Chapter 27 Noninvasive Blood Pressure Monitors Introduction Frequent blood pressure determinat ions during anes thesia a id drug t it rat ion and

f luid management and provide warning of condi tions that could affec t patient safe ty

(1,2). There are a number of devices that au tomatical ly measure blood pressure

noninvasively ( indirec tly). The origina l automated devices were stand-alone

moni tors . Now, the majority of blood pressure monitors for operat ing room use are

incorporated in to a physiologic moni tor o r anesthesia workstation .

Intermittent Blood Pressure Monitors Inte rmittent noninvasive measurement of b lood pressure requires a distensib le cuff

or b ladder enclosed in an unyielding cover. The cu ff is inf lated, and blood f low

through the underlying artery is obstruc ted. The cu ff is then def la ted in a controlled

manner, causing the pressure appl ied to the artery to decrease. Pulsa tions are

detec ted, and the resul ts are displayed or recorded as blood pressure. Most

devices also display pu lse rate.

Standards An American standard on automated sphygmomanometers was pub lished in 2002

(3). I t s tates that blood

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pressure measurements determined by the device must be equivalen t to those

obtained by a tra ined observer using the auscul tatory method or by di rect

moni toring us ing an intra -arte rial catheter within the l imits of the standard.

An inte rnational s tandard was published in 1999 (4). I t l imi ts the maximum cuff

pressure obtainable during normal use to 300 mm Hg fo r equipment speci fied fo r

adults and 150 mm Hg fo r equipment specif ied fo r neonates. In addi tion , means

must be present to prevent the cuff f rom remaining inf lated for long periods of t ime.

Operating Principles The majority of automated noninvasive moni to rs employ osci llometry (5,6). The cuf f

is inf lated above the point where pressure osci l la tions are present. As the cuf f is

deflated, pressure pulsat ions (osci l lat ions) caused by movement of the arterial wal l

are transmitted to the cuff . The magni tude of these oscil lat ions increases to a

maximum, then decreases (Fig. 27 .1). The monitor measures these osci llations.

Af ter the determinat ion is complete, the remaining air in the cuff is rapidly

exhaus ted.

During the f irs t cycle, the cuff is inf lated to a predetermined pressure that is held

constant wh ile the monitor tries to detect oscil lat ions. I f sign if icant pulsat ions are

st i l l p resent, the cuff is inf la ted further. When f i rs t turned ON, most automatic blood

pressure monitors s top inf la tion at 160 to rr or less (7). On pediatric uni ts , a lower

pressure is used. If the ini tia l cuff pressure is g reater than necessary to de termine

systolic pressure , the monitor may decrease this pressure during the next cycle. If

the init ia l pressure is too low, the moni to r wi l l inc rease it during the nex t cycle .

When no oscil lat ions are detected, the pressure in the cuff is dec reased in a

stepwise or l inear manner (Fig. 27.1).

Non invasive blood pressure (NIBP) moni tors rely on measurement, extrapo la tions .

and c linical ly tes ted algori thms to arrive at values fo r mean, systol ic , and dias tol ic

pressures (6,7,8). The point of maximum ampl itude corresponds to the mean

pressure (5,9,10,11). Systol ic and diastolic blood pressures are then ca lcula ted

f rom the inc reasing and dec reasing magni tude of the oscil lat ions according to an

empirically derived algorithm. These algori thms are proprietary and vary from

manufac turer to manufac turer and sometimes from device to device (10).

Consequently , different devices do not always yield the same pressures (12).

With oscil lometry, accurate bladder placement is not necessary, and the risk that

the cuff bladder wi l l become dis lodged is minimal . These moni tors do not requ ire a

low-noise envi ronment, are not sens it ive to electrosurgical interference, and work

wel l when there is periphera l vasoconstric t ion . Venous engorgement has li t tle effec t

on accuracy (7). These monitors can re ject mos t art ifac ts caused by pat ien t

movement o r external interference, based on ana lysis o f the shape of the

oscil lometric pulse waveform and the timing of the osci l lometric pulses (5).

