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'''Editor(s)-In-Chief:''' {{ATI}}, [[C. Michael Gibson, M.S., M.D.]] [mailto:charlesmichaelgibson@gmail.com]; {{AE}}; {{AE}} {{Rim}}
==Overview==
A [[ventilation/perfusion scan]] (otherwise known as [[V/Q scan]] or lung [[scintigraphy]]) is a study which shows whether an area of the lung is being ventilated with [[oxygen]] and [[perfused]] with blood. In the setting of a pulmonary embolism (PE), [[perfusion]] can be obstructed due to the formation of a clot. The [[ventilation/perfusion scan]] is less commonly used due to the more widespread availability of computed tomography ([[CT]]) technology, however it may be useful in patients who have an allergy to [[iodinated contrast]]. It may also be useful in [[pregnancy|pregnant]] patients in an attempt to minimize radiation exposure.  The diagnostic value of the results of the [[V/Q scan]] is improved when combined with the clinical pretest probability of PE. A high probability scan coupled with a high clinical pretest probability of PE is diagnostic for PE, while a normal scan regardless of the clinical pretest probability excludes PE. For the majority of the cases of suspected PE, however, the [[ventilation/perfusion scan]] does not establish the diagnosis nor exclude PE and further tests are required.<ref name="pmid2332918">{{cite journal| author=PIOPED Investigators| title=Value of the ventilation/perfusion scan in acute pulmonary embolism. Results of the prospective investigation of pulmonary embolism diagnosis (PIOPED). | journal=JAMA | year= 1990 | volume= 263 | issue= 20 | pages= 2753-9 | pmid=2332918 | doi= | pmc= | url=http://www.ncbi.nlm.nih.gov/entrez/eutils/elink.fcgi?dbfrom=pubmed&tool=sumsearch.org/cite&retmode=ref&cmd=prlinks&id=2332918  }} </ref>


{{CMG}}; '''Associate Editors-in-Chief:''' [[User:Ujjwal Rastogi|Ujjwal Rastogi, MBBS]] [mailto:urastogi@perfuse.org]
==[[Ventilation/Perfusion Scan]]==
===Principle===
Technetium (Tc)-99m labeled macro-aggregated [[albumin]] particles, when injected, fill small fractions of the pulmonary capillaries. This helps in the assessment of lung perfusion at the tissue level via [[scintigraphy]]. When any branch of the pulmonary artery is occluded, the particles will not reach the capillaries rendering the area "cold" on imaging.


==Overview==
===Indications===
[[Ventilation/perfusion scan]] (or V/Q scan or lung [[scintigraphy]]), show whether or not a lung area is being ventilated and perfused with blood (perfusion can be stopped due to obstruction caused by a clot). This type of examination is used less often because of the more widespread availability of CT technology, however, it may be useful in patients who have an allergy to [[iodinated contrast]]or in [[pregnancy]] due to lower radiation exposure than CT.
The utilization of ventilation/perfusion scanning has declined since the advent of more widespread availability of CT technology, however it may be useful in particular subgroups of patients, such as:
# Patients who have a known allergy to iodinated contrast. To read more about contrast allergy, click [[Contrast medium#Allergy Reactions|here]].
# In pregnant patients to minimize exposure to radiation.
# For patients who are in a hospital lacking CT technology.
 
===Interpretation===
A normal or near normal V/Q scan excludes the diagnosis of PE.<ref name="pmid2332918">{{cite journal| author=PIOPED Investigators| title=Value of the ventilation/perfusion scan in acute pulmonary embolism. Results of the prospective investigation of pulmonary embolism diagnosis (PIOPED). | journal=JAMA | year= 1990 | volume= 263 | issue= 20 | pages= 2753-9 | pmid=2332918 | doi= | pmc= | url=http://www.ncbi.nlm.nih.gov/entrez/eutils/elink.fcgi?dbfrom=pubmed&tool=sumsearch.org/cite&retmode=ref&cmd=prlinks&id=2332918  }} </ref><ref name="pmid18757870">{{cite journal| author=Torbicki A, Perrier A, Konstantinides S, Agnelli G, Galiè N, Pruszczyk P et al.| title=Guidelines on the diagnosis and management of acute pulmonary embolism: the Task Force for the Diagnosis and Management of Acute Pulmonary Embolism of the European Society of Cardiology (ESC). | journal=Eur Heart J | year= 2008 | volume= 29 | issue= 18 | pages= 2276-315 | pmid=18757870 | doi=10.1093/eurheartj/ehn310 | pmc= | url=http://www.ncbi.nlm.nih.gov/entrez/eutils/elink.fcgi?dbfrom=pubmed&tool=sumsearch.org/cite&retmode=ref&cmd=prlinks&id=18757870  }} </ref> When the result of the V/Q scan is not normal or near normal, the results of the V/Q scan should be interpreted in combination with the pretest probability of PE. Shown below is a table that summarizes the interpretation of the different results of the V/Q scan.<ref name="pmid18757870">{{cite journal| author=Torbicki A, Perrier A, Konstantinides S, Agnelli G, Galiè N, Pruszczyk P et al.| title=Guidelines on the diagnosis and management of acute pulmonary embolism: the Task Force for the Diagnosis and Management of Acute Pulmonary Embolism of the European Society of Cardiology (ESC). | journal=Eur Heart J | year= 2008 | volume= 29 | issue= 18 | pages= 2276-315 | pmid=18757870 | doi=10.1093/eurheartj/ehn310 | pmc= | url=http://www.ncbi.nlm.nih.gov/entrez/eutils/elink.fcgi?dbfrom=pubmed&tool=sumsearch.org/cite&retmode=ref&cmd=prlinks&id=18757870  }} </ref>
 
