The ventilatory effects of auto-positive end-expiratory pressure development during cardiopulmonary resuscitation.

OBJECTIVE: Auto-positive end-expiratory pressure (auto-PEEP) is a physiologic phenomenon defined as the positive alveolar pressure that exists at the end of expiration. Normally, the alveolar pressure is near zero at the end of expiration. However, certain ventilatory and/or physiologic paradigms can cause the development of auto-PEEP during cardiopulmonary resuscitation (CPR). Auto-PEEP has a detrimental cardiovascular effect similar to that of positive end-expiratory pressure that is intentionally applied to the ventilatory circuit in a mechanically ventilated patient. The connection between auto-PEEP and its cardiovascular effects, however, may go undetected. In this study, the effect that ventilatory factors have on auto-PEEP in a simulation of patients with lung disease undergoing CPR was delineated. DESIGN: A case control study. SETTING: Laboratory of a university hospital anesthesia department. SUBJECTS: A baseline quantification of breathing patterns that occur during CPR was obtained by recording observed respiratory rate and relative tidal volume during treatment of in-hospital cardiac arrests. MEASUREMENTS AND MAIN RESULTS: A test lung was set up to mimic a series of different airway resistances and lung compliances as would be seen in different types of pulmonary pathology. A sensitivity analysis was performed on each of the factors of respiratory rate, tidal volume, and inspiratory/expiratory ratio as to the effect each of these factors has on the development of auto-PEEP. Our study suggests that in various combinations of airway resistances and lung compliances, auto-PEEP can be generated to substantial levels depending on the methods of ventilation performed. CONCLUSION: We conclude from our findings that ventilation techniques during CPR may need to be altered to avoid the development of what may be a hemodynamically significant level of auto-PEEP.

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.

The effect of right internal jugular vein cannulation on intracranial pressure.

Access to the central venous circulation is often necessary in patients who have elevated intracranial pressure. It has been suggested that a disadvantage of the internal jugular vein approach to the central circulation may be an elevated intracranial pressure. The purpose of this prospective study was to evaluate the effect of right internal jugular vein cannulation on intracranial pressure in patients who are at risk of intracerebral hypertension. Eleven adult patients studied in the intensive care unit were evaluated. The population included those patients who were admitted to the neurosurgical intensive care unit requiring intracranial pressure monitoring and central venous access. With the intracranial pressure monitor in place, patients were put in supine and 30 degrees head-up positions while intracranial pressure was recorded. The Queckenstedt maneuver was performed on all patients. A central venous line was then placed in the right internal jugular vein, and intracranial pressure was recorded. The Queckenstedt maneuver was again performed in the study population, and intracranial pressure measurements were recorded for the right, left, and bilateral compression of the internal jugular vein. The results of the intracranial pressure measurements before and after placement of the central venous line were statistically analyzed using single-factor analysis of variance over time. The mean Glasgow coma and Apache II scores for the study groups were 8 +/- 4 and 15 +/- 6, respectively. There were no significant differences in heart rate; cerebral perfusion pressure; or systolic, mean, or diastolic pressures throughout the study period. There was no statistical difference found between the intracranial pressures at any time point throughout the study. Furthermore, no difference was found in percentage change from baseline intracranial pressure data throughout the study period. Our results suggest that cannulation of the right internal jugular vein is a safe approach to the central circulation in patients at risk of intracranial hypertension. A description of the possible accommodating mechanisms are outlined.

Cost-benefit analysis of nasal cannulae in non-tracheally intubated subjects.

We evaluated four nasal cannulae used to deliver oxygen and measure PETC02 in a non-tracheally intubated, healthy population. The effect of various oxygen flow rates on PETC02 and respiratory rate (RR), as well as the cost and relative patient comfort of the cannulae, was compared. In this controlled study, 20 healthy volunteers tested the cannulae using oxygen flow rates of 0 (breathing room air), 2, 4, and 6 L/min. The volunteers rated the comfort of each cannula on a scale from 1 (uncomfortable) to 10 (comfortable). Hospital costs of the cannulae were also compared. All of the cannulae, except the Hospitak nasal cannula (HOS), demonstrated an oxygen flow dependency with respect to PETC02 and RR. The HOS cannula was judged by the study population to be the most comfortable and was the second least expensive cannula tested. From a qualitative standpoint, the carbon dioxide waveform recorded with the HOS cannula was the one least affected by oxygen flow. Of the nasal cannula systems evaluated in this study, the HOS system demonstrated the best cost-benefit ratio, performing well clinically while being comfortable to wear and relatively inexpensive. These conclusions are specific to a healthy population and not to patients with lung disease,those who smoke, or those having a higher ASA classification status. Our evaluation suggests that comfort and clinical performance of nasal cannulae may well depend on device design.

Effect of measurement error on calculated variables of oxygen transport.

The effect of measurement error in pH, PCO2, and PO2 on mathematically derived variables of oxygen transport in patients was delineated by comparing calculated oxygen saturations from a blood-gas machine with measured saturations from a CO-oximeter and further by modeling the error in a computer simulation. Twenty-one critically ill patients aged 30-84 yr were studied. A total of 80 arterial and 80 mixed venous blood gas samples were collected. The intraclass correlation results between measured and calculated arterial (SaO2) and mixed venous (Sv-O2) oxygen saturations were 0.59 and 0.68, respectively. The product-moment correlation for SaO2 was 0.75 and for Sv-O2 was 0.77. The percent error in calculating and measuring oxygen saturation was found to be greater at low PO2 values, whereas percent error of calculating oxygen consumption increased as the PO2 increased. Measurement repeatability at high PO2 is better than at low PO2 for both measured and calculated methods. We conclude from this comparison that measured and calculated SaO2 and Sv-O2 values are not interchangeable. Each can introduce substantial error in calculating oxygen consumption through error propagation and error amplification.

Respiratory dysfunction in muscular dystrophy and other myopathies.

Respiratory muscle weakness occurs in a wide spectrum of acute and chronic myopathic disorders, and may cause a restrictive ventilatory defect and life-threatening ventilatory failure. Therapeutic intervention is based on a thorough knowledge of the course of each disease and meticulous monitoring of respiratory function. Comprehensive management of patients with myopathic disorders requires an understanding of the pathophysiology and specific manifestations of respiratory involvement in each disease.