View Figure

Figure 27.1 A, B: Sequence of oscillometric blood pressure determination. Cuff deflation may be linear (A) or in steps (B). Pressure oscillations increase in magnitude, then decrease. The oscillations are analyzed to determine systolic, mean, and diastolic pressures. (Reprinted with permission from Nellcor Puritan Bennett, Pleasanton, CA.)

Equipment These moni to rs use electrici ty, either l ine or ba ttery, as the power source . Many

units are equipped wi th a connec tion fo r a printer or data management system.

Some can d isplay trends. Many manufacturers offe r different models for dif ferent-

s ize patients.

Inflatable Cuff Cuffs come in dif ferent s izes. Disposable and latex-f ree cuffs are avai lable. Some

have an ant imicrobial agent

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on the cuff fabric to help prevent bacterial growth and mi ldew. Some cuffs are color

coded by s ize. The inte rnational s tandard requires that cuf fs are marked wi th an

indicat ion of the l imb ci rcumference for wh ich they are appropriate (4).

Pressure Tubing(s) One or two tubings serve as a means to inflate the cuff and sense the pressure

wi th in the cuff . A Y-piece near the cuff can be used to mix s ingle- and two-hose

systems (13). Blood pressure tub ings shou ld no t have Luer connec tions , because

these may allow a pressure tube to be connec ted to another device such as an

intravenous l ine (14).

Cuff Inflation and Deflation Mechanism The cuff inf lation mechanism consists of a pump and a reservoir that holds a

quant ity of ai r that can be quick ly released into the cuff . A bleed valve def lates the

cuff . The speed and manner of def la tion vary from manufacturer to manufacturer.

Some adjus t the rate as a funct ion of pressure.

Sensing Mechanism A pressure transducer detects the pressure in the cuff and transduces i t into an

electrical current that can be measured. The pressure-sensing mechanism is bu il t

into the cuf f in most mon itors. An autozero valve tha t periodically opens the

transducer to a tmosphere, automatica lly establ ishing zero pressure, may be

present (7 ).

Timing Circuitry The t iming ci rcui try regulates the f requency of blood pressure determinations. Most

instruments a lso a llow manua l cycling . Most o ffer a short-term automatic (STAT)

mode that allows a number of determinat ions to be made in quick succession.

Control Circuitry The control c i rcuitry de termines such parameters as the maximum pressure in the

cuff , artifact reject ion , electrosurgical interference suppression, def lat ion rate, and

automatic cutoff . The overpressure swi tch may be set d if ferently fo r adul t and

pedia tric models. Some machines have automatic cuff deflat ion when the cycle time

exceeds a predetermined l imi t.

Alarms Monitors vary according to which parameter(s ) wi l l t rigger an alarm. Some moni to rs

automatical ly set alarms around the ini t ia l values. Some have defaul t alarm limi ts

that can be changed by the operator. Some moni to rs ac tivate an alarm when they

are unab le to determine the b lood pressure.

Accuracy Factors Affecting Blood Pressure Determinations

Cuff/Arm Relationship

Accurate blood pressure measurement depends on the relat ionsh ip be tween arm

c i rcumference and cuff width (10). Typically, a cuff that is too small fo r the patien t's

arm c ircumference wi l l overes timate blood pressure, whereas a cuff that is too

large wil l underestimate blood pressure (10,15,16). I t is important that dif ferent

s izes cuffs are readily available. If a suff ic iently large cuff is no t available, i t may

be possible to place a cu ff more distally on the l imb. Current recommendations are

that a cu ff 's width should be approximately 40% to 50% of the c ircumference (125%

to 150% of the diameter) o f the l imb on wh ich i t is used a t the l imb's midpo in t

(17,18,19). Cuff length has l it tle effec t as long as the bladder enci rc les at least

50% of the arm's c i rcumference (17).

In the past, it was the practice in pediatric patien ts to use a cuff that was two th irds

to three fourths of the upper arm length . This resul ted in underes timation o f blood

pressure (20).