{| style="cellpadding=0; cellspacing= 0; width: 600px;"
|-
| style="padding: 0 5px; font-size: 100%; background: #4682B4; color: #FFFFFF" align=center |'''V/Q Scan''' ||style="padding: 0 5px; font-size: 100%; background: #4682B4; color: #FFFFFF" align=center |'''Clinical Probability''' || style="padding: 0 5px; font-size: 100%; background: #4682B4; color: #FFFFFF" align=center |'''Interpretation'''
|-
|style="font-size: 100; padding: 0 5px; background: #B8B8B8" align=left | Normal
|style="font-size: 100; padding: 0 5px; background: #B8B8B8" align=left | Any probability
|style="font-size: 100; padding: 0 5px; background: #B8B8B8" align=left | PE excluded
|-
|style="font-size: 100; padding: 0 5px; background: #B8B8B8" align=left | Low probability scan
|style="font-size: 100; padding: 0 5px; background: #B8B8B8" align=left | Low
|style="font-size: 100; padding: 0 5px; background: #B8B8B8" align=left | PE excluded
|-
|style="font-size: 100; padding: 0 5px; background: #B8B8B8" align=left | High probability scan
|style="font-size: 100; padding: 0 5px; background: #B8B8B8" align=left | High
|style="font-size: 100; padding: 0 5px; background: #B8B8B8" align=left | PE confirmed
|-
|style="font-size: 100; padding: 0 5px; background: #B8B8B8" align=left colspan="2"|Any other combination of V/Q scan and clinical probability
|style="font-size: 100; padding: 0 5px; background: #B8B8B8" align=left |Further tests are needed
|}


==Ventilation/perfusion scan==
===Supportive Trial Data===  
This type of examination is used less often because of the more widespread availability of CT technology, however, it may be useful in patients
* Data regarding the use of the V/Q scan for the evaluation of patients with suspected PE is provided by the findings of the [[Pulmonary embolism landmark trials#Trials assessing the diagnosis|PIOPED]] study. The rates of confirmed PE by [[angiography]] for each of the categories of the V/Q scan are:<ref name="pmid2332918">{{cite journal| author=PIOPED Investigators| title=Value of the ventilation/perfusion scan in acute pulmonary embolism. Results of the prospective investigation of pulmonary embolism diagnosis (PIOPED). | journal=JAMA | year= 1990 | volume= 263 | issue= 20 | pages= 2753-9 | pmid=2332918 | doi= | pmc= | url=http://www.ncbi.nlm.nih.gov/entrez/eutils/elink.fcgi?dbfrom=pubmed&tool=sumsearch.org/cite&retmode=ref&cmd=prlinks&id=2332918  }} </ref>
# Who have an allergy to iodinated contrast. To read more about contrast allergy, click [[Contrast medium#Allergy Reactions|here]].
** High probability scan: 88%
# In pregnancy due to lower radiation exposure than CT.
** Intermediate probability scan: 33%
# Hospitals lacking CT facility or inexperienced medical staff.
** Low probability scan: 16%
** Normal or near normal probability scan: 9%


