The IVC undergoes a change in diameter during the respiratory cycle. Investigators compared the degree of sonographic IVC respirophasic diameter change with CVP in 73 patients and found >= 50% change in diameter predicted a CVP< 8 mmHg with 91% sensitivity (95% CI 71% to 99%) and 94% specificity (95% CI 84% to 99%). The positive predictive value was 87% (95% CI 66% to 97%), and the negative predictive value was 96% (95% CI 86% to 99%). Presumably the rather arbritrary CVP of 8 was chosen because of its importance as a target for goal directed therapy in sepsis guidelines. A more meaningful endpoint such as a fluid responsive cardiac output might be a more clinically relevant application of this technique, which had been demonstrated previously.
Emergency Department Bedside Ultrasonographic Measurement of the Caval Index for Noninvasive Determination of Low Central Venous Pressure
Ann Emerg Med. 2010 Mar;55(3):290-5
One hundred and thirty-two adults underwent propofol sedation in the emergency department and were randomised into a group in which treating physicians had access to the capnography and a blinded group in which they did not. All patients received supplemental oxygen (3 L/minute) and opioids greater than 30 minutes before. Propofol was dosed at 1.0 mg/kg, followed by 0.5 mg/kg as needed.
Hypoxia (defined as SpO2 less than 93%) was observed in 17 of 68 (25%) subjects with capnography and 27 of 64 (42%) with blinded capnography (p=.035; difference 17%; 95% confidence interval 1.3% to 33%). Capnography identified all cases of hypoxia before onset (sensitivity 100%; specificity 64%), with the median time from capnographic evidence of respiratory depression to hypoxia 60 seconds (range 5 to 240 seconds).
The journal comments: ‘this study provides compelling evidence that capnography can aid in the detection of respiratory depression and reduce hypoxia during procedural sedation.’
However in an accompanying article outlining a pro-con debate for introducing capnography as standard practice in ED procedural sedation, the point is made that the safety benefit purported in this and similar studies is decreased hypoxemia, according to thresholds ranging from 90% to 95%, lasting from 5 to 15 seconds. In the clinical context, many of these events are self-limiting or resolve with minimal interventions such as airway repositioning or supplemental oxygen, and other more clinically relevant outcomes are rarely examined (perhaps due to the rarity of genuinely adverse events in ED procedural sedation by emergency physicians).
Does end tidal CO2 monitoring during emergency department procedural sedation and analgesia with propofol decrease the incidence of hypoxic events? A randomized, controlled trial
Ann Emerg Med. 2010 Mar;55(3):258-64
The Intensive Care Society has published guidelines on the use of capnography in critical care. The recommendations are:
- Capnography should be used for all critically ill patients during the procedures of tracheostomy or endotracheal intubation when performed in the intensive care unit.
- Capnography should be used in all critically ill patients who require mechanical ventilation during inter-hospital or intra- hospital transfer.
- Rare situations in which capnography is misleading can be reduced by increasing staff familiarity with the equipment, and by the use of bronchoscopy to confirm tube placement where the tube may be displaced but remains in the respiratory tract.
- Capnography offers the potential for non-invasive measurement of additional physiological variables including physiological dead space and total CO2 production.
- Capnography is not a substitute for estimation of arterial CO2.
- Careful consideration should be given to the type of capnography that should be used in an ICU. The decision will be influenced by methods used for humidification, and the advantages of active or passive humidification should be reviewed.
- Capnometry is an alternative to capnography where capnography is not available, for example where endotracheal intubation is required in general ward areas.
Link to Full Guideline Document
ScvO2 values are obtained by measuring the oxygen saturation in venous blood returning to the heart, and reflect the balance between oxygen delivery and oxygen consumption.
Low (<70%) ScvO2 values were targeted by Rivers in his Early Goal Directed Therapy study: by improving the macrocirculation with fluids, vasoactive drugs, and packed red cells the aim is to improve oxygen delivery to tissues, and therefore a higher oxygen saturation is found in the venous blood returning to the heart in adequately resuscitated patients. The story is more complex, however, as mechanisms of oxygen supply (macrocirculatory flow), distribution (microcirculatory flow), and processing (mitochondrial function) must all function at an adequate level to maintain normal physiology.
Although low ScvO2 values may be a marker for macrocirculatory failure, high ScvO2 values may reflect microcirculatory or mitochondrial failure.
A multicentre study demonstrated a higher mortality on patients whose ScvO2 in the ED was high (90-100%) compared with those with a normal ScvO2.
Mortality associated with three groups according to their highest recorded ScvO2 in the ED was:
Hypoxia group (ScvO2 <70%) – 40% mortality (95% CI 29-53)
Normoxia group (ScvO2 71-89%) – 21% mortality (95% CI 17-25)
Hyperoxia group (ScvO2 90-100%) – 34% mortality (95% CI 25-44)
The study design could not control for many potential confounders, but this opens the door for further study, and reminds us that the unthinking pursuit of a single physiological target may miss the bigger clinical picture.
Multicenter Study of Central Venous Oxygen Saturation (ScvO2) as a Predictor of Mortality in Patients With Sepsis
Annals of Emergency Medicine 2010;55(1):40-46
In 180 intubated trauma patients in the ED, there was little correlation between arterial carbon dioxide tension (PaCO2) and end-tidal carbon dioxide levels (ETCO2) (R2 = 0.277). In fact, in those patients ventilated to the ‘normal range’ of 35-40 mmHg (4.6-5.2 kPa), PaCO2 was over 50 mmHg 30% of the time. Slightly reassuring that in isolated brain injury the correlation was better (r2 = 0.52)
The Utility of Early End-Tidal Capnography in Monitoring Ventilation Status After Severe Injury
J Trauma. 2009 Jan;66(1):26-31
Arterial lines in the field? You’ve got to be nuts, or…..French! Yep, the SAMU boys and girls put in 94 arterial lines in pre-hospital cases over two years, and found big differences between invasive and non-invasive arterial pressures in systolic and diastolic pressures. What about mean pressures though, which we’d expect to be more closely correlated? They didn’t say. An interesting paper, but I don’t think I can use it.
Emerg Med J. 2009 Mar;26(3):210-2
Invasive arterial blood pressure monitoring in an out-of-hospital setting: an observational study
Bispectral index monitoring (BIS) was applied to 57 intubated patients transported by a Helcopter Emergency Medical Service (HEMS), demonstrating (1) that the patients were adequately sedated, (2) BIS works in helicopters, and (3) there is enormous scope for publishing work related to the retrieval environment – anything is of interest!
Bispectral index monitoring in helicopter emergency medical services patients
Prehosp Emerg Care. 2009 Apr-Jun;13(2):193-7