Measuring systolic arterial blood pressure. Possible errors from extension tubes or disposable transducer domes

Non-invasive versus arterial pressure monitoring in the pre-hospital critical care environment: a paired comparison of concurrently recorded measurements

BACKGROUND

Blood pressure monitoring is important in the pre-hospital management of critically ill patients. Non-invasive blood pressure (NIBP) measurements are commonly used but the accuracy of standard oscillometric cuff devices may be affected by extremes of physiology and adverse conditions (e.g. vibration) during transport. This study aimed to quantify the accuracy of NIBP measurements amongst patients requiring pre-hospital critical care.

METHODS

A retrospective cohort study was undertaken using data from patients treated by a pre-hospital critical team between 1st May 2020 and 30th April 2023 that had NIBP measured concurrently with invasive blood pressure (IBP) arterial manometry. An acceptable difference was determined a priori to be < 20mmHg for systolic blood pressure (SBP) and diastolic blood pressure (DBP), and < 10mmHg for mean arterial pressure (MAP). The primary outcome was "pairwise agreement", i.e. the proportion of paired observations that fell within this range of acceptability. Bland-Altman plots were constructed together with 95% limits of agreement to visualise differences between pairs of data. Associations with patient age, reason for critical care, transport status, haemodynamic shock, severe hypertension, and arterial catheter position were explored in univariate analyses and by fitting multivariable logistic regression models.

RESULTS

There were 2,359 paired measurements from 221 individual patients with a median age of 57. The most frequent reason for transport was cardiac arrest (79, 35.7%). Bland-Altman analyses suggested unacceptably wide limits of agreement with NIBP overestimating both SBP and MAP during hypotension and underestimating these values during hypertension. Haemodynamic shock (SBP < 90mmHg) was independently associated with reduced pairwise agreement for SBP (adjusted odds ratio [aOR] 0.52, 95% CI 0.35 to 0.77), DBP (aOR 0.65, 95% CI 0.42 to 0.99) and MAP (aOR 0.53, 95% CI 0.36 to 0.78) and severe hypertension (SBP > 160mmHg) with reduced pairwise agreement for SBP (aOR 0.17, 95% CI 0.11 to 0.27). There was no association between patient transport and agreement between the methods for SBP, DBP, or MAP.

CONCLUSIONS

Non-invasive blood pressure measurements are often inaccurate in the pre-hospital critical care setting, particularly in patients with haemodynamic instability. Clinicians should be cautious when interpreting NIBP measurements and consider direct arterial pressure monitoring when circumstances allow.

The knowledge and practice of maintaining the patency of arterial catheters

BACKGROUND

Maintaining the patency of arterial catheters-routinely inserted in critically ill patients in intensive care units (ICUs)-is essential for obtaining physiological measurements and enabling blood sampling.

AIM

This study aims to evaluate current ICU nurse knowledge and practice of maintaining the patency of arterial catheters and explore the factors that influence nurses' knowledge level.

DESIGN

This was a cross-sectional survey conducted in China.

METHODS

This research was conducted in 20 tertiary hospitals in Beijing, China between March and June 2020. The data were collected by electronic questionnaire, which was designed in accordance with the literature and consisted of 28 questions. Descriptive and inferential statistics were used to analyse the data.

RESULTS

A total of 576 completed questionnaires were returned. The mean score of nurses' knowledges was 3.66 ± 1.35, which is a moderate level. There was a statistically significant difference between the mean scores of nurses with different professional titles and work experiences (mean 3.58 vs 4.04/7; mean 3.50 vs 3.58 vs 3.94/7). Considering ICU nurses' practice of maintaining the patency of arterial catheters, 376(65.3%)nurses replaced the pressure transducer as per the manual, and 347 (60.2%) nurses zeroed the pressure transducer once per shift. More than 90% ICU nurses aligned the transducer with the heart surface marker during zeroing procedures. Furthermore, 79.9% of nurses performed fast-flush tests routinely, 459 (85.9%) nurses flushed the arterial catheter routinely, and 80% of nurses evaluated the patency of the arterial catheter every shift.