The ventilation/perfusion ratio (V/Q) Scan: The PIOPED data suggested that normal perfusion scans are almost never associated with recurrent pulmonary embolism, even if anticoagulation is withheld.
* Shown below is the sensitivity and specificity of V/Q scan for the detection of PE:<ref name="pmid2332918">{{cite journal| author=PIOPED Investigators| title=Value of the ventilation/perfusion scan in acute pulmonary embolism. Results of the prospective investigation of pulmonary embolism diagnosis (PIOPED). | journal=JAMA | year= 1990 | volume= 263 | issue= 20 | pages= 2753-9 | pmid=2332918 | doi= | pmc= | url=http://www.ncbi.nlm.nih.gov/entrez/eutils/elink.fcgi?dbfrom=pubmed&tool=sumsearch.org/cite&retmode=ref&cmd=prlinks&id=2332918  }} </ref>
:* High-prob scans, however only identified 41% of patients with PE.
:* In the setting of a high pre-test probability, a high-prob scan revealed PE in 95% of cases.
:* Unfortunately, 41% of all scans in PIOPED were interpreted as intermediate, and an additional 16% were interpreted as low-probability.
:** Upon angiography, however, 30 and 14% of these patients respectively were found to have PE.
:** Based on these numbers, there has been a huge movement to abolish the low-probability, and intermediate-probability categories, and have readings either be high-probability, normal, or non-diagnostic.
:* It should also be realized that the false-positive rate for high-prob scans was 14%, and that 72% of patients in PIOPED had a clinical – scan combination that required further investigation.
* It has been suggested by some authors that patients with an intermediate pre-test probability of PE a venous ultrasound would provide the same justification for anticoagulation as would a confirmed PE.
'''The following table summarizes the possible outcome of V/Q scan:'''


{| border="1"
{|
|+
|-
! [[Pulmonary embolism ventilation/perfusion scan|V/Q Scan]] !! [[Pulmonary embolism diagnosis#Wells score|Clinical Probability]] !! Diagnosis
| style="padding: 0 5px; font-size: 100%; background: #4682B4; color: #FFFFFF" align=center |'''Category of ventilation/perfusion scan'''|| style="padding: 0 5px; font-size: 100%; background: #4682B4; color: #FFFFFF" align=center |'''Sensitivity''' || style="padding: 0 5px; font-size: 100%; background: #4682B4; color: #FFFFFF" align=center |'''Specificity'''
|-
|-
| Normal
|style="font-size: 100; padding: 0 5px; background: #B8B8B8" align=left | High probability scan ||style="font-size: 100; padding: 0 5px; background: #B8B8B8" align=left| 41% || style="font-size: 100; padding: 0 5px; background: #B8B8B8" align=left| 97%
| any probability
| PE excluded<ref name="pmid19634076">{{cite journal |author=Hoeper MM |title=Definition, classification, and epidemiology of pulmonary arterial hypertension |journal=Semin Respir Crit Care Med |volume=30 |issue=4 |pages=369–75 |year=2009 |month=August |pmid=19634076 |doi=10.1055/s-0029-1233306 |url=http://www.thieme-connect.com/DOI/DOI?10.1055/s-0029-1233306 |accessdate=2011-12-06}}</ref>
|-
|-
| Low probability scan
|style="font-size: 100; padding: 0 5px; background: #B8B8B8" align=left| High or intermediate probability scan || style="font-size: 100; padding: 0 5px; background: #B8B8B8" align=left| 82% || style="font-size: 100; padding: 0 5px; background: #B8B8B8" align=left| 52%
| Low
| PE excluded
|-
|-
| High probability scan
|style="font-size: 100; padding: 0 5px; background: #B8B8B8" align=left| High, intermediate, or low probability scan || style="font-size: 100; padding: 0 5px; background: #B8B8B8" align=left| 98% || style="font-size: 100; padding: 0 5px; background: #B8B8B8" align=left| 10%
| High
| PE confirmed
|-
|-
| Variable result/Non diagnostic
| Variable
| Serial [[Deep vein thrombosis ultrasound|lower extremity USG]] or [[Pulmonary embolism other imaging findings#Angiography|Pulmonary angiography]]
|}
|}
===Comparison with CT Pulmonary angiography===
* Spiral CT scanning is now a standard modality to non-invasively diagnose PE<ref name="pmid15136509">{{cite journal |author=Schoepf UJ, Goldhaber SZ, Costello P |title=Spiral computed tomography for acute pulmonary embolism |journal=Circulation |volume=109 |issue=18 |pages=2160–7 |year=2004 |month=May |pmid=15136509 |doi=10.1161/01.CIR.0000128813.04325.08 |url=http://circ.ahajournals.org/cgi/pmidlookup?view=long&pmid=15136509 |accessdate=2011-12-05}}</ref>.
*:* Initial studies reported sensitivities for diagnosing emboli to the segmental level (4th order branch) as high as 98%, however subsequent studies have found sensitivities to be lower.
*:* The sensitivity is higher with more proximal clot.
*:* Although smaller clot, in the subsegmental arteries, is certainly not as physiologically important as the larger, more proximal clot, they may be important predictors of future, larger clots.
*:* The study consisting of 142 patients concluded that the sensitivity and specificity of CT angiography are higher than that of V/Q scans, as is the inter-observer agreement<ref name="pmid9356627">{{cite journal |author=Mayo JR, Remy-Jardin M, Müller NL, Remy J, Worsley DF, Hossein-Foucher C, Kwong JS, Brown MJ |title=Pulmonary embolism: prospective comparison of spiral CT with ventilation-perfusion scintigraphy |journal=Radiology |volume=205 |issue=2 |pages=447–52 |year=1997 |month=November |pmid=9356627 |doi= |url=http://radiology.rsnajnls.org/cgi/pmidlookup?view=long&pmid=9356627 |accessdate=2011-12-06}}</ref>.
*:*:* They recommend getting a CT angiography as the next test following an indeterminate V/Q scan, however caution that if the pre-test probability is ‘sufficiently high’ a standard angiogram should still be obtained after a negative CT angiography.
* A cost-effective analysis using spiral CT angiography for the diagnosis of PE showed the following result<ref name="pmid8816516">{{cite journal |author=van Erkel AR, van Rossum AB, Bloem JL, Kievit J, Pattynama PM |title=Spiral CT angiography for suspected pulmonary embolism: a cost-effectiveness analysis |journal=Radiology |volume=201 |issue=1 |pages=29–36 |year=1996 |month=October |pmid=8816516 |doi= |url=http://radiology.rsnajnls.org/cgi/pmidlookup?view=long&pmid=8816516 |accessdate=2011-12-05}}</ref>.
*:* The use of CT angiography in a diagnostic algorithm was by far and away a more cost-effective strategy.
*:* If the sensitivity of CT angiography was < 85%, conventional angiography was associated with a lower mortality, but still remained a more expensive strategy.


