Critical Care Medicine Information Sheet 2003

Critical Care Medicine Information Sheet 2003 Respiratory Critical Care 1. Measurement of Hypoxemia a. Alveolar-arterial oxygen difference (A-a gradient) i. A-a gradient = PAO 2 - PaO 2 ii. A-a gradient = [(PB - PH 2 0) (FIO 2 ) - PCO 2 / R] - PaO 2 iii. A-a gradient = [713 (FIO 2 ) - PCO 2 / 0.8] - PaO 2 iv. A-a gradient = [713 (FIO 2 ) - 1.25 (PCO 2 )] - PaO 2 v. A-a gradient: 1. On room air normal always < 25 2. On 100% FIO 2 on mechanical ventilation normal 100 3. Formula for determining normal A-a gradient based on age: 4. Normal A-a gradient = [Patient age + 4] / 4 b. PaO 2 /FIO 2 ratio i. PaO 2 off blood gas / FIO 2 ii. Normal > 200 iii. Useful trend to follow when patient on mechanical ventilation; results affected less by changes in FIO 2 then when using A-a difference to monitor hypoxemia. c. PAO 2 : PaO 2 ratio (A.K.A. A/a ratio) i. Same as for B.3 ii. PaO 2 off ABG iii. PAO 2 calculated from known FIO 2 and measured PCO 2 off ABG where, PAO 2 = 713 (FIO 2 ) - 1.25 (PCO 2 ) iv. Normal PaO 2 / PAO 2 0.8 d. Normal PaO 2 corrected for age in patients with normal Lungs: i. Arterial PaO 2 = 100 - (0.4 X age in years) 2. Oxygen Saturation (Oxygen Dissociation Curve Determinants) Shift Right ( affinity, P50, delivery) Acidosis Hypercapnea Hyperthermia Elevated RBC MCHC 2,3 DPG Abnl Hgb (ss) Exercise Propranolol Shift Left ( Affinity, P50, delivery) Alkalosis Hypocapnea Hypothermia Reduced RBC MCHC 2,3 DPG Abnl Hgb (fetal, meth, sulf) Carbon monoxide 1

3. Ventilator Equations and Adjustments: a. Minute ventilation = tidal volume (liters) X respiratory rate (# per minute) (normal < 10 liters/min) b. Adjusting ventilator rate: i. Desired f = [(actual f) (actual PaCO 2 )] / desired PaCO 2 c. Adjusting tidal volume: i. Desired V T = [(actual PaCO 2 ) (actual V T )] / desired PaCO 2 d. Adjusting FIO 2 : i. Desired FIO 2 = [(actual FIO 2 ) (desired PaO 2 )] / actual PaO 2 e. Static pulmonary compliance (Cst): i. C stat = V T exhaled / P plat - PEEP ii. Normal = 50-100 cm H 2 O iii. Stiff noncompliant lung < 50 cm H 2 O (example ARDS) iv. Highly compliant non-stiff lung > 100 cm H 2 O (example COPD) f. Pressure generated to overcome airway resistance: i. P aw = PIP - P plat g. Mean airway pressure: i. Mean P aw = K (PIP - PEEP) (T 1 / TCT) + PEEP 1. This equation shows the relationship of PIP, PEEP and time on mean airway pressure. h. Bohr Equation (V D / V T ): i. V D / V T = PaCO 2 - P E CO 2 / PaCO 2 4. Strategies to reduce auto-peep in bronchospastic patients on mechanical ventilation. a. Controlled ventilation: sedation ± paralysis as needed. b. β-agonist, Ipatropium, steroids, magnesium sulfate, aersolized lidocaine. c. Increase peak inspiratory flow rate (70-120 Liters / min). d. Reduce tidal volumes (5-8-ml/kg)--[permissive hypercapnea]. e. Reduce respiratory rate [permissive hypercapnea]. i. Permissive hypercapnea add sodium bicarbonate drip if arterial ph < 7.20 and titrate the drip to maintain arterial ph > 7.20. ii. Add extrinsic PEEP (never more than 10 cm H 2 0) to counteract intrinsic PEEP (auto-peep). 2

