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passed through bone and tissue as well. The presence of carboxyhemoglobin and methemoglobin, which have differing specific wavelengths, causes an overestimation of oxygen saturation (Choice B). Cyanide toxicity would cause an inability to use oxygen as an electron acceptor in aerobic metabolism but would not change the SpO2 (Choice C). Motion artifact has long been a reason for inaccuracy with peripheral sensors, and despite several attempts at correction, this remains a common cause of alarm that is not associated with actual patient decline (Choice D). Other causes of pulse oximetry inaccuracy include methylene blue, indocyanine green, indigo carmine, and black, blue, or green nail polish (Choice E).Answer: BChan ED, Chan MM, Chan MM. Pulse oximetry: understanding its basic principles facilitates appreciation of its limitations. Respir Med. 2013; 107(6):789–99. doi: 10.1016/j.rmed.2013.02.004. Epub 2013 Mar 13. PMID: 23490227.

      6 Following an MVC, A 47‐year‐old woman sustains a grade 4 splenic laceration and pelvic and femur fracture, for which she went immediately to the operating room. Following her operation, she remains intubated and is transferred to the surgical ICU for continued resuscitation and monitoring. She has a right internal jugular central line placed for CVP monitoring, as well as a right radial arterial line for monitoring of stroke volume variability. Which of the following measurements of stroke volume variability is consistent with insufficient intravascular volume?2%5%10%15%Stroke volume variability is an unreliable predictor of intravascular volume status.Stroke volume variability is a naturally occurring phenomenon in which the arterial pulse pressure falls during inspiration and rises during expiration due to changes in the intrathoracic pressure secondary to negative pressure ventilation (i.e., spontaneous breathing). Variations over 10 mmHg have been referred to as pulses paradoxus. Reverse pulsus paradoxus is a same phenomenon with controlled mechanical ventilation. Stroke volume variability (SVV) is calculated from the percent variability in stroke volume between inspiratory and expiratory portions of the ventilator cycle. The differences in intrathoracic pressure from the ventilator directly affect the amount of venous return to the right ventricle, which determines the subsequent stroke volume (increased intrathoracic pressure decreases the amount of venous return, subsequently decreasing the stroke volume). This effect is magnified in states of volume deficiency, as the lower circulating volume is more subjected to the changes in intrathoracic pressure. A SVV of >13% has been shown to correlate well to a deficit in overall intravascular volume, and thus predicts an increase in stroke volume in response to fluid administration. Stroke volume variability can be measured from the arterial line using a variety of monitoring devices.Answer: DPinsky MR. Functional hemodynamic monitoring. Crit Care Clin. 2015; 31(1):89–111. doi: 10.1016/j.ccc.2014.08.005. PMID: 25435480; PMCID: PMC4250574.Shi R, Monnet X, Teboul JL. Parameters of fluid responsiveness. Curr Opin Crit Care. 2020; 26(3):319–26. doi: 10.1097/MCC.0000000000000723. PMID: 32332283.