*:* The use of CT angiography in a diagnostic algorithm was by far and away a more cost-effective strategy.
* For a [[prevalence]] of PE of 33% (confirmed by [[angiography]]), the [[positive predictive value]] (PPV) and [[negative predictive value]] (NPV) of the ventilation/perfusion scan for PE are as follows:<ref name="pmid2332918">{{cite journal| author=PIOPED Investigators| title=Value of the ventilation/perfusion scan in acute pulmonary embolism. Results of the prospective investigation of pulmonary embolism diagnosis (PIOPED). | journal=JAMA | year= 1990 | volume= 263 | issue= 20 | pages= 2753-9 | pmid=2332918 | doi= | pmc= | url=http://www.ncbi.nlm.nih.gov/entrez/eutils/elink.fcgi?dbfrom=pubmed&tool=sumsearch.org/cite&retmode=ref&cmd=prlinks&id=2332918  }} </ref>
*:* If the sensitivity of CT angiography was < 85%, conventional angiography was associated with a lower mortality, but still remained a more expensive strategy.
** [[PPV]] for high probability scan: 88%
*According to the International Commission on Radiological Protection (ICRP), the radiation exposure for an average sized adult, from a lung scan with 100 MBq of Tc-99 m MAA is 1.1 mSv.
** [[NPV]] for intermediate probability scan: 84%-88%
**The radiation exposure of a spiral CT is 2–6 mSv.
** [[NPV]] for a normal or near normal scan: 91%-96%
**The radiation exposure of a plain chest X-ray is approximately 0.05 mSv.
 
* PIOPED demonstrated that the value of the V/Q scan is improved when combined with the clinical pretest probability of PE.  For example, among patients with suspected PE and high probability V/Q scan, the rates of confirmed PE by [[angiography]] were 56%, 88%, and 96% in cases of low, intermediate, and high clinical pre-test probability of PE respectively.<ref name="pmid2332918">{{cite journal| author=PIOPED Investigators| title=Value of the ventilation/perfusion scan in acute pulmonary embolism. Results of the prospective investigation of pulmonary embolism diagnosis (PIOPED). | journal=JAMA | year= 1990 | volume= 263 | issue= 20 | pages= 2753-9 | pmid=2332918 | doi= | pmc= | url=http://www.ncbi.nlm.nih.gov/entrez/eutils/elink.fcgi?dbfrom=pubmed&tool=sumsearch.org/cite&retmode=ref&cmd=prlinks&id=2332918  }} </ref>
 
===Comparison with CT Pulmonary Angiography (CTPA)===
====Benefits of V/Q Scan over CTPA====
* Less radiation exposure
* Diagnostic test of choice in an institution lacking a CT facility, or with inexperienced staff
 
====Benefits of CTPA over V/Q Scan====
* Cost effectiveness
* CT may also identify right heart dysfunction, or provide an alternative diagnosis.


