Multicenter study of the feasibility and safety of using the Memotherm carotid arterial stent for extracranial carotid artery stenosis

2002 ◽  
Vol 96 (5) ◽  
pp. 830-836 ◽  
Author(s):  
Adnan I. Qureshi ◽  
M. Fareed K. Suri ◽  
Gishel New ◽  
Daniel C. Wadsworth ◽  
Joan Dulin ◽  
...  
Keyword(s):  
Carotid Artery ◽  
Primary Endpoint ◽  
Stent Placement ◽  
Content Type ◽  
Residual Stenosis ◽  
Major Cardiovascular Events ◽  
Long Term Follow Up ◽  
The Mean ◽  
The Impact ◽  
Fine Print

Object. Carotid artery (CA) angioplasty with stent placement has been proposed as an alternative technique for revascularization in cases of CA stenosis. In this report the authors review the results of a multicenter Phase I study in which they evaluated the safety and feasibility of using a new self-expanding nitinol stent, the Bard Memotherm, to treat CA stenosis. Methods. Enrollment was limited to patients in whom there was either 50% or greater symptomatic or 70% or greater asymptomatic stenosis of the internal CA. The primary endpoint was a technically successful implantation procedure (delivery of the stent to the target site and retrieval of the delivery device), resulting in less than 30% residual stenosis demonstrated on immediate postprocedure (control) angiograms, and no incidence of mortality, ipsilateral stroke, Q-wave myocardial infarction, or other major cardiovascular events immediately after or within 30 days following the procedure. Stent placement was attempted for 73 lesions in 71 patients (mean age 71.3 ± 8.5 years), 43 (61%) of whom were men. The mean degree of stenosis was 82.6 ± 9%. The stenosis was symptomatic in 27 (37%) and asymptomatic in 46 (63%) of 73 lesions. In four procedures the stent could not be delivered or released. The mean residual stenosis observed on angiograms was 3.8 ± 6.9% in the 69 lesions treated with the Bard Memotherm stent; residual stenosis was greater than 30% in one of the 69 procedures. The primary endpoint was achieved in 65 (89%) of the 73 procedures. One patient experienced a major ischemic stroke and another patient died of intracerebral hemorrhage. The overall 1-month stroke rate was 2.7% for 73 attempted procedures. One patient died of pneumonia and acute respiratory distress syndrome, which occurred 3 weeks after the stent procedure and was unrelated to the procedure. Conclusions. The Memotherm stent can be used to treat patients with CA stenosis and is associated with a low periprocedure complication rate. Long-term follow-up studies are underway to determine the impact of stent placement on the risk of ipsilateral ischemic events.

Patient presentation, angiographic features, and treatment of strangulation-induced dissection of the internal carotid artery

2000 ◽  
Vol 92 (3) ◽  
pp. 481-487 ◽  
Author(s):  
Adel M. Malek ◽  
Randall T. Higashida ◽  
Van V. Halbach ◽  
Christopher F. Dowd ◽  
Constantine C. Phatouros ◽  
...  
Keyword(s):  
Domestic Violence ◽  
Carotid Artery ◽  
Internal Carotid Artery ◽  
Stent Placement ◽  
Internal Carotid ◽  
Artery Dissection ◽  
Percutaneous Angioplasty ◽  
Consistent Finding ◽  
Content Type ◽  
Fine Print

