Test Taking Tip
Critical Care problems generally can be solved by applying physiology and pharmacology principles to the question (or patient). Understanding the sympathetic and parasympathetic nervous system assists in the diagnosis of cardiac issues and their treatment. Understanding respiratory physiology and mechanics greatly assists in solving ventilator-related questions.
What is the formula for mean arterial pressure (MAP)?
MAP = DBP + 1/3 × (SBP – DBP)
What is the normal range for cardiac output (CO)?
4 to 8 L/min
What is the normal range for cardiac index (CI)?
2.5 to 4 L/min
What is the normal range for systemic vascular resistance (SVR)?
800 to 1400 dyn · s/(cm5)
What is the normal range for SVR index (SVRI)?
1500 to 2400 dyn · s/(cm5)
What is the normal range for pulmonary capillary wedge pressure (PCWP)?
7 to 15 mm Hg
What is the normal range for central venous pressure (CVP)?
2 to 6 mm Hg
What is the normal range for pulmonary artery (PA) pressure?
20 to 30/6 to 15 mm Hg
What is the normal range for mixed venous oxygen saturation (SvO2)?
70% ± 5%
What is SvO2?
The oxygen saturation of blood in the right ventricle/pulmonary artery that serves as an indirect measure of peripheral oxygen supply and demand
What factors influence SvO2?
Oxygen delivery and oxygen extraction
Approximate percentage of CO that goes to the brain:
Approximate percentage of CO that goes to the heart:
What complications can occur during central venous catheter placement?
Arterial puncture, air or catheter embolism, dysrhythmias, hemorrhage or arteriovenous fistulization, pneumothorax, pseudoaneurysm formation
What should you immediately do if a patient receives an air embolus?
Roll the patient to the left and place head down to keep air in right atrium/ventricle; attempt to aspirate air with central catheter/pulmonary artery catheter
Relative contraindications for pulmonary artery catheter placement:
Left bundle branch block, previous pneumonectomy
Treatment for hemoptysis after pulmonary artery catheter placement:
Immediately pull the pulmonary artery catheter slightly back and reinflate balloon, increase positive end expiratory pressure (PEEP) to help tamponade the bleeding, mainstem intubate the nonaffected side; attempt can be made to place a fogarty catheter down the affected side; if recalcitrant, may need thoracotomy and lobectomy
What West zone of the lung is the desired location for a pulmonary artery catheter?
Zone III (Pa > Pv > PA); pressure in the arteries (Pa), pressure in the veins (Pv), pressure in the alveoli (PA)
Which portion of the lung has the highest V/Q ratio and which portion has the lowest V/Q ratio?
V/Q ratio highest in upper lobes
V/Q ratio lowest in lower lobes
At what point in the respiratory cycle is the PCWP most accurate in a ventilated patient?
At what point in the respiratory cycle is the PCWP most accurate in a nonventilated patient?
What conditions may make the wedge pressure unreliable?
Aortic regurgitation, high PEEP, mitral stenosis, mitral regurgitation, poor left ventricular compliance, pulmonary hypertension, pulmonary disease (acute respiratory distress syndrome [ARDS]), tamponade, pneumothorax
Pulmonary artery catheters allow the direct measurement of which physiologic parameters?
CVP, right atrial pressure, pulmonary arterial pressure, right ventricular end-diastolic pressure, pulmonary artery wedge pressure, SVO2
Arrhythmias and conduction defects, knotting or catheter entrapment, pulmonary infarction, pulmonary artery rupture, valvular damage.
What is an intra-aortic balloon pump?
A mechanical device that consists of a cylindrical balloon that actively deflates in systole increasing forward blood flow by reducing afterload, and actively inflates in diastole increasing blood flow to the coronary arteries resulting in decreased myocardial oxygen demand and increased CO
When does the balloon from an intra-aortic balloon pump inflate on ECG?
T wave (diastole)
When does the balloon from an intra-aortic balloon pump deflate on ECG?
P wave or start of Q wave (systole)
Indications for intra-aortic balloon pump placement:
Bridge to heart transplant for patients with left ventricular failure; cardiogenic shock, percutaneous coronary angioplasty, post cardiothoracic surgery, preoperative use for high-risk patients (unstable angina with stenosis >70% main coronary artery), reversible intracardial mechanical defects complicating infarction, unstable angina pectoris, ventricular dysfunction with ejection fraction <35%
Absolute contraindications to intra-aortic balloon pump placement:
Aortic regurgitation, aortic dissection, severe aortoiliac occlusive disease
Relative contraindications to intra-aortic balloon pump placement:
Prosthetic vascular grafts in the aorta, aortic aneurysm, aortofemoral grafts
Desired location for the tip of the intra-aortic balloon pump catheter?