Another mis leading criterion for de termin ing proper cuff s ize is a label such as

infant, child , pediatric , smal l adu lt , adult , and large adult . These te rms are open to

inte rpretat ion by the user and can lead to inappropriate cuff select ion (20).

Site The s ite where the cuff is placed wi l l af fect the measured pressure. As the s ite

moves more peripheral ly , the systolic pressure tends to increase and diastol ic

pressure to decrease (21). Increased vascular tone may resul t in an inc rease in

pulse pressure. Vascular disease and peripheral vasoconstric tion may cause

reduced pressures at dis ta l locat ions.

The forearm or wrist may be used when the upper arm is inaccessible and/or a

standard blood pressure cuff does not f it (22,23,24,25). The ankle may also be

used (26 ,27). The mean and dias tol ic pressures measured at the ankle are

comparable with those measured in the arm but the sys tolic pressure is higher. A

poor corre la tion was found between blood pressures measures on the arm and ca lf

of women who were undergoing cesarean section (28).

Arm Position I f the cuff and the pat ient's heart a re not a t the same level , a correction mus t be

made. For each 10 cm of vert ical he ight, 7.5 mm Hg (for every inch 1.80 mm Hg)

above or below the heart level should be added to or subtracted f rom the pressures

measured (21).

Arrhythmias

The osci llometric techn ique is vulnerable to error in pat ients wi th arrhythmias

(29,30,31,32).

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Improper Calibration and Maintenance Studies performed wi th instruments in common use have shown prob lems wi th

ca libration , main tenance, and accuracy (10). Errors in the algorithm may occur

(33,34).

Correlation with Other Methods of Blood Pressure Determination Many feel tha t the s tandard of comparison shou ld be the pressure measured

di rectly in an artery, most commonly the radia l. However, indi rect pressure readings

wi l l never exac tly match invasive pressures. One reason is that s imu ltaneous

measurements often cannot be made. Another reason is that di rect and ind irect

moni tors measure dif ferent physical p roperties. The di rec t method measures both

the sys tol ic and diastol ic pressures and calcula tes the mean. Osci l lometry

measures the mean and calcu lates the sys tol ic and diastolic p ressures.

Many s tud ies have compared pressures obtained wi th NIBP moni to rs wi th those

obtained di rect ly (16,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52). In

general, correlat ion of di rectly measured b lood pressures wi th indi rect ly measured

blood pressures is best in normal hea lthy pa tients and least accurate at the

ex tremes of b lood pressure. Al though errors can occur in either d irect ion,

oscil lometric dev ices tend to overestimate low pressures and underestimate high

pressures. The STAT mode does not denigrate values and is subject to the same

variations as measurements taken under normal ci rcumstances (53).

Most studies have concluded that an NIBP monitor is not suff ic ient ly accurate if

vasoactive drugs are being administered or in c ri tica lly i l l pa tients al though i t may

be usefu l fo r trending purposes . Another problem is that wi th moderate to severe

hypotension , the automated machine of ten cycles repeatedly before indicat ing a

failure to measure (2).

Proper Use I f possible , the cuff should no t be app lied to the l imb in which an intravenous

infusion is placed because infusion of f lu ids and drugs wi l l be s lowed or blocked

and the increased pressure may cause blood to f low retrograde into the infusion se t

tubing or ex travasat ion (7). Check valves in the intravenous tub ing between the

f luid bag and the patient usually prevent backflow in to the tubing. A s imple method

to avoid, or at least reduce, this problem is to route the infusion tubing under or

through the blood pressure cuff (54). Other means to prevent this problem have

been suggested (55,56,57). In some cases , it may be possib le to place the cuf f

dis tal to the infusion si te .

I f used on the arm, the cuff should be p laced as high as possible, and the inflation

tubes should exi t proximally (58,59). I f the upper arm is very large or conical ly

shaped, it may be better to measure blood pressure at another locat ion such as the

ankle , th igh, or forearm. The cuff should not be appl ied over a superfic ial nerve,

bony prominence, or join t.