✓ Domestic violence leading to strangulation by an abusive spouse can cause carotid artery dissection. This phenomenon is rare and has been described in only three previous instances. The authors present their management strategies in three additional cases.Three young women aged 24 to 43 years were victims of manual strangulation committed by their spouses 3 months to 1 year before presentation. Two of the patients suffered delayed cerebral infarctions before presentation and angiography demonstrated focal, mirror-image severe residual stenoses in the high-cervical internal carotid artery (ICA), which were characteristic of a healed chronic dissection; there was no evidence of fibromuscular dysplasia. One of these patients underwent unilateral percutaneous angioplasty with stent placement, and the other underwent bilateral percutaneous angioplasty. Both patients have recovered from their strokes and remain clinically stable at 8 and 20 months posttreatment, respectively. The third patient presented with bilateral ischemic frontal watershed infarctions resulting from an occluded left ICA and a severely narrowed right ICA. Given the extent of the established infarctions, this case was managed with a long-term regimen of anticoagulation medications, and the patient remains neurologically impaired.These cases illustrate the susceptibility of the manually compressed ICA to traumatic injury as a result of domestic violence. They identify bilateral symmetrical ICA dissection as a consistent finding and the real danger of delayed stroke as a consequence of strangulation. Endovascular therapy in which percutaneous angioplasty and/or stent placement are used can be useful in treating residual focal stenoses to improve cerebral perfusion and to lower the risk of embolic or ischemic stroke.

Cerebral hemodynamics in patients with carotid artery occlusion and contralateral moderate or severe internal carotid artery stenosis

2001 ◽  
Vol 94 (4) ◽  
pp. 559-564 ◽  
Author(s):  
Fabrizio Vernieri ◽  
Patrizio Pasqualetti ◽  
Marina Diomedi ◽  
Patrizia Giacomini ◽  
Paolo M. Rossini ◽  
...  
Keyword(s):  
Regression Analysis ◽  
Carotid Artery ◽  
Cerebral Hemodynamics ◽  
Cerebrovascular Reactivity ◽  
Content Type ◽  
Collateral Pathways ◽  
Collateral Vessels ◽  
Ica Stenosis ◽  
The Impact ◽  
Fine Print

Object. The purpose of this study was to evaluate cerebral hemodynamics in patients suffering from occlusion of the carotid artery (CA) and contralateral CA stenosis. Methods. Using transcranial Doppler ultrasonography, the cerebrovascular reactivity to hypercapnia in the middle cerebral arteries was evaluated by calculating the breath-holding index (BHI) of 69 symptomatic patients suffering from internal CA (ICA) occlusion and moderate or severe contralateral ICA stenosis. To evaluate which variables influenced BHIs ipsilateral to the site of ICA occlusion, a multiple stepwise linear regression analysis was performed that included the following factors: patient age, percentage of contralateral ICA stenosis, contralateral BHI, number of collateral pathways, and presence of hypertension, diabetes, smoking, and hyperlipidemia. An analysis of variance was conducted to evaluate the impact of the type of collateral vessels on the BHI. A regression analysis showed that the BHI ipsilateral to the site of ICA occlusion could be accounted for by the contralateral BHI (which was entered at the first step of the analysis, p < 0.001) and by the number of collateral pathways (which was entered at the second step, p = 0.033). Neither the degree of contralateral ICA stenosis nor the other variables could be added to improve the model. The analysis demonstrated that the absence of collateral pathways and the presence of the anterior communicating artery (ACoA) alone were associated with lower BHI values than those found in the presence of two or three collateral vessels, regardless of the presence of an anterior collateral pathway. Conclusions. On the basis of these data one can infer that the cerebral hemodynamic status of patients with occlusive disease of the CA is influenced by individual anatomical and functional characteristics. Because improvement in contralateral hemodynamics after surgical correction of an ICA stenosis can only be expected in the presence of an ACoA, the planning of strategies for influencing cerebral blood flow distal to an ICA occlusion and, in particular, the consideration of a contralateral carotid endarterectomy, should be preceded by a careful evaluation of the intracranial hemodynamic adaptive status of the patient. Particular attention should be paid to cerebrovascular reactivity and the number and type of collateral vessels that are present.