5. Protocol for Mechanical Ventilation in patients with early ARDS. a. Controlled ventilation: sedation ± paralysis as needed. b. Use lower tidal volumes for the baby lung seen in ARDS (e.g., 5-8 ml/kg of ideal body weight and maintain plateau pressure ± 35 cm H 2 O). This is a lung protective strategy designed to prevent over distention of normal alveoli and to reduce pulmonary and systemic inflammation caused by volume and/or pressure induced over distention of the alveoli (alveolar volume/pressure trauma). [JAMA. 1999; 282:54-61; Amato NEJM 1998; 338:347-354; ARDS Network Trial NEJM 5/00]. Studies show that in large groups of ARDS patients, the lower inflexion point on the pressure volume curve ranges from (mean) 12.6-13.6 ± 2.8-3.9 (SD). For this reason, initially set PEEP at 12-15 cm H 2 O or set PEEP at 2-3 cm H 2 O higher than the lower inflection point on the pressure volume curve. This is a lung protective strategy to prevent repetitive opening and closure of alveolar units sheer trauma. [JAMA. 1999; 282:54-61, Amato NEJM 1998;338:347-354; ARDS Network Trial NEJM 5/00]. c. Respiratory rate is adjusted to < 10-15/min. d. Permissive hypercapnea- add bicarbonate drip if arterial ph < 7.20 and titrate drip to maintain arterial ph > 7.20. e. The standard inspiratory to expiratory ratio of 1:2 should be used; however, in some cases Pressure Control Ventilation with inverse ratio ventilation [I:E ratio s of 1:1; 1.5:1; or 2:1] may be required. Be aware that hemodynamic instability may occur during inverse ratio ventilation especially in patients with pre-existing heart disease. A reduction in preload caused by high intrathoracic pressure occurs. Therefore, a physician must always be present at the bedside during initiation of PCV or when making a change to inverse ratio ventilation. f. Any patient on mechanical ventilation who develops a pneumothorax should have a chest tube placed. Prophylactic chest tubes may be justified in some patients. g. Be aware that the development of pneumomediastinum or subcutaneous air signals that the patient is at high risk for developing a pneumothorax. A standard CXR does not always show the pneumothorax (it may be located anteriorly). If a patient is hemodynamically stable, a CT scan of the chest will demonstrate a pneumothorax. Rule of thumb: In unstable patients, if in doubt as to whether or not a pneumothorax is present, place a chest tube. h. Calculation of idea body weight (Kg) for use in determining tidal volume settings: i. Ideal body weight (kg) males = 50 + 0.91[Height (cm) 152.4] ii. Ideal body weight (kg) females = 45.5 + 0.91[Height (cm) 152.4] 3

6. Differential Diagnosis for Failure to Wean From Mechanical Ventilation a. WEANS NOW: i. W - Work of breathing = NIF, VC, Tobin Index (frequency / TV), BICORE machine ii. E - Endotracheal or tracheotomy tube size. 1. Note: Airway resistance is related to diameter and length of the breathing tube. Airway resistance may be decreased by using a larger diameter tube and/or a shorter breathing tube. iii. A - Acid/base; abdominal distention; atelectasis; anxious/agitated; alkalosis. 1. Note: metabolic alkalosis shifts oxygen hemoglobin dissociation curve to the left and impairs the CNS respiratory drive. iv. N - Nutrition and electrolytes (Mg 2+, PO 4, K + ) v. S - Secretions due to sinusitis; bronchitis; pneumonia; aspiration; CHF (systolic or diastolic dysfunction); TE fistula; GE reflux; aspiration. vi. N - Neuromuscular status. Neuromuscular disease; neuromuscular blockers; steroids; aminoglycosides; endocrine (hypothyroid). vii. O - Occult obstruction (bronchospasm) 1. Consider: β-agonist; ipatropium, steriods, leukotriene inhibitors, theophyline viii. W - Wakefulness. Is the patient over sedated? Is the patient able to follow commands and participate in his/her care? Cardiovascular Critical Care 1. Hemodynamic monitoring a. Normal Values: i. Cardiac output = 4-8 liters / minute ii. Cardiac Index = CO / BSA = 2.2-4.0 l / m 2 / min iii. Stroke Volume = CO / heart rate iv. Cardiac Output = Heart rate X stroke volume (1/min) v. Right atrium pressure = 0-8 mmhg vi. Central venous pressure (CVP) = 0-8 mmhg vii. Pulmonary artery systolic (PAS) = 15-30 mmhg viii. Pulmonary artery diastolic (PAD) = 5-12 mmhg ix. Mean pulmonary artery pressure (MPAP) < 20 mmhg x. Pulmonary artery wedge pressure (PAOP or PCWP): 1. < 6 = dehydrated 2. 6-15 = normal 3. 18-20 = mild congestion 4. 20-25 = moderate congestion 5. 25 = severe congestion 4

