A 52-year-old female with a history of nonischemic cardiomyopathy and reduced ejection fraction undergoes emergent spine surgery after being involved in a motor vehicle collision. There is significant blood loss during the case requiring transfusion of blood products. The patient arrives to the ICU intubated, sedated, and requiring vasopressor support.
A pulmonary artery catheter is placed to guide management and the mixed venous oxygen saturation is noted to be low.
Which of the following is not a potential cause of decreased oxygen delivery in this patient?
Correct Answer: C
The mixed venous oxygen saturation is the oxygen saturation of blood sampled at the proximal pulmonary artery and reflects the balance between global delivery and global uptake of oxygen. Oxygen delivery is the product of arterial oxygen content and CO: DO2 = CaO2 x CO
In blood, oxygen is carried in two forms: the majority bound to hemoglobin and the remainder dissolved in plasma. Therefore, the arterial content of oxygen is expressed by the following equation representing both components:
CaO2 = 1.34x Hb x SaO2 + 0.003 x PaO2
CaO2 = mL of O2 per 100 mL blood
Oxygen-combining capacity: 1.34 mL of O2 per gram of hemoglobin Hb = grams of hemoglobin per 100 mL blood
SaO2 = fraction of Hb saturated with O2
PaO2 = oxygen tension
Solubility: 0.003 mL of O2 per 100 mL plasma for each mm Hg PaO2 Factors that will decrease oxygen delivery include decreased hemoglobin (A), decreased CO due to heart failure or hypovolemia (B), hypoxia (D), and abnormalities such as carbon monoxide poisoning or methemoglobinemia that affect the oxygen-carrying capacity of hemoglobin. Shivering (C) could potentially decrease mixed venous oxygen saturation through increased metabolic demand and oxygen uptake but should not affect oxygen delivery.
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A 56-year-old male with acute on chronic systolic heart failure and septic shock from pneumonia is admitted to the ICU. A recent transthoracic echo reveals moderate tricuspid regurgitation and an ASD with significant left to right shunting. A pulmonary artery catheter is placed to guide hemodynamic management.
Which of the following would most consistently underestimate CO if measured by thermodilution?
Correct Answer: D
CO measurement using thermodilution is the gold standard in current practice owing to its ease of use, safety, and reproducibility over time. To run thermodilution CO, 10 mL of saline cooler than blood is rapidly injected into the RA, and the change in temperature after injection is measured by a thermistor in the PA and is integrated over time. The area under the curve of this injectate is inversely proportional to CO. To provide a more reliable measurement, the test is run three times and values within 10% of each other are averaged. An initial steep positive deflection with high amplitude represents rapid delivery of the cold injectate to the thermistor causing a rapid maximal change in temperature. A steep negative deflection with prompt return to baseline represents forward flow and resolution of temperature change.
Low CO states demonstrate an attenuated rise and fall and overall larger area under the curve. Left to right shunting may overestimate CO by diluting out the injectate, whereas right to left shunting overestimates CO by allowing the injectate to quickly bypass the pulmonary circulation. Right-sided valvular lesions can also overestimate or underestimate CO making thermodilution unreliable. A larger than programmed injectate would cause a larger area under the curve than expected and thus underestimates actual CO. Warmer than programmed injectate would cause a smaller area under the curve than expected and thus overestimates actual CO. Injectate that is warmer than programmed into the computer would be the only answer that would consistently overrestimate CO. Other sources of error include extremely low flow states causing injectate heat loss from slow transit, rapid fluid administration causing temperature fluctuation, improper injection technique, improper placement, and thermistor clot.
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An 80-year-old female was admitted to the ICU from the emergency department with the diagnosis of septic shock secondary to urosepsis. She was noted to be in her usual state of good health 12 hours earlier, but was then found at home lying in bed with altered mental status by her daughter. In the emergency department she was found to be hypotensive with a mean arterial blood pressure (MAP) of 45, lethargic, and was mostly incoherent on physical examination. She was given 2 L of IV fluids with improvement in her BP and mental status. Blood cultures were obtained and a urinalysis revealed E. coli in her urine. She was started on broad-spectrum antibiotics and was admitted to the ICU for further management of her urosepsis.
The resident has seen the patient on admission and is concerned because she continues to require aggressive volume resuscitation to maintain her pressures and is now on a norepinephrine infusion at high doses. On physical examination she is cold to the touch, quite pale, and has a moderately distended abdomen. Your resident is concerned that her initial diagnosis may be incorrect, is worried about a retroperitoneal hemorrhage causing hypovolemic shock, and elects to place a pulmonary artery catheter to help further elucidate the etiology of her shock.
Which of the following hemodynamic parameters obtained from the pulmonary artery catheter would be most helpful in distinguishing hypovolemic shock from septic shock?
