Effect of sample dilutions on arterial blood gas determinations

Anatomical Considerations in Obtaining Arterial Access

Arterial cannulation and catheterization are routine parts of medical care in intensive care units for neo nates, children, and adults. In this review I describe the anatomical landmarks that facilitate successful catheteri zation of the radial, ulnar, brachial, axillary, femoral, pos terior tibial, dorsalis pedis, umbilical, and superficial temporal arteries. Criteria for selection of each of these arteries for catheterization are discussed. Risks of arte rial puncture or catheterization include hemorrhage, thrombosis, infection, and vascular compromise to sur rounding structures. These risks are reviewed with ref erence to each of the arteries described.

Continuous intravascular blood gas monitoring: development, current techniques, and clinical use of a commercial device

This review focuses on the development, current techniques, and clinical use of continuous intravascular blood gas monitoring (CIBM) devices in anaesthesia and intensive care. The operating principles, range of application, performance, limitations, costs, and impact on patient treatment and outcome, are discussed. Studies of early and currently available CIBM devices were analysed. At present, the Paratrend 7+ (PT7+) for adults and Neotrend (NT) for newborns are the only commercially available CIBM systems. The PT7+ contains three optical sensors to measure PO(2), PCO(2) and pH, as well as a thermocouple to measure temperature. The NT is a modification of the PT7+ to continuously monitor PO(2), PCO(2), pH and temperature in newborns. Under laboratory conditions, good performance over a wide range of blood gas values was observed with the Paratrend 7 (PT7. Performance in the clinical setting was not as satisfactory, especially for PO(2) values. However, the performance and accuracy of CIBM devices appear to be sufficient for clinical use and they are being used clinically in selected patient groups. Several factors affecting the performance of CIBM are considered.

A discard volume of twice the deadspace ensures clinically accurate arterial blood gases and electrolytes and prevents unnecessary blood loss

OBJECTIVE

To determine the blood discard volume, as a multiple of deadspace, that is required for accurate arterial blood gas and electrolyte testing from arterial catheters.

DESIGN

Prospective, controlled, crossover trial.

SETTING

Eighteen-bed intensive care unit of a metropolitan teaching hospital.

PATIENTS

A total of 84 critically ill patients with a 20-gauge, radial arterial cannulae, pressure monitoring transducer set, and stable oxygenation.

INTERVENTIONS

System deadspace (priming volume from sampling port to catheter tip) was established. Patients had six 0.5-mL arterial blood samples taken sequentially in random order using discard volumes of 1, 1.5, 2, 2.3, and 3.6 times the deadspace (experimental values) and 5.5 times the deadspace (control).

MEASUREMENTS AND MAIN RESULTS

Samples were analyzed for Pao(2), Sao(2), pH, Paco(2), HCO(3)-, Na+, and K+. We performed repeated-measures analysis of variance with post hoc linear contrasts and compared mean experimental and control values. The smallest discard volumes that provided measurements that were statistically equal to control were twice the deadspace (Pao(2), p =.563; Sao(2), p =.371) and 3.6 times the deadspace (pH, p =.107; Paco(2), p=.519; HCO(3)-, p =.10). All discard volumes tested provided results that were statistically different from control for Na+ (p <.003) and K+ (p <.001).

CONCLUSIONS

Many results were statistically different from control, although the actual discrepancies were very small. At clinically relevant levels of measurement, there was minimal variation between values obtained after a discard volume of twice the deadspace and control values. The level of error was clinically acceptable and within or close to the precision limits of the blood gas analyzer. Slight fluctuation in patient variables during sampling could also have contributed to the error. A blood discard volume of twice the deadspace is recommended for all variables. This will provide clinically accurate results and avoid the deleterious effects of unnecessary blood loss.

The effect of draw-up volume on the accuracy of electrolyte measurements from neonatal arterial lines

OBJECTIVES

Contamination by infusate of blood samples withdrawn from arterial lines has been recognized but not well documented for neonates. The aim of this study was to investigate, using in vitro and in vivo studies, the effects of different draw-up volumes (withdrawn from the line prior to the sample being taken) on the concentration of sodium.

METHODS

In-vitro study: The tip of an umbilical artery catheter (dead space 0.6 mL), infused with half normal saline containing 1 unit/mL of heparin was placed in a beaker of normal saline. The line was flushed with 1 mL of this infusate just before each sample was taken. Volumes from 0.5 mL to 2.0 mL of infusate/normal saline were withdrawn in 0.1 mL increments from a three-way tap and discarded. A sample was then taken from the line into a blood gas syringe for analysis of the sodium concentration by the 860 Blood Gas Analyzer (Chiron Diagnostics, Bayer, Scoresby). Control samples were taken from the beaker. In-vivo study: A 22 gauge intravenous catheter was inserted into a vein of an adult male volunteer. The dead space was also 0.6 mL. The line was flushed with 5 mL of half-normal saline immediately before sampling. Draw-up volumes of 0.6, 0.9, 1.3, and 1.6 mL were withdrawn and discarded. 10 mL was used as a control. A 0.5-mL blood sample was then taken and the electrolyte concentrations analysed immediately.

RESULTS

In-vitro: A minimum draw-up volume of 1.3 mL was required before the sodium concentration was not significantly different from the control samples. In-vivo: A minimum draw-up volume of 1.6 mL was required before the sodium concentration was not significantly different from the control samples. There were similar trends in the effect of draw-up volume for glucose, calcium, potassium, chloride and lactate.

CONCLUSION

: A minimum volume of 1.6 mL should be withdrawn from neonatal arterial lines (dead space 0.6 mL) before taking blood for analysis.

On the dynamic performance of the Abbott Safeset blood-conserving arterial line system

OBJECTIVE

Critically ill patients frequently have indwelling arterial lines placed during their Intensive Care Unit stay. The lines are used to monitor blood pressure continuously, administer drugs and to draw blood for a variety of physiologic tests. Several blood-conserving arterial line systems have been developed to eliminate the need to discard blood in the process of obtaining undiluted and uncontaminated blood samples. The purpose of this study was to evaluate the dynamic performance of one such system the Abbott Clinical Care System Safeset blood conserving arterial line system - in comparison to a conventional arterial line system.

METHODS

We studied ninety-nine patients who had indwelling arterial lines placed during surgery and who were admitted to our Surgical Intensive Care Unit (SICU). The patients were randomly placed into one of two groups. The control group received a conventional indwelling arterial line system; the experimental group received the Abbott Safeset system. We measured the damping coefficient and resonant frequency daily in order to evaluate and compare the dynamic performance of the two systems. We also measured discard volumes (in the control group) and blood sample sizes during the patients' stays in the SICU.

RESULTS

The two patient groups were similar in regards to demographics and baseline clinical characteristics. A median 3 ml of blood per draw and 17.5 ml of blood per patient was discarded in purging the conventional arterial line system while, by design, no blood was discarded with the experimental system. There was no difference between the two groups with regard to damping coefficient. Both systems were underdamped. However, the conventional arterial line system had a significantly higher resonant frequency (16.7 Hz) compared to the Safeset system (12.5 Hz).

CONCLUSIONS

Because the Abbott Safeset blood-conserving arterial line system is underdamped and has a lower resonant frequency compared to the traditional arterial system, it may overestimate systolic blood pressure, particularly in patients with high heart rates.

Adverse occurrences in the pediatric intensive care unit

This article is intended to provide a review of the literature describing adverse occurrences affecting children in the intensive care unit. In addition, information related to the care of critically ill children is inferred from studies of adult or neonatal patients. Finally, areas in need of future investigation are suggested.