xi. Body surface area (m 2 ) = [(height (cm) x weight (kg)) / 3600] Where means take the square root of the number in brackets b. Essential Formulae: i. Mean arterial pressure (MAP): 1. MAP = 1/3 (AP systolic - AP diastolic ) + AP diastolic 2. MAP = normal > 70 3. MAP < 70 = shock in the majority of patients. ii. Mean pulmonary artery pressure: 1. PAP mean = 1/3 (PA systolic - PA diastolic ) + PA diastolic iii. Systemic vascular resistance (SVR) 1. SVR = [MAP CVP / CO] x 80 (nl = 700-1400 dynes - sec - cm- 5 ) a. Where MAP = mean arterial pressure b. CVP = central venous pressure c. CO = cardiac output d. 80 = Conversion factor used to convert wood units to dynes - sec - cm- 5 iv. Pulmonary vascular resistance (PVR) 1. PVR = [PAP mean - PCWP / CO] x 80 (nl < 200 dynes - sec - cm- 5 ) a. Where PAP mean = mean pulmonary artery pressure b. PCWP = pulmonary capillary wedge pressure c. CO = cardiac output d. 80 = conversion factor used to convert wood units to dynes - sec - cm- 5 c. Hemodynamic monitoring (short form) calculation i. Theoretical, calculated, end-pulmonary capillary oxygen content (CcO 2 ), assuming 100% hemoglobin saturation with oxygen: 1. CcO 2 = 1.39 (Hgb) ii. Total arterial oxygen content (CaO 2 ): 1. CaO 2 = 1.39 (Hgb) (SaO 2 ) iii. Total venous oxygen content (CvO 2 ) 1. CvO 2 = 1.39 (Hgb) (SvO 2 ) iv. Arterial venous oxygen content difference (AVDO 2 ) 1. AVDO 2 = CaO 2 - CvO 2 (nl < 5) 5

v. Oxygen delivery (DO 2 in ml/min) 1. DO 2 = CO (CaO 2 ) (10) vi. Oxygen consumption (VO 2 in ml/min) 1. VO 2 = CO (AVDO 2 ) (10) vii. Intrapulmonary shunt fraction (Q S /Q T ). AKA: arterial-venous admixture (nl < 10%) 1. Qs/Q T = CcO 2 - CaO 2 / CcO 2 - CvO 2 viii. Extraction ratio (nl 22 32%) 1. VO 2 / DO 2 ix. Caloric requirements (kcal/24 hours): 1. 7.1 (VO 2 ) x. Crude method of estimating intrapulmonary shunt fraction in patients on mechanical ventilation who are on 100% FIO 2 : 1. Normal PaO 2 ~600-650 mmhg 2. For every drop in PaO 2 of 100 mmhg from the normal corresponds to a 5% shunt. 3. Example: PaO 2 = 230 mmhg on 100% FIO2 corresponds to an estimated 20% intrapulmonary shunt. 2. Adverse consequences of PEEP a. Elevation in airway pressure; increased risk of barotrauma. b. Decreased venous return to right heart decreased right ventricular preload decreased cardiac output. c. Decreased left ventricular filling (preload) decreased cardiac output (due to increased PA, PVR, and RV afterload). d. Direct compression of heart by overextended lungs thus limiting LV filling decreased cardiac output. e. Decreased LV output due to PEEP induction of increased RV afterload with subsequent RV distention and bulging of intraventricular septum into LV cavity (causes LV diastolic dysfuntion). 3. Beneficial effects of PEEP: a. Increases FRC improves / prevents atelectasis. b. Decreases work of breathing by reducing hypoxemia. c. Reduces LV preload and afterload. d. Improves hypoxemia by: i. Recruiting atelectatic alveoli and increasing FRC 6

ii. Redistribution of lung H 2 O from alveolar space into the interstitium iii. Improving static lung compliance iv. Decreasing intrapulmonary shunting. e. May be used to counter the effects of intrinsic (auto) PEEP. f. Peep at or above the lower inflection point on the pressure volume curve has a protective effect on the lungs in patients with ARDS. g. Special note: PEEP does not prevent the development of ARDS. 4. Nitroprusside Information a. Thiocyanate toxicity i. Most common in patients with renal failure. ii. Thiocyanate level < 10 mg/dl considered safe. iii. Manifest as: fatigue, muscle weakness, nausea, vomiting, confusion, seizures, coma. iv. Treatment: hemodialysis b. Cyanide toxicity i. Use of nitroprusside for > 3 days. ii. Most common with severe liver failure. iii. Manifest as severe lactic acidosis. iv. Treatment: 1. IV sodium nitrate 2. IV thiosulfate 3. IV vitamin B 12 Critical Care Acid-Base: 2. Stepwise approach: a. Determine electrolyte and ABG values concomitantly. b. Compare the calculated and measured plasma HCO 3 concentration to rule out laboratory error using H + = 24 (PaCO 2 / HCO 3 ) where normal H + = 40 and corresponds to a normal ph = 7.40. An increase in H + results in a lower ph and vise versa. c. Compute anion gap (Na (Cl + HCO3). d. Calculate degree of compensation (see below). e. Compare the change in plasma sodium and chloride concentration; anion gap and bicarbonate concentration; chloride and bicarbonate concentration. 3. Determinants of compensation: a. Metabolic acidosis: i. PaCO 2 = 1.5 (HCO 3 ) + 8 ii. PaCO 2 = last two digits of ph iii. PaCO 2 = 1.0 1.5 per 1 meq/l HCO 3 7