Staged stent-assisted angioplasty for symptomatic intracranial vertebrobasilar artery stenosis

2002 ◽  
Vol 97 (6) ◽  
pp. 1294-1301 ◽  
Author(s):  
Elad I. Levy ◽  
Ricardo A. Hanel ◽  
Bernard R. Bendok ◽  
Alan S. Boulos ◽  
Mary L. Hartney ◽  
...  
Keyword(s):  
Endovascular Treatment ◽  
Stent Placement ◽  
Atherosclerotic Disease ◽  
High Grade ◽  
Content Type ◽  
Related Risk ◽  
Vertebrobasilar Artery ◽  
Neurological Morbidity ◽  
The Mean ◽  
Fine Print

Object. Medically refractory symptomatic vertebrobasilar atherosclerotic disease has a poor prognosis. Studies have shown that longer (≥ 10 mm), eccentric, high-grade (> 70%) stenoses portend increased procedure-related morbidity. The authors reviewed their experience to determine whether a staged procedure consisting of angioplasty followed by delayed (≥ 1 month later) repeated angioplasty and stent placement reduces the morbidity associated with endovascular treatment of symptomatic basilar and/or intracranial vertebral artery (VA) stenoses. Methods. The authors retrospectively reviewed the medical records in a consecutive series of eight patients who underwent planned stent-assisted angioplasty for medically refractory, symptomatic atherosclerotic disease of the intracranial posterior circulation between February 1999 and January 2002. Staged stent-assisted angioplasty was planned for these patients because the extent and degree of stenosis of the VA and/or basilar artery (BA) lesion portended an excessive procedure-related risk. The degree of stenosis, recent onset of symptoms (unstable plaque), vessel tortuosity, and lesion length and morphological features were contributing factors in determining procedure-related risk. Patient records were analyzed for location and degree of stenosis, preprocedural regimen of antiplatelet and/or anticoagulation agents, devices used, procedure-related complications, and clinical and radiographic outcomes. Among the patients in whom staged stent-assisted angioplasty was planned, vessel dissection, which necessitated immediate stent placement, occurred during passage of the balloon in one of them. In a second patient, the stent could not be maneuvered through the tortuous VA. In a third patient, the VA and BAs remained widely patent after angioplasty alone, and therefore stent placement was not required. Significant complications among the eight patients included transient aphasia and hemiparesis in one and a groin hematoma that necessitated surgical intervention in another; there was no permanent neurological morbidity. The mean stenosis before treatment was 78%, which fell to 54% after angioplasty, and the mean residual stenosis after stent placement was 30%. At the last follow-up examination, none of the treated patients had further symptoms attributable to the treated stenosis. Conclusions. The novel combination of initial angioplasty followed by delayed endoluminal stent placement may reduce the neurological morbidity associated with endovascular treatment of long, high-grade stenotic lesions. Attempting to cross high-grade stenoses with higher-profile devices such as stents may result in an embolic shower. Furthermore, neointimal proliferation and scar formation after angioplasty result in a thickened fibrous layer, which may be protective during delayed stent deployment. Larger-scale studies involving multiple centers are needed to elucidate further the lesion morphological characteristics and patient population most likely to benefit from staged procedures.

Carotid artery angioplasty and use of stents in high-risk patients with contralateral occlusions

1999 ◽  
Vol 90 (6) ◽  
pp. 1031-1036 ◽  
Author(s):  
Robert A. Mericle ◽  
Stanley H. Kim ◽  
Giuseppe Lanzino ◽  
Demetrius K. Lopes ◽  
Ajay K. Wakhloo ◽  
...  
Keyword(s):  
High Risk ◽  
Carotid Artery ◽  
Carotid Endarterectomy ◽  
Stent Placement ◽  
Death Rate ◽  
Minor Stroke ◽  
Content Type ◽  
Symptomatic Carotid ◽  
Fine Print

