Blood Pressure Monitoring

Routine Monitoring of Critically I11 Patients

The explosion in computer use and technology during the past several decades has dramatically changed criti cal care. All vital signs can now be monitored accu rately, noninvasively, and continuously. We examined the predominantly noninvasive methods of monitor ing temperature, arterial blood pressure, heart rate and rhythm, respiratory mechanics, and gas exchange that can be used continuously on almost every patient in the intensive care unit. We conclude that temperature should be measured either intermittently with a rectal thermometer or continuously with a rectal, bladder, or great vessel thermocouple or thermistor probe. Arterial pressure should be measured either intermittently with a sphygmomanometer and cuff or continuously with an indwelling arterial catheter, except in specific situa tions when automated indirect monitoring may be use ful. Electrocardiographic rhythm monitoring has been clearly shown to improve prognosis in patients after acute myocardial infarction and should be universal in all intensive care units. Ischemia monitoring may prove beneficial, but its role has not been clearly defined. Re spiratory inductance plethysmography is effective in measuring respiratory rate, tidal volume, and breathing pattern. Pulse oximetry is useful in detecting occult hy poxemia. It should be continuous on most patients. Transcutaneous oxygen and carbon dioxide measure ment has a limited role in monitoring gas exchange and perfusion. Capnography also has a limited role in the intensive care unit but is more helpful in the operating room.

Relating external compressing pressure to mean arterial pressure in non-invasive blood pressure measurements

Arterial volume clamping uses external compression of an artery to provide continuous non-invasive measurement of arterial blood pressure. It has been assumed that mean arterial pressure (MAP) corresponds to the point where unloading leads to the maximum oscillation of the arterial wall as reflected by photoplethysmogram (PPG), an assumption that has been challenged. Five subjects were recruited for the study (three males, mean age (SD) = 32 (15) years). The PPG waveform was analysed to identify the relationship between the external compressing pressure, PPG pulse amplitude and MAP. Two separate tests were carried out at compression step intervals of 10 mmHg and 2 mmHg, respectively. No significant differences were found between the two tests. The bias between the compressing pressure and the MAP was -4.7 ± 5.63 mmHg (p < 0.001) showing a normal distribution. Further research is needed to identify optimal algorithms for estimation of MAP using PPG associated with arterial compression.

Noninvasive techniques for blood pressure measurement are not a reliable alternative to direct measurement: a randomized crossover trial in ICU

INTRODUCTION

Noninvasive blood pressure (NIBP) monitoring methods are widely used in critically ill patients despite poor evidence of their accuracy. The erroneous interpretations of blood pressure (BP) may lead to clinical errors.

OBJECTIVES

To test the accuracy and reliability of aneroid (ABP) and oscillometric (OBP) devices compared to the invasive BP (IBP) monitoring in an ICU population.

MATERIALS AND METHODS

Fifty adult patients (200 comparisons) were included in a randomized crossover trial. BP was recorded simultaneously by IBP and either by ABP or by OBP, taking IBP as gold standard.

RESULTS

Compared with ABP, IBP systolic values were significantly higher (mean difference ± standard deviation 9.74 ± 13.8; P < 0.0001). Both diastolic (-5.13 ± 7.1; P < 0.0001) and mean (-2.14 ± 7.1; P=0.0033) IBP were instead lower. Compared with OBP, systolic (10.80 ± 14.9; P < 0.0001) and mean (5.36 ± 7.1; P < 0.0001) IBP were higher, while diastolic IBP (-3.62 ± 6.0; P < 0.0001) was lower. Bland-Altman plots showed wide limits of agreement in both NIBP-IBP comparisons.

CONCLUSIONS

BP measurements with different devices produced significantly different results. Since in critically ill patients the importance of BP readings is often crucial, noninvasive techniques cannot be regarded as reliable alternatives to direct measurements.

Discrepancies between direct and indirect blood pressure measurements using various recommendations for arm cuff selection

OBJECTIVE

The current recommendation for choosing an appropriate size cuff for measuring blood pressure (BP) is a bladder width to equal 40% of the upper arm circumference (UAC). However, most physicians use the older two-thirds or three-fourths upper arm length (UAL) recommendations to choose a cuff. The aim of this study was to verify the disparity in cuff size by using two-thirds UAL, three-fourths UAL, and 40% UAC criteria for cuff selection and to compare the indirectly measured BP by these criteria with directly measured radial intra-arterial BP.