b. Metabolic alkalosis: i. PaCO 2 = 0.9 (HCO 3 ) + 9 ii. PaCO 2 = 0.5 1.0 mm per 1 meq/l HCO 3 c. Respiratory acidosis and alkalosis (acute acid-base changes based on PCO 2 and HCO 3 ): i. H + =0.8 ( PaCO 2 ) ii. For every or of PCO 2 by 1 = ph by 0.008 iii. For every or of HCO 3 by 1 = ph by 0.015 d. Estimate of baseline PCO 2 in patients with Acute Respiratory Acidosis: i. Estimated baseline PCO 2 = 2.4 (admission measured HCO 3 22) 4. Increased anion gap metabolic acidosis a. Anion gap = Na + - [Cl - + HCO 3 - ] = Normal < 15 i. Differential Diagnosis (MUD PIES): 1. M - Methanol 2. U - Uremia 3. D - DKA 4. P - Paraldehyde 5. I - Ischemia (lactic acidosis); INH 6. E - Ethanol; ethylene glycol 7. S - Salicylates 5. Normal anion gap metabolic acidosis a. Differential diagnosis (HARD UP): i. H - Hyperalimentation ii. A - Acid Ingestion; Addison s; hypoaldosteronism; acetazolamide, aldactone iii. R - RTA; early renal failure iv. D - Diarrhea; diuretics (e.g., spironolactone, diamox) v. U - Uretosigmoidostomy vi. P - Posthypocapnea; pancreatitis 6. Differentiating acidosis in alcohol ingestion: Substance Target Osm Gap Ketones Breath Acidosis Urine Ethanol Liver ± +++ 0 Methanol Eyes 0 --- +++ Ethylene glycol Kidney 0 --- +++ Oxalate crystals Isopropyl ± + 0 Alcoholic ketoacidosis N ± + + 8

7. Osmolar Gap: a. Osmolar gap = [measured osmolality] [calculated osmolality] b. Osmolar gap = normal < 10 c. Calculated osmolality = 2 [Na + ] + [glucose / 18] + [BUN / 2.8] d. Differential Diagnosis of increased osmolar gap: i. Lactic acid ii. Ethylene glycol iii. Ethanol iv. Isopropanol v. Methanol e. If osmolar gap > 25 think ethylene glycol and methanol. f. You will generally see an anion gap > 20 and an osmolar gap > 25 in ethylene glycol and methanol poisoning. 8. Osmolar gap and ingestion of an unknown alcohol-glycol: Osm Normal ph No acetone Ethanol acetone Isopropanol Acidosis Methanol Ethylene glycol 9. Calcium relationship to albumin a. Corrected calcium = observed calcium + 0.8 (4.0 - albumin) 10. Water/salt balance: a. Fractional excretion of Na + i. FENA = [(Urine Na + ) (Plasma Cr) / (Plasma Na + ) (Urine Cr)] X 100 b. Water deficit (L) = [(0.6) (wt in kg) ((Observed Na + / 140) 1)] i. Infuse ½ of deficit over 24 hours then the remainder over the next 2-3 days. Critical Care Nutrition 1. Energy (kcal/gram) a. Lipid = 9.1 kcal/gram b. Protein = 4.0 kcal/gram c. Glucose = 3.75 kcal/gram 2. Ideal body weight (kg) males = 50 + 0.91 [Height (cm) 152.4] 3. Ideal body weight (kg) females = 45.5 + 0.91 [Height (cm) 152.4] 4. Short form basal energy expenditure (BEE) equation: a. BEE (kcal/day) = 25 x wt (kg) b. Stress factors i. Burns: 9

1. 1-20% TBSA 1.2-1.5 2. 20-40% TBSA 1.5-2.0 3. 40% TBSA 1.8-25 a. Sepsis 1.2-1.7 b. Trauma 1.1-1.5 c. Fever 1.0 x each degree C of fever above 38C c. Rule of thumb calculation of caloric needs: i. 25-30 kcal/kg/day for most patients is sufficient. d. Swan-Ganz guided measurement of energy expenditure. i. kcal/24 hours = VO 2 x 7.1 5. Protein a. In general need one gram protein / kg / day i. Nitrogen intake (grams) = protein intake (grams) / 6.25 ii. Nitrogen balance (grams) = [protein intake (grams) / 6.25] [24 hour urine nitrogen (grams) + 4] 10