Object. The risks associated with carotid endarterectomy (CEA) are increased in the presence of contralateral carotid artery (CA) occlusion. The 30-day stroke and death rate for patients in the North American Symptomatic Carotid Endarterectomy Trial (NASCET) who had contralateral CA occlusion was 14.3%. The authors analyze their experience with angioplasty and/or stent placement in patients with contralateral CA occlusion to determine the safety and efficacy of endoluminal revascularization in this subgroup.Methods. Twenty-six procedures were evaluated in 23 patients with high-grade CA stenosis and contralateral CA occlusion. The first 15 procedures were evaluated retrospectively, and the next 11 prospectively. All patients had severe medical comorbidities and were considered too high risk for CEA, even without considering the contralateral occlusion. Clinical follow-up review was performed an average of 18 months later (median 15 months).Conclusions. The average ipsilateral CA stenosis according to NASCET criteria was 78% preprocedure and 5% postprocedure. There were no changes in neurological or functional outcome immediately postoperatively in any patient. The 30-day postoperative stroke and death rates were zero. However, there was one symptomatic femoral hematoma that resolved without surgery. At follow up, there were three patients who had suffered stroke or death. One patient died secondary to respiratory arrest at 2 months; one died secondary to prostate carcinoma at 12 months; and one patient experienced a minor stroke contralateral to the treated artery at 41 months. Despite the substantial preoperative risk factors in patients in this series, the 30-day stroke and death rate for angioplasty and/or stent placement appears to be lower than that of CEA in patients with contralateral occlusions.

Carotid endarterectomy performed after progressive carotid stenosis following angioplasty and stent placement

1997 ◽  
Vol 87 (6) ◽  
pp. 940-943 ◽  
Author(s):  
Fernando L. Vale ◽  
Winfield S. Fisher ◽  
William D. Jordan ◽  
Cheryl A. Palmer ◽  
Jiri Vitek
Keyword(s):  
Carotid Artery ◽  
Carotid Endarterectomy ◽  
Stent Placement ◽  
Vessel Wall ◽  
Pathological Examination ◽  
Metallic Stent ◽  
Content Type ◽  
Symptomatic Disease ◽  
Ica Stenosis ◽  
Fine Print

✓ Carotid endarterectomy (CEA) is the treatment of choice for asymptomatic and symptomatic disease causing greater than 60% internal carotid artery (ICA) stenosis. Recently, percutaneous transluminal angioplasty (PTA) with stent placement has been investigated as a therapeutic option for the treatment of ICA stenosis. In this report the authors document CEA performed after PTA with stent placement and describe the pathological findings. A standard CEA was performed. The surgical intervention was more difficult secondary to the following variables: the length of the exposure necessary to dissect out the metallic stent, the difficulty with opening and cutting the artery, and the care required to remove the stent to avoid vessel wall perforation. Pathological examination of the specimen demonstrated classic atherosclerotic changes revealing persistence of native disease. The metallic stent was embedded within the plaque. Many questions remain unanswered regarding the physiological and biological changes that occur in the carotid vessel wall after PTA with stent placement. It is concluded that CEA of a stent-containing carotid artery is feasible and should be considered as an alternative when recurrent stenosis occurs after PTA.

Multicenter clinical trial of the NexStent coiled sheet stent in the treatment of extracranial carotid artery stenosis: immediate results and late clinical outcomes

2003 ◽  
Vol 99 (2) ◽  
pp. 264-270 ◽  
Author(s):  
Adnan I. Qureshi ◽  
Charlene Knape ◽  
John Maroney ◽  
M. Fareed K. Suri ◽  
L. Nelson Hopkins
Keyword(s):  
Carotid Artery ◽  
Long Term Results ◽  
Extracranial Carotid Artery ◽  
Content Type ◽  
Residual Stenosis ◽  
End Point ◽  
Major Adverse Events ◽  
Fine Print

