‘Big 7’ (i.e. goal –directed) • HR • Preload • Afterload • Contractility • Hgb • SaO2 • ↓V̇O2

12. Decreasing Oxygen Consumption • Control of fever • Unloading respiratory muscles • NIV vs Intubation • Sedation • Paralytics

13. Goal = Nl SvO2 • Vast majority of shock states respond to EARLY optimization of balancing oxygen delivery and consumption, BUT…

14. Exceptions to the rule • The pattern of rising lactate despite normal (or even more ominous, high) SvO2 frequently implicates three clinical scenarios… • Cellular hypoxia (? Role for steroids) • DIC (0bstruction of the micro-vasculature does not allow delivery of oxygenated blood to tissues on other side) (? Role for rhAPC) • Uncontrolled source of shock (typically ongoing hemorrhage, infection, etc)

15. Hyperlactatemia • Type A- • Classic- due to hypoxia and anaerobic metabolism • Type B- • Drugs (metformin, HARRT, etc), Cancer, ETOHism, HIV • Type δ- encountered in short-gut syndrome with overproduction of δ-isomer of lactate (not assayed by typical lactate measurements clinically).

16. Question #1 • Which of the following values is NOT a determinant of ḊO2? 1. Hbg 2. SvO2 3. Preload 4. PaO2 5. Heart rate

17. Question #2 • Disutilization of oxygen at the mitochondrial level prompting anaerobic metabolism and lactate production independent of O2 delivery, can be termed… 1. Circulatory Hypoxia 2. Cytopathic Hypoxia 3. Anemic Hypoxia 4. Hypoxemic Hypoxia 5. Obstructive Hypoxia

18. Question #3 • Optimizing which of the following is NOT a treatment option for shock? 1. Urine Output 2. Contractility 3. Work of breathing 4. Arterial Oxygen Saturation 5. Hgb

19. Question #4 • Which of the following is NOT an example of obstructive shock? 1. Tension pneumothorax 2. Atrial myxoma 3. Pulmonary embolism 4. Pericardial tamponade 5. Papillary muscle rupture

20. Question #5 • Which of the following is NOT a cause of Type B lactic acidosis? 1. HAART 2. Alcoholism 3. Lymphoma 4. short-gut syndrome 5. metformin

21. Bibliography • Oxygen delivery. Hameed SM. Aird WC. Cohn SM. Critical Care Medicine. 31(12 Suppl):S658-67, 2003 Dec.