      7 Pulse pressure variation (PPV), which quantifies the changes in arterial pulse pressure during mechanical ventilation, is one of the dynamic variables that can predict fluid responsiveness. In which of the following ventilated patients would PPV most reliably predict fluid responsiveness?A 58‐year‐old woman with ARDS following an MVC.A 93‐year‐old man with atrial fibrillation following total colectomy for stercoral ulcer.A 27‐year‐old woman with abdominal compartment syndrome following a 63% total body surface area (TBSA) burn resuscitation.A 68‐year‐old man with impending uncal herniation with a set respiratory rate of 35 breaths/min.A 17‐year‐old man with complete transection at C4 following an MVC.Aortic pulse pressure is directly proportional to left heart stroke volume and aortic compliance. The variation in the pulse pressure (pulse pressure variation or PPV) should thus reflect stoke volume changes as a result of the respiratory cycle. With more intravascular volume, the respiratory influence on preload should diminish, and the variation in pulse pressure should similarly decrease. Several studies showed that PPV accurately predicts fluid responsiveness when patients are under controlled mechanical ventilation. Nevertheless, in many conditions encountered in the ICU, the interpretation of PPV is unreliable (spontaneous breathing, cardiac arrhythmias) or doubtful (low VT). However, there are certain instances in which the respiratory cycle does not significantly alter preload. If the patient has ARDS, he should be on low tidal volume lung‐protective ventilation. Per ARDSNET protocol, a goal tidal volume of 6 mL/kg ideal body weight should be utilized in patients with ARDS. However, low tidal volumes have a less pronounced influence on preload, and several studies have shown that tidal volumes of at least 8 mL/kg are necessary for a reliable variation due to hypovolemia (Choice A). Atrial fibrillation (Choice B) likewise will alter pulse pressure due to changes in length between beats, thus changing the amount of time available for atrial/ventricular filling. This will result in intrinsic pulse pressure variation independent of volume status. In true abdominal compartment syndrome (Choice C), the increased abdominal pressure will prevent an adequate respiratory cycle, and similar to a patient with a set low tidal volume, this will result in significantly decreased effect on preload variation from hypovolemia. Increased respiratory rates (Choice D) can result in low PPV even in cases of sufficient intravascular volume, and thus PPV is not reliable when the heart rate/respiratory rate ratio is less than 3.6. The final patient (Choice E) may have altered hemodynamics secondary to neurogenic shock; however, this will not change the effect of respiratory cycle on ventricular filling. In medical critically ill patients, although no randomized controlled trial has compared PPV‐based fluid management with standard care, the Surviving Sepsis Campaign guidelines recommend using fluid responsiveness indices, including PPV, whenever applicable. In conclusion, PPV is useful for managing fluid therapy under specific conditions where it is reliable. The kinetics of PPV during diagnostic or therapeutic tests is also helpful for fluid management.Answer: EYang X, Du B. Does pulse pressure variation predict fluid responsiveness in critically ill patients? A systematic review and meta‐analysis. Crit Care. 2014; 18(6):650. doi: 10.1186/s13054‐014‐0650‐6. PMID: 25427970; PMCID: PMC4258282.Teboul JL, Monet X, Chemla D, Michard F. Arterial pulse pressure variation with mechanical ventilation. Am J Res Crit Care Med. 2019; 199(1):22–32. doi: 10.1164/rccm.201801‐0088CI.

      8 A 32‐year‐old man is in your ICU following splenectomy and left hemicolectomy after he was kicked by a horse. On postop day 6, the patient is febrile, tachycardic, and hypotensive. Blood cultures are sent, and while awaiting results from gram stain and culture, you place invasive support and monitoring lines with a right internal jugular triple lumen central line and a left radial arterial line. In trying to evaluate whether his stroke volume will increase following fluid administration, you use ultrasound and observe his inferior vena cava during several inspiratory cycles. In regard to IVC diameter and respiratory variation, which of the following is true regarding IVC collapsibility?An IVC diameter variation of < 5% predicts an increase in SV in response to fluid administration.An IVC diameter variation of 5–8% predicts an increase in SV in response to fluid administration.An IVC diameter variation of 8–10% predicts an increase in SV in response to fluid administration.An IVC diameter variation of > 36% predicts an increase in SV in response to fluid administration.While used historically, the data do not clearly indicate that there is a consistent correlation between IVC variation and SV response to fluid administration.While IVC collapsibility or distensibility throughout the respiratory cycle (caval index) has been used for years as an adjunct to determine volume responsiveness in critically ill patients, the practice has demonstrated fairly poor reliability. In a systematic review and meta‐analysis performed by Orso et al., 26 studies were evaluated that investigated the predictiveness of the caval index in both adult and pediatric intubated and non‐intubated patients. While the caval index in intubated patients was more predictive than in non‐intubated patients, there was not sufficient correlation among studies to show that IVC diameter throughout the respiratory cycle was a reliable method of predicting volume responsiveness.Answer: ELong E, Oakley E, Duke T, Babl FE ; Paediatric Research in Emergency Departments International Collaborative (PREDICT). Does respiratory variation in inferior vena cava diameter predict fluid responsiveness: A systematic review and meta‐analysis. Shock. 2017; 47(5):550–9.Orso D, Paoli I, Piani T, Cilenti FL, Cristiani L, Guglielmo N. Accuracy of ultrasonographic

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