Padding shou ld be placed around the sk in under the cuff to prevent sk in creasing,

petechiae, blis tering, nerve damage, and problems f rom residual c leaning materials

(60).

Prior to apply ing the cuff , all res idual air should be expelled. The cuff should be

applied snugly enough to al low only one f inger to be s lipped under i t . Too t ight a

cuff may cause discomfort and venous distent ion. Too loose a cuff may cause

falsely elevated readings and may prevent the moni tor f rom determining the blood

pressure . The cuff should be placed and severa l inflat ions made whi le the pat ient is

awake to elic i t complaints. Nothing should be allowed to press against the cuff

during measurements .

Blood pressure measurements should be made no more f requently than c linical ly

necessary. In one study, i t was shown that except when rapid changes in b lood

pressure are ant ic ipated, a one- or two-minute cycle t ime offers no advantage over

a five-minute cycle time, but the shorter time may increase the complicat ion rate

(61).

The cuff site and ex tremity should be inspec ted periodically during prolonged

applica tions . It is good practice is to occasionally swi tch the cuff to another s i te , if

possible.

Complications Damage to Underlying Tissues Petechiae, erythema, edema, thrombophlebitis , and skin avuls ion at the cuff s ite

have been reported (62 ,63,64,65,66,67,68,69). Predispos ing factors to the

development of pe techial hemorrhages include patients tak ing anti -inf lammatory

drugs , s tero ids , or an ticoagulants and patients with th in or redundant sk in (70).

Petechia of ten occur under fo lds in the cuff . Wrapping the space under the cuff with

co tton may avoid these prob lems .

Neuropathies Neuropath ies of the median, ulnar, and radial nerves have been reported fol lowing

use of an automatic NIBP moni to r (58,59,69,71,72). All resolved spontaneously.

Locating the cuff c lose to the axi l la and away from the elbow wi l l help to prevent

radial nerve injury. Excessive movement makes i t diff icul t for the device to

determine pressures, so it wil l cycle more of ten , possibly to higher p ressures. This

can contribute to nerve injuries.

Compartment Syndrome Cases of compartment syndrome assoc ia ted wi th prolonged use have been reported

(73,74,75). In one case, the patien t had hyperact iv ity and tremor. In another case,

P.841

the blood pressure was labi le, causing the monitor to cycle longer and wi th higher

pressures than usual . In a thi rd case, the cuff was appl ied across the antecubital

fossa.

Mechanical Problems Failure of an NIBP monitor is not uncommon and may be associated with serious

morbidity or mortali ty (2,76). A common problem is a leaking cuff , hose, or

connec tor (7,76,77).

Artifacts Oscil lometric devices are sensi tive to both intrinsic and extrins ic motions (79).

Intr insic motion artifacts are caused by movements such as del iberate patient

motion, sh ivering, tremors, convulsions , rest lessness, o r v igorous sk in preparat ion

(79,80). Extrins ic motion art ifacts are caused by act ions that compress the cuff ,

such as bumping by personne l or equipment (7) or massaging the arm where the

blood pressure cuff is loca ted (81). Synchronizat ion with the e lec trocardiogram can

reduce art ifacts (82).

Blood pressure readings were reported in a patient where the cuff was under the

patient 's head (83). It is surmised that head movements caused enough motion in

the cuff to p roduce the art ifac tua l readings . In another case, a cuff became

unfas tened, but the bladder remained between the arm and the thorax (84). I t was

st i l l able to detect osci l lations and d isplayed a f ic ti tious pressure.

Air Embolism I f the l ine used to in flate the cuff is connected to an intravenous l ine , a serious ai r

embolus can resul t (85).

Advantages Automaticity A major advantage of NIBP dev ices is that they can determine pressures regularly

and f requently on an automatic basis. During busy t imes, this is very he lpful , saving

t ime and allowing c l in ic ians to perform other tasks.