==References==
==References==
{{reflist|2}}
{{Reflist|2}}
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Latest revision as of 23:53, 29 July 2020
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Editor(s)-In-Chief: The APEX Trial Investigators, C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: ; Associate Editor(s)-in-Chief: Rim Halaby, M.D. [2]

Overview

A ventilation/perfusion scan (otherwise known as V/Q scan or lung scintigraphy) is a study which shows whether an area of the lung is being ventilated with oxygen and perfused with blood. In the setting of a pulmonary embolism (PE), perfusion can be obstructed due to the formation of a clot. The ventilation/perfusion scan is less commonly used due to the more widespread availability of computed tomography (CT) technology, however it may be useful in patients who have an allergy to iodinated contrast. It may also be useful in pregnant patients in an attempt to minimize radiation exposure. The diagnostic value of the results of the V/Q scan is improved when combined with the clinical pretest probability of PE. A high probability scan coupled with a high clinical pretest probability of PE is diagnostic for PE, while a normal scan regardless of the clinical pretest probability excludes PE. For the majority of the cases of suspected PE, however, the ventilation/perfusion scan does not establish the diagnosis nor exclude PE and further tests are required.[1]

Ventilation/Perfusion Scan

Principle

Technetium (Tc)-99m labeled macro-aggregated albumin particles, when injected, fill small fractions of the pulmonary capillaries. This helps in the assessment of lung perfusion at the tissue level via scintigraphy. When any branch of the pulmonary artery is occluded, the particles will not reach the capillaries rendering the area "cold" on imaging.

Indications

The utilization of ventilation/perfusion scanning has declined since the advent of more widespread availability of CT technology, however it may be useful in particular subgroups of patients, such as:

  1. Patients who have a known allergy to iodinated contrast. To read more about contrast allergy, click here.
  2. In pregnant patients to minimize exposure to radiation.
  3. For patients who are in a hospital lacking CT technology.

Interpretation

A normal or near normal V/Q scan excludes the diagnosis of PE.[1][2] When the result of the V/Q scan is not normal or near normal, the results of the V/Q scan should be interpreted in combination with the pretest probability of PE. Shown below is a table that summarizes the interpretation of the different results of the V/Q scan.[2]

V/Q Scan Clinical Probability Interpretation
Normal Any probability PE excluded
Low probability scan Low PE excluded
High probability scan High PE confirmed
Any other combination of V/Q scan and clinical probability Further tests are needed

Supportive Trial Data

  • Data regarding the use of the V/Q scan for the evaluation of patients with suspected PE is provided by the findings of the PIOPED study. The rates of confirmed PE by angiography for each of the categories of the V/Q scan are:[1]
    • High probability scan: 88%
    • Intermediate probability scan: 33%
    • Low probability scan: 16%
    • Normal or near normal probability scan: 9%
  • Shown below is the sensitivity and specificity of V/Q scan for the detection of PE:[1]
Category of ventilation/perfusion scan Sensitivity Specificity
High probability scan 41% 97%
High or intermediate probability scan 82% 52%
High, intermediate, or low probability scan 98% 10%
  • PIOPED demonstrated that the value of the V/Q scan is improved when combined with the clinical pretest probability of PE. For example, among patients with suspected PE and high probability V/Q scan, the rates of confirmed PE by angiography were 56%, 88%, and 96% in cases of low, intermediate, and high clinical pre-test probability of PE respectively.[1]

Comparison with CT Pulmonary Angiography (CTPA)

Benefits of V/Q Scan over CTPA

  • Less radiation exposure
  • Diagnostic test of choice in an institution lacking a CT facility, or with inexperienced staff

Benefits of CTPA over V/Q Scan

  • Cost effectiveness
  • CT may also identify right heart dysfunction, or provide an alternative diagnosis.

References

  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 PIOPED Investigators (1990). "Value of the ventilation/perfusion scan in acute pulmonary embolism. Results of the prospective investigation of pulmonary embolism diagnosis (PIOPED)". JAMA. 263 (20): 2753–9. PMID 2332918.
  2. ↑ 2.0 2.1 Torbicki A, Perrier A, Konstantinides S, Agnelli G, Galiè N, Pruszczyk P; et al. (2008). "Guidelines on the diagnosis and management of acute pulmonary embolism: the Task Force for the Diagnosis and Management of Acute Pulmonary Embolism of the European Society of Cardiology (ESC)". Eur Heart J. 29 (18): 2276–315. doi:10.1093/eurheartj/ehn310. PMID 18757870.

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