Object. Carotid angioplasty with stent placement has been proposed as an alternative method for revascularization of carotid artery (CA) stenosis. A novel stent with a coiled sheet design (NexStent; EndoTex Interventional Systems, Inc., Cupertino, CA) has been introduced because it provides improved flexibility over conventional tubular designs during navigation through tortuous arterial segments. The authors report the results of a multicenter study in which they evaluated the safety, feasibility, and effectiveness of this stent in the treatment of CA stenosis. Methods. Enrollment was limited to patients with 70% or more symptomatic or asymptomatic stenosis of the internal CA. The primary end point was a technically successful implantation procedure (delivery of the stent to the target site and retrieval of the delivery device) resulting in less than 30% residual stenosis on angiograms obtained immediately postprocedure and no death or ipsilateral stroke immediately after or within 30 days following the procedure. The secondary effectiveness end point was prevention of an ipsilateral stroke within 31 days to 1 year postprocedure. The tertiary effectiveness end point was less than 70% residual stenosis demonstrated on CA Doppler ultrasonography performed at 6 and 12 months. Forty-four patients (mean age 71 years, range 51–89 years; 30 patients were men and 18 [41%] had symptomatic stenotic lesions) were treated using 45 NexStents (two stents were placed in one patient). Residual stenosis on postprocedure angiograms was less than 30% in all patients. The 1-month primary end point was achieved in 41 patients (93%). The three major adverse events that occurred within 30 days were one death related to congestive heart failure and two ipsilateral strokes, one of which resulted in death. No ipsilateral stroke occurred between 1 and 12 months postprocedure among 35 patients who completed the follow-up evaluation. Asymptomatic recurrent stenosis was identified in one patient at 6 months postprocedure, requiring the performance of repeated angioplasty. Conclusions. Placement of the NexStent was feasible and resulted in the effective treatment of patients with CA stenosis, with promising long-term results.

Endovascular therapy for stenosis of the petrous or cavernous portion of the internal carotid artery: percutaneous transluminal angioplasty compared with stent placement

2003 ◽  
Vol 98 (3) ◽  
pp. 491-497 ◽  
Author(s):  
Tomoaki Terada ◽  
Mitsuharu Tsuura ◽  
Hiroyuki Matsumoto ◽  
Osamu Masuo ◽  
Tomoyuki Tsumoto ◽  
...  
Keyword(s):  
Carotid Artery ◽  
Stent Placement ◽  
Percutaneous Transluminal Angioplasty ◽  
Internal Carotid ◽  
Treated Group ◽  
Transluminal Angioplasty ◽  
Content Type ◽  
Percutaneous Transluminal ◽  
Fine Print

Object. The effects of percutaneous transluminal angioplasty (PTA) and stent placement for stenosis of the petrous or cavernous portion of the internal carotid artery (ICA) were compared. Methods. Twenty-four patients with symptomatic, greater than 60% stenosis of the petrous or cavernous portion of the ICA were treated using PTA or stent placement; 15 were treated with PTA and nine with stent insertion. Initial and follow-up results (> 3 months posttreatment) were compared in each group. Stenotic portions of the ICA were successfully opened in 13 of 15 patients in the PTA group, and in all nine patients in the stent-treated group. In one case in the PTA group stent delivery was attempted; however, the device could not pass through the vessel's tortuous curve, and PTA alone was performed in this case. Postoperatively, the mean stenotic ratio decreased from 72.1 to 29.6% in the PTA group, and from 75.6 to 2.2% in the stent-treated group. In four patients in the PTA group, stenoses greater than 50% were demonstrated on follow-up angiography performed at 3 to 6 months after PTA. In the stent-treated group, no restenosis was encountered, although in one case acute occlusion of the stent occurred; the device was recanalized with PTA and infusion of tissue plasminogen activator. This case was the only one of the 24 in which any neurological deficits related to the endovascular procedure occurred. Stent placement brought a greater gain in diameter than did PTA at the initial and late follow-up period; this gain was statistically significant. Conclusions. Stent placement is more effective than PTA for stenosis of the petrous or cavernous portion of the ICA from the viewpoint of initial and late gain