Automatic devices eliminate mos t of the factors that cause errors when blood

pressures are determined manua lly, such as variable concentration, reaction t imes,

hearing acuity, ambient no ise, confusion of aud itory and v isua l cues, variable

deflation rates, background noise, variab le interpretation o f sounds, preference for

certain d igi ts , and b ias due to knowledge of previous readings . They may e liminate

some of the mistrust that some ind iv iduals have when o thers measure the blood

pressure manual ly .

Simplicity Automatic NIBP moni tors are simple to use. They do not require ex tensive training

to set up or main tain.

Noninvasiveness In comparison wi th di rect measurements, NIBP moni tors are less expensive,

s impler, and avoid the risks (ischemic damage, emboli) associated wi th d irect

techniques.

Reliability NIBP monitors are rel iable devices that generally do not require a lot of

maintenance or experience a lo t of downtime.

Usefulness Some NIBP devices wil l work in certain patien ts (such as those with obes ity or

sc leros is and infants), in whom an accurate pressure cannot be obta ined manua lly.

They wil l read accurately through bulky dressings (86). Pulse rates between 40 and

200 beats per minute usua lly do not h inder readings (78).

Versatility

Oscil lometric blood pressure measurements can be made on a variety of si tes.

Monitor Integration NIBP monitors can be integrated with other moni tors . This in tegration may allow the

number of false alarms to be reduced. For example, i t may be possible to set the

pulse oximeter so tha t it does not alarm for a certa in period of t ime af te r the NIBP

cuff is inf lated.

Disadvantages Unsuitable Situations While non-cont inuous blood pressure moni toring is he lpful in es tab lish ing trends, i t

is unsu itable fo r detecting rapid changes in blood pressure (87). If rap id changes

are ant ic ipa ted, a continuous method should be used.

Patient Discomfort Patient discomfort is of ten associa ted wi th a prolonged cycle t ime. Cycle time wil l

be prolonged with a large cuff , hypertensive patien ts, poor periphera l ci rcula tion, a

leak in the monitor, low blood pressure, dysrhythmias, or motion artifacts.

Clinical Limitations Automated blood pressure devices do not work wel l with extreme heart ra tes and

pressures. In extreme condi tions , even mean pressure may not be measurable .

Studies involv ing ambulance and helicopter transport have shown that these

moni tors are not as rel iable as needed (88,89,90).

Noncontinuous Measurements Af ter a b lood pressure measurement has been made, the value is usual ly d isplayed

unti l the next measurement.

P.842

Most moni tors give some indica tion of the age of the disp layed value. Some

c linic ians do not set the blood pressure monitor to automatic un ti l af te r induct ion is

comple te. On occasion, there is a prolonged period unt i l the automatic mode is

used (91 ,92). During th is t ime, the original b lood pressure wil l be disp layed.

Continuous Noninvasive Blood Pressure Instruments Continuous or near-continuous blood pressure moni toring is desi rable in patients

wi th cardiac or cerebrovascular disease, in crit ically i l l pat ients, and in surgical

procedures in which periods of hemodynamic instabi li ty a re ant ic ipated.

Vasotrac The Vasotrac consists of a wris t module and a moni tor/display connec ted by a

cable (93). The sensor is placed over the radial artery at the wris t. A pressure-

sensing mechanism appl ies variable pressure d irectly above the arte ry. The

counterpressure in the artery produces a pressure waveform. Analys is of the

waveform is used to calculate the sys to lic , d ias to lic , and mean arterial pressures.

Blood pressure measurements, pulse rate, and an arteria l waveform display are

updated several times a minute and displayed on a moni tor. It is available in a

handheld vers ion.

When the Vasotrac was compared with di rec t arterial pressure , good correlation

was found (93,94,95,96,97,98). I t was found to be accura te when deliberate

hypotension was used (98,99). However, in l iver transplantation patients, the

Vasotrac was not suff ic ien tly accura te to subs ti tu te for di rec t arterial blood

pressure monitoring (100).

Blood pressure can be detected down to 40 mm Hg and up to 240 mm Hg. It is

accurate wi th arrhythmias. The Vasotrac sys tem is susceptible to patient arm

movement, but the waveform can alert the caregiver to artifacts .