1 to 2 cm below the top of the aortic arch just distal to the left subclavian
What is preload?
End diastolic length of cardiac myocytes, which is linearly related to end-diastolic volume (EDV) and filling pressure
What is afterload?
Resistance against ventricular contraction (SVR)
What is contractility?
The force of myocardial contraction
What 3 things determine stroke volume?
Left ventricular EDV (LVEDV) (preload), contractility, afterload
How is stroke volume calculated?
LVEDV – left ventricular end-systolic volume
How is the ejection fraction calculated?
What is the normal O2 delivery to consumption ratio?
5 to 1
Heart rate (HR), increased ventricular wall tension
What is the normal range for the alveolar:arterial gradient?
10 to 15 mm Hg
What shifts the oxygen-hemoglobin dissociation curve to the left?
Decrease 2,3-diphosphoglycerate (DPG)
What shifts the oxygen-hemoglobin dissociation curve to the right?
What is the definition of shock?
Inadequate perfusion to meet the body’s metabolic needs
What are the 5 broad categories of shock?
Hypovolemic, obstructive, neurogenic, septic, cardiogenic
What is the formula for the O2 content of blood (CaO2)?
CaO2 = [Hb × SaO2 × 1.34] + [0.003 × PaO2]; Hb is hemoglobin in g/dL, SaO2 is arterial O2 saturation (%), PaO2 is partial pressure of O2 (mm Hg) in arterial blood
What is the formula for O2 delivery (DO2)?
DO2 = Cao2 × CO; [(Hb × Sao2 × 1.34) + (0.003 × PaO2)] × (HR × SV); Hb is hemoglobin in g/dL, SaO2 is arterial O2 saturation (%), PaO2 is partial pressure of O2 (mm Hg) in arterial blood, HR is heart rate (beats/min [bpm]), SV stroke volume (mL)
VO2 = CO × (CaO2 – CVO2) × 10
(This can be rearranged to estimate the mixed venous saturation; the Fick equation)
How many milliliters of oxygen will a gram of hemoglobin bind when hemoglobin is fully saturated with oxygen?
1.34 mL of oxygen is bound to each gram of hemoglobin. (1.39 mL of oxygen is normally bound to hemoglobin, but you subtract the 3%–5% of circulating hemoglobin present as methemoglobin and carboxyhemoglobin.)
What does the solubility coefficient of 0.03 mL/L/mm Hg in the oxygen delivery equation signify?
0.03 mL of O2 dissolves in 1 L of water when the PO2 is 1 mm Hg at normal body temperature (37°C)
Manipulation of what factors can increase oxygen delivery?
Greatest increase of oxygen delivery with increasing hemoglobin content and SaO2; can also increase oxygen delivery by raising CO by increasing either HR or stroke volume
What is the equation for oxygen extraction ratio?
O2 ER = VO2/DO2
Usually expressed as a percentage; the normal value is 0.25 to 0.3 or 25% to 30%
Treatment for ventricular fibrillation/pulseless ventricular tachycardia:
1 shock (monophasic 360 J; biphasic, 100–200 J); cardiopulmonary resuscitation with additional countershocks if shockable rhythm obtained; epinephrine 1 mg IV and repeat every 3 to 5 minutes, or vasopressin, 40 units IV (to replace the first or second dose of epinephrine); consider amiodarone (300 mg IV), lidocaine (1–1.5 mg/kg), magnesium (1–2 g IV); if no shockable rhythm, revert to asystole/pulseless electrical activity algorithm
Treatment for asystole/pulseless electrical activity:
Verify with lead rotation; epinephrine 1 mg IV and repeat every 3 to 5 minutes, or vasopressin 40 units IV (to replace the first or second dose of epinephrine); consider atropine 1 mg IV every 3 to 5 minutes, up to 3 doses; if shockable rhythm, revert to ventricular fibrillation/ventricular tachycardia algorithm
Treatment for an unstable patient with bradycardia (HR <60 bpm):
Transcutaneous pacing; if transcutaneous pacing not immediately available, give atropine 1 mg and epinephrine 2 to 10 mg/min
In which patients with atrial fibrillation is anticoagulation unnecessary?