Shock is the physiologic state that results from cell dysfunction or death due to inadequate oxygen delivery or uptake. There are multiple causes of shock, including low circulatory volume (hypovolemic shock), severe vasodilation (septic or anaphylactic shock), low CO from heart failure (cardiogenic shock), or obstruction to forward blood flow (obstructive shock). In all cases, shock is manifested by low MAP. The diagnosis of the type of shock may be challenging, but it is important as the treatment of shock differs between the various etiologies. The hallmark of septic shock is low blood pressure due to profound vasoplegia from bacterial endotoxin. Early septic shock is characterized by high CO, low circulating volume, and low SVR due to vasoplegia, low cardiac filling pressures (CVP and PCWP), and an elevated mixed venous oxygen content due to the inability of cells to utilize delivered oxygen due to poisoning from the bacterial infection. The hallmark of hypovolemic shock, in contrast, is low MAP due to low effective circulating volume only. In this case, the body’s usual homeostatic mechanisms to increase blood pressure by increasing both CO and SVR are intact, and both of these numbers will be elevated. Owing to volume loss, the cardiac filling pressures (CVP and PCWP) will all be low. Therefore, the greatest contrast to septic shock is an elevated SVR.
Note that . Raising both CO and SVR will work to raise blood pressure back toward normal. In septic shock the primary problem is low SVR, so the body has no way to raise vascular resistance and can only rely on increased CO to move MAP toward a more normal value. Because of this difference, patients in septic shock will usually feel warm to the touch, whereas patients in hypovolemic shock will usually feel cold to the touch because of the high SVR state.
Of note, the use of a pulmonary artery catheter has never been shown to improve survival in randomized controlled clinical trials of patients in shock.
A 52-year-old female undergoes spine fusion surgery with 2 L of intraoperative blood loss. On POD#3 she develops new onset atrial fibrillation with plans to undergo transesophageal echocardiography (TEE) before cardioversion. After topicalization with 20% benzocaine spray, sedation with fentanyl and versed, the TEE rules out clot and confirms otherwise normal heart function. Electrical cardioversion is successful, but shortly afterward the patient becomes dyspneic and cyanotic. Despite adequate spontaneous ventilation with a non-rebreather mask, the patient remains cyanotic with a pulse oximetry saturation of 85%.
Which study would be most helpful in confirming the diagnosis?
Acquired methemoglobin is a form of hemoglobin where heme is oxidized to the ferric Fe 3+ state. Affected hemoglobin is unable to reversibly bind oxygen causing a functional anemia. Normally, hemoglobin displays increased affinity to oxygen causing a left shift in the hemoglobin dissociation curve. Classic features of methemoglobinemia include cyanosis with a normal PaO2 , decreased SaO2 , and “chocolate brown blood.” Symptomatic methemoglobinemia usually occurs when levels exceed 10% of total hemoglobin. Patients with anemia are typically more sensitive to the effects of methemoglobinemia. Common acquired causes of methemoglobinemia are drugs such as dapsone, lidocaine, prilocaine, benzocaine, metoclopramide, nitroglycerin, and sulfonamides. Other substances such as antifreeze, aniline dyes, hydrogen peroxide, nitrates, nitrites, paraquat, and resorcinol can also cause this.
Deoxyhemoglobin absorbs more red light and oxyhemoglobin absorbs more infrared light. Pulse oximetry works by emitting these two wavelengths and then calculates how much of each is absorbed, thus determining the percent saturation. Methemoglobin, however, absorbs both red and infrared light equally making pulse oximetry inaccurate. At increasing levels, pulse oximetry reading will approximate 85%. Cooximetry measures the absorbance of additional wavelengths specific to other dyshemoglobins such as methemoglobin and carboxyhemoglobin. Methylene blue is the preferred treatment. Methylene blue has a potent, reversible inhibitory effect on MAO, so consideration of an alternative treatment like Vitamin C should be given if patients are at risk for serotonin syndrome. Given this patient’s high suspicion for methemoglobinemia based on sequence of events and clinical signs, blood gas with co-oximetry is most helpful with diagnosis. CBC, ultrasound, chemistry panel, and EEG may help rule out other less likely contributors but will not confirm the diagnosis.
As part of a clinical study, you are measuring oxygen consumption using indirect calorimetry in a ventilated patient in the ICU and want to compare your measurements to a calculated value. Radial artery and pulmonary artery catheters are placed, and the following measurements are obtained:
Assuming minimal contribution from dissolved oxygen, what is the patient’s calculated oxygen consumption?
Correct Answer: A
The Fick equation relates CO, oxygen consumption, and the arteriovenous oxygen content difference as follows:
CO = VO2 / (CaO2 - CvO2)
Rearrange to solve for oxygen consumption yields:
VO2 = CO x (CaO2 - CvO2)
Oxygen content of arterial or venous blood is given by the equation:
Ca/vO2 = 1.34 x Hb x Sa/vO2 + 0.003 x Pa/vO2
Excluding the dissolved oxygen term, CaO2 and CvO2 can be calculated from the information given:
CaO2 = 1.34 x 11.7 x 0.97 = 15.2 ml O2 / 100 mL blood
CvO2 = 1.34 x 11.7 x 0.72 = 11.3 ml O2 / 100 mL blood
Therefore:
VO2 = 5100 mL blood / min x (15.2 - 11.3 ml O2 / 100 mL blood) = 199 mL O2 / min , or approximately 0.2 L/min (A).
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