Al though the Vasotrac does not provide cont inuous blood pressure measurement, i t

provides much more f requent measurements (3 to 4 times per minute) than the

inte rmittent method. This may decrease the need for intra-arterial cannulat ion.

T-Line Tensymeter The T-Line Tensymeter u ti lizes arterial tonometry, in which a peripheral artery is

s light ly compressed and the pulse wave is sensed by a transducer placed over i t. I t

consists of th ree parts: A disposable wris t spl in t, a disposab le radial artery sensor,

and a reusable bracelet. The pat ient 's heigh t and weight must be entered into the

moni tor.

Fi rs t, the wris t res traint is applied. The radial pulse is palpated and marked. The

sensor is aligned over the point of maximum pu lsation . Then, the bracelet is

connec ted over this . The device then automatical ly and cont inuously de termines the

point of maximum pulsat ion and exerts s l ight pressure on the artery. Once the

optimum s ite is loca ted, the system does a dynamic pressure search to determine

the mean arterial pressure . The pulsa tion is converted into a disp layed waveform,

and systolic and diastolic blood pressures are computed by using the waveform, the

patient 's body mass index, and an algori thm. This provides cont inuous, beat-to-beat

NIBP measurements.

This dev ice requires less t ime to set up than invasive arteria l catheter placement

and avoids the risks assoc iated wi th an invasive arterial catheter. One study found

that the blood pressures agreed c lose ly wi th s imul taneous pressures measured

invasively f rom a contralateral radial arte ry (101).

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P.844

Questions For the fol lowing quest ions, answer

i f A, B, and C are correct

i f A and C are correct

i f B and D are correct

is D is correct

i f A, B, C, and D are correct.

1. During the measurement cycle to determine the blood pressure,

A. The point of maximum ampli tude is considered the sys tol ic pressure

B. Systolic and d ias to lic pressures may be estimated by ident ifying the region

where ampli tudes of osci l lat ions increase and decrease rapidly

C. The d iastolic pressure is the po in t where oscil la tions cease

D. Mean pressure is de termined and the sys tol ic and diastolic p ressures are

ca lcula ted

View Answer2. Which statements regarding accuracy of noninvasive blood pressure monitors are correct? A. Since most use oscil lometric technology, there is c lose agreement between

different ins truments

B. Placement of the cuff over the arte ry is important

C. Patient movement causes a great deal of a rt ifac t to an osci llometric techn ique

D. The posi tion of the cuff in re la tion to the heart is important

View Answer3. Accuracy of the automatic blood pressure monitor may be affected by A. Elec trosurgical interfe rence

B. Venous engorgement

C. Peripheral vasoconstric tion

D. Cuff s ize

View Answer4. Which factors characterize the re lationship between the size of the cuff and the s ize of the arm? A. Unders ized cuffs g ive falsely low readings

B. The cuff wid th should be 125% to 150% of the diameter of the l imb on which it is

used

C. Overs ized cuffs give falsely high readings

D. The cuff length has li t tle effec t on accuracy

View Answer5. Factors that can affect the determination of blood pressure by the auscultatory method include A. Rate of cuff def la tion

B. Rate of cuff inf lation

C. Hearing acuity

D. Standard ized concept o f sys tolic and diastol ic end points

View Answer6. Which techniques constitute proper cuff placement?

A. The cuff should be p laced tigh tly on the l imb

B. A loose ly appl ied cuff wil l usual ly cause falsely elevated readings

C. The cuff should be kept on the same arm even during long procedures

D. The limb should be wrapped wi th padding where the cuf f is to be placed

View Answer7. Patients at risk for developing petechiae include

A. Those on anticoagulants

B. Those tak ing steroids

C. Those taking nonsteroida l ant i- inflammatory drugs

D. Those wi th redundant skin

View Answer8. Contributing causes of a compartment syndrome include A. Prolonged use

B. Hyperact ive patien t

C. Low p lacement of the cuff

D. Stable blood pressures

View Answer