METHODS

A prospective, cross-sectional, observational study was conducted in 65 hemodynamically stable patients, aged 5 days to 22 years. Direct BP measurements were obtained from a radial intra-arterial catheter. Indirect BP measurements were taken from the same arm as the arterial catheter with a mercury sphygmomanometer and standard-size arm cuffs. UAL and UAC of each patient were measured, and the 3 cuffs closest to two-thirds and three-fourths UAL and 40% UAC were used. For each cuff, 3 direct and 3 indirect BP measurements were taken. Student t test was used to compare mean systolic and diastolic BP for direct and indirect measurements.

RESULTS

A total of 172 observations were recorded, including 56 by two-thirds UAL, 55 by three-fourths UAL, and 61 by 40% UAC criteria. There was no significant difference between the means of the ideal cuff size by 40% UAC criterion and the actual cuffs used from the available standard cuffs. However, because of an unavailability of the larger cuffs for UAL criteria, the actual cuffs used were significantly smaller than ideal. Comparison of direct and indirect BP measurements revealed no significant difference in systolic BP when the latter was obtained by 40% UAC criterion. However, the difference in diastolic BP was significant. With available cuffs, a significant difference in systolic as well as diastolic BP was seen with both UAL criteria.

CONCLUSIONS

Practitioners are likely to use significantly larger arm cuffs when following the two-thirds and three-fourths UAL criteria than when following the 40% UAC criterion. Of the 3 criteria for cuff selection, systolic BP by 40% UAC criterion most accurately reflects directly measured radial arterial pressure. However, the 40% UAC cuff significantly overestimates the diastolic pressure. Using available cuffs for indirect measurements by two-thirds and three-quarters UAL criteria significantly underestimates systolic as well as diastolic BP when compared with radial intra-arterial BP.

Femoral arterial catheterization in paediatric burn patients

Despite the obvious drawbacks associated with prolonged arterial cannulation, arterial catheters are frequently required in paediatric burn patients with respiratory failure to facilitate monitoring of oxygenation, ventilation and blood pressure, and to allow for the painless performance of phlebotomy. Burn patients with respiratory failure frequently require prolonged arterial catheterization, often for more than 72 h, and many of their cannulation sites are burned. We report our favourable 3-year experience with 81 femoral arterial catheters in 36 paediatric burn patients. When small diameter pressure monitoring catheters are used, femoral cannulation sites are associated with a low rate of mechanical and infectious complications.

Dynamic response of a neonatal catheter-manometer system in situ

OBJECTIVE

The purpose of this study was to develop, validate, and apply a flush-pulse method to determine the dynamic response of a neonatal catheter-manometer system (CMS) in situ.

METHODS

In the flush-pulse method, the opened fast-flush valve of the CMS is closed; as a result, the fluid column in the CMS is impacted. This procedure can be done without affecting the net flow of infusion fluid. We validated the method in laboratory conditions by comparing 14 paired results obtained with this method to the results obtained using a generally accepted step-response method. The measurable values are the resonance frequency (fr) and the damping coefficient (delta). The analysis of the flush-pulse method in situ is complicated by the patient's blood pressure wave. A remedy for this problem that is based on the first derivative of the pressure signal has been developed. The flush-pulse method is applied 14 times in situ.

RESULTS

In laboratory settings, the fr ranged from 12.5 to 64.0 Hz and delta ranged from 0.14 to 0.32. The correlation coefficient was 0.99 for fr and 0.91 for delta. We found four overdamped systems in situ (delta > 1). In other systems fr values between 8.5 and 41.0 Hz and delta values between 0.16 and 0.72 were observed. The dynamic response in situ appeared to deteriorate with time due to routine intensive care procedures.

CONCLUSIONS

The flush-pulse method proved to be a valid test for determining the dynamic response. The results obtained in situ emphasize the need for a regular evaluation of the dynamic response of the neonatal CMS in order to assess the shape of the pressure wave.

Amniotic fluid embolus

A case report of a 27-yr-old healthy patient for Caesarean section under epidural anaesthesia is presented. The patient suffered an acute cardiorespiratory collapse when the infant's head was being delivered through the anterior abdominal wall. The patient remained cyanosed after proper tracheal intubation and pulmonary ventilation with 100% O2. Hypotension was difficult to treat but returned to normal 25 min after the event. A pulmonary artery catheter inserted three hours after the event showed normal pressures and a high cardiac output. The patient suffered permanent neurological damage. The differential diagnosis is discussed and current concepts of the aetiology and management of amniotic fluid embolism reviewed.

Noninvasive monitoring

Noninvasive monitors are finding increased use in the intensive care unit both as labor-saving tools and as a means to reduce complications associated with invasive techniques. The current technology allows for the replacement of a number of invasive devices with a noninvasive counterpart capable of providing similar information. The potential reduction in morbidity and mortality realized with a greater reliance on noninvasive monitors should result in widespread application of these modalities.