CONCLUSIONS

This study found that the practices of ICU nurses varied, and their knowledge of how to maintain the patency of arterial catheters was moderate and could be improved. ICU nurses should be trained effectively to develop a unified standard of arterial catheter management.

RELEVANCE TO CLINICAL PRACTICE

Training programmes on arterial catheter management for ICU nurses are essential for improving knowledge and practice.

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.

Feasibility of in vivo pressure measurement using a pressure-tip catheter via transventricular puncture

Pressure-tip catheters (PTCs) are used to evaluate ventricular mechanics during surgical repair of congenital heart disease in children. Studies in infants require miniaturized sensors. We compared the safety and accuracy of a 2-Fr ultraminiature PTC with a 5-Fr PTC. In 10 piglets (weight 19-22 kg), a 5-Fr PTC was inserted through a 3-mm apical puncture with a #11 blade. A 20-gauge angiocatheter was inserted using a separate site. A 2-Fr PTC was threaded through the angiocatheter lumen. The angiocatheter was withdrawn, leaving the 2-Fr PTC within the left ventricle (LV). Left ventricular pressure (LVP) changes were measured during three inferior vena caval occlusions. Reliability coefficients demonstrated correlation between the 2-Fr PTC and 5-Fr PTC for LV end-diastolic pressure (0.90-0.95), peak LVP (0.92-0.99), and the maximal (0.87-0.93) and minimal (0.89-0.94) first derivatives of LVP. Bland-Altman analysis demonstrated agreement for all variables. Blood loss was trivial with pressure manipulation and catheter placement and removal. Pressure measurements using the 2-Fr PTC were accurate and comparable with those from the 5-Fr PTC. Transventricular placement of a 2-Fr PTC is feasible and should allow evaluation of ventricular mechanics during surgical repair of congenital heart disease.

Accuracy of intravascular microcatheter pressure measurements: an experimental study

Intravascular pressure measurements are considered useful for the monitoring and assessment of endovascular treatment effects in intracranial vascular malformations. Experimental data on the accuracy of these measurements are limited. A flow phantom with defined intraluminal pressures and pulsatility flow waveforms was used in this study. Microcatheters commercially available for neuroendovascular procedures (length 140-155 cm), with different outer (0.5-0.83 mm) and inner (0.3-0.53 mm) diameters, were introduced into the phantom in the direction of flow. In a static experiment, pressure values from 0 to 75 mmHg were applied, and in the dynamic part of the experiment mean pressure values from 25 to 65 mmHg, with a pulsatile amplitude from 70 to 170 mmHg were employed. In the static experiment, there was a linear relationship between the pressure values obtained through the microcatheters and the local transducer of the flow phantom. The pulsatile experiments showed increased damping of the pressure waveforms with decreasing inner diameter of the microcatheters. However, the mean pressure values remained accurate. This experimental study has shown that mean pressure values can be accurately measured through microcatheters from 0.3-0.5 mm inner diameter and more than 140 cm in length. In vivo pressure measurements during interventional procedures are therefore reproducible and can be used for monitoring of embolization effects in patients.

Correction of pressure waveforms recorded by fluid-filled catheter recording systems: a new method using a transfer equation

BACKGROUND

Pressure measuring systems using fluid-filled catheters can result in the recording of distorted pressure waveforms. It results in phase delay, overestimation of systolic and, to a lesser extent, of diastolic pressure. We designed and evaluated a method to correct this pressure waveform distortion using an appropriate transfer equation obtained from the dynamic response of the fluid-filled catheter. This transfer equation is based on the principle that a fluid-filled catheter recording system is considered as an underdamped dynamic system fully characterized by its natural frequency (omega n) and damping ratio (zeta).

METHODS

Pressure waveforms, simultaneously recorded in vitro or in vivo by a fluid-filled catheter (Pc) and a micromanometer-tipped catheter (Pref), were used to validate the method. Dynamic response of the catheter used was obtained from a fastflush test. The corrected signal (Ppred) was obtained using omega n, zeta and the following transfer equation: d2Pc/dt2 + 2 omega n zeta dPc/dt + omega n 2Pc = C Ppred (t) After correction of Pc, Ppred was compared, using a linear regression, with Pref taken as reference.

RESULTS

Our results showed that Ppred was fitted to Pref with excellent coefficient correlation (0.99). The mean error and the standard error of estimate were respectively -1.16 mmHg and 1.4 mmHg.

CONCLUSION

This new method can convert the distorted pressure waveforms transmitted by any fluid-filled catheters into high-fidelity signals. It suppresses the phase delay and the over-estimation of systolic pressure induced by fluid-filled catheters.

Correctly selecting a liquid-filled nasogastric infant feeding catheter to measure intraesophageal pressure

Polyvinyl chloride (PVC) nasogastric feeding catheters are used clinically to measure intraesophageal pressure as an estimate of pleural pressure for calculating lung compliance in infants. The accuracy of pressure measurement of 4 French gauge (FG) catheter sizes and three brands of liquid-filled catheter manometer systems (CMS) was evaluated by determining their resonance-frequency amplitude and phase properties. All CMS were underdamped and resonated. No CMS exhibited a uniform mean frequency response above 11 Hz. The maximum respiratory rate (Frr) within which CMS could potentially measure dynamic intraesophageal pressure within a 5% error limit was determined (Frr): the highest mean Frr recorded reliably in large-diameter catheters was 82 breaths/min. Significant CMS differences in accuracy existed between catheter FG sizes and between catheters of similar diameters but differing brands. Correlation (r2) between catheter inner diameter and CMS Frr was 0.66 across brands. In conclusion, intraesophageal PVC liquid-filled feeding catheters are suitable for estimating pleural pressures in subjects mechanically ventilated without sharp inspiratory waveforms or high respiratory rates. Quantitative frequency response characterization of different nasogastric catheter brands and different diameters is mandatory prior to their utilization.

Analysis and comparison of pressure gradients and ratios for predicting iliac stenosis

Over the past 35 years five different pressure variables and their numeric criteria have been recommended to determine the hemodynamic significance of iliac artery stenosis. To analyze and compare the five variables, systolic and mean radial and femoral artery pressures were measured intraoperatively in 144 legs at rest and in 119 (83%) of these during hyperemic flow augmentation with papaverine. Iliac artery diameter stenosis measured from preoperative two-view arteriograms was 48 +/- 37% (mean +/- 1 SD). Resting systolic and resting mean pressure gradients (radial minus femoral artery pressure), hyperemic mean pressure gradients, hyperemic systolic pressure ratios (femoral/radial), and the percentage change from rest to hyperemia of the systolic pressure ratios were measured. For completeness a sixth variable, the hyperemic systolic pressure gradient, was also measured. High-grade (75%) stenosis is predicted with 95% confidence by resting pressure gradients > or = 52 mm Hg systolic and > or = 16 mm Hg mean and resting systolic pressure ratios < or = 0.61. Hyperemia is unnecessary and not useful for predicting > 50% stenosis. The rest-to-hyperemia percentage changes in systolic pressure ratios give poor results. Moderate (50%) stenosis is predicted with 95% confidence by resting pressure gradients > or = 34 mm Hg systolic and > or = 7 mm Hg mean, hyperemic mean pressure gradients > or = 30 mm Hg, and systolic pressure ratios < or = 0.73. Most published criteria have low accuracy, low predictive value, and a low optimal percentage of stenosis range. Simple pressure gradients give optimal results.

Accuracy of four indirect methods of blood pressure measurement, with hemodynamic correlations

In 38 adults undergoing cardiac surgery, 4 indirect blood pressure techniques were compared with brachial arterial blood pressure at predetermined intervals before and after cardiopulmonary bypass. Indirect blood pressure measurement techniques included automated oscillometry, manual auscultation, visual onset of oscillation (flicker) and return-to-flow methods. Hemodynamic measurements or calculations included heart rate, cardiac index, stroke volume index, and systemic vascular resistance index. Indirect and intraarterial blood pressure values were compared by simple linear regression by patient and measurement period. Measurement errors (arterial minus indirect blood pressure) were calculated, and stepwise regression assessed the relationship between measurement error and heart rate, cardiac index, stroke volume index, and systemic vascular resistance index. Indirect to intraarterial blood pressure correlation coefficients varied over time, with the strongest correlations often occurring at the first and last measurement periods (preinduction and 60 minutes after cardiopulmonary bypass), particularly for systolic blood pressure. Within-patient correlations between indirect and arterial blood pressure varied widely--they were consistently high or low in some patients. In other patients, correlations were especially weak with a particular indirect blood pressure method for systolic, mean, or diastolic blood pressure; in some cases indirect blood pressure was inadequate for clinical diagnosis of acute blood pressure changes or trends. The mean correlations between indirect and direct blood pressure values were, for systolic blood pressure: 0.69 for oscillometry, 0.77 for auscultation, 0.73 for flicker, and 0.74 for return-to-flow; for mean blood pressure: 0.70 for oscillometry and 0.73 for auscultation; and for diastolic blood pressure: 0.73 for oscillometry and 0.69 for auscultation. The mean measurement errors (arterial minus indirect values) for the individual indirect blood pressure methods were, for systolic: 0 mm Hg for oscillometry, 9 mm Hg for auscultation, -5 mm Hg for flicker, 7 mm Hg for return-to-flow; for mean: -6 mm Hg for oscillometry, and -3 mm Hg for auscultation; and for diastolic: -9 mm Hg for oscillometry and -8 mm Hg for auscultation. Mean measurement error for systolic blood pressure was thus least with automated oscillometry and greatest with manual auscultation, while standard deviations ranging from 9 to 15 mm Hg confirmed the highly variable nature of single indirect blood pressure measurements. Except for oscillometric diastolic blood pressure, a combination of systemic hemodynamics (heart rate, stroke volume index, systemic vascular resistance index, and cardiac index) correlated with each indirect blood pressure measurement error, which suggests that particular numeric ranges of these variables minimize measurement error.(ABSTRACT TRUNCATED AT 400 WORDS)

A comparison of brachial, femoral, and aortic intra-arterial pressures before and after cardiopulmonary bypass

Following recent evidence that brachial and femoral artery pressures are more reliable than radial artery pressures after cardiopulmonary bypass, thirty-one adults had simultaneous pre- and post-bypass measurements of brachial, femoral, and ascending aortic pressures. Two minutes after cardiopulmonary bypass, brachial artery systolic pressure and mean arterial pressure fell significantly below corresponding pressures in the femoral artery and aorta. Five minutes after cardiopulmonary bypass, only brachial artery systolic pressure was still less than femoral and aortic systolic pressures. By ten minutes after bypass, all significant pressure differences had resolved except between brachial and femoral artery systolic pressures. Clinically significant (greater than or equal to 5 mmHg) aortic-to-brachial reductions in mean arterial pressures occurred in six (19%) patients at two minutes and in three (10%) patients at five and ten minutes after bypass. Equivalent aortic-to-femoral mean pressure diminution occurred in two (6%) patients at two minutes and one (3%) patient at five and ten minutes after bypass. Neither systemic vascular resistance nor body temperatures contributed significantly to post-bypass central-to-peripheral pressure reductions. Immediately following bypass, femoral artery pressures reproduce central aortic pressures more reliably than do radial or brachial artery pressures.

A comparison of radial, brachial, and aortic pressures after cardiopulmonary bypass

Previous investigations have identified falsely low radial artery pressures after cardiopulmonary bypass (CPB). The present study investigates the relationship among radial, brachial, and aortic arterial pressures in 33 cardiac surgical patients following CPB. Two minutes after separation from CPB, clinically important (greater than or equal to 10 mmHg) underestimation of systolic aortic pressures occurred in 17 of 33 (52%) radial artery catheters, while occurring in seven of 33 (21%) brachial artery catheters. Radial artery mean pressure underestimated aortic mean pressure by greater than or equal to 5 mmHg in 21 of 33 (61%) patients two minutes after CPB, while an equivalent aortic-to-brachial artery mean arterial pressure difference occurred in nine of 33 (27%) patients. The incidence of aortic-to-radial mean arterial pressure differences greater than or equal to 5 mmHg decreased to 40% (four of ten patients) by ten minutes after CPB, although interpretation is complicated by decreased availability of aortic pressure measurements. Multivariate analysis failed to identify factors predisposed to central-to-peripheral pressure gradients. Radial and brachial arterial pressures were compared both before and after CPB in all 33 patients. Brachial artery systolic and mean pressures were higher than corresponding radial artery measurements two minutes after CPB (P less than 0.05), followed by gradual resumption of a normal brachial-to-radial pressure relationship over 60 minutes. Either vasospasm in the brachial and radial arteries or profound arteriolar vasodilation in the upper extremity might cause the observed central-to-peripheral arterial pressure differences. The progressive central-to-peripheral decrease in mean arterial pressure favors the latter mechanism.(ABSTRACT TRUNCATED AT 250 WORDS)

Physical characteristics and clinical evaluation of a new disposable fibreoptic transducer-tipped catheter system

A new disposable fibreoptic transducer-tipped catheter manometer system was evaluated to assess its accuracy, stability of accuracy under prolonged simulated intra-arterial conditions, and dynamic characteristics. Maximum errors observed in the measurement of static pressure using a sample of five catheters (with one display unit) were 2 mmHg at 0 mmHg reference pressure, 2 at 20 mmHg, 4 at 40 mmHg, 4 at 100 mmHg and 9 at 200 mmHg. An immersion artifact caused a shift in baseline of up to 2 mmHg. Exposure of the transducer to 24 hours of simulated intra-arterial conditions (pulsatile pressure at 40 degrees C) resulted in errors of up to 7 mmHg for pressures up to 100 mmHg, and 11 mmHg for 200 mmHg, which were largely attributable to a drift in baseline pressure (up 6 mmHg by 24 hours). Consistent overestimation by the system suggested inappropriate gain setting within the display unit which, however, is not user-adjustable. The system exhibited uniform frequency response up to 33 Hz.

An evaluation of blood pressure measurement

The accuracy of routine measurements by nursing staff of systemic arterial, central venous, pulmonary artery and pulmonary capillary wedge pressures was determined. There was a significant difference between direct mean arterial blood pressure measurements and routine indirect measurements by the nursing staff in the pressure range of 50--100 mmHg, whereas there was no significant difference between direct and indirect measurements when indirect measurements were made by specially trained hypertension clinic personnel. However, there was a good correlation between direct and indirect measurements in each instance, indicating that changes in blood pressure could be adequately followed by both groups. Systems commonly used to measure blood pressure directly were tested. Limits in frequency response preclude the routine direct measurement of systolic or diastolic blood pressures. If direct systolic and diastolic pressure measurements are required, it is necessary to check the performance of the amplifier and recording system, attach the transducer to the patient, and determine and adjust, if necessary, the natural frequency and damping coefficient of each system before each measurement. However, it is suggested that a knowledge of systolic and diastolic pressure measurements seldom improves patient management, and if mean pressures are accepted, reliable routine measurements may be obtained by the nursing staff. The digital display of the systems tested may be accepted for mean arterial pressure, but for accurate mean central venous and pulmonary capillary wedge pressure measurements, it is necessary to interpret the trace on a chart recorder; pulmonary artery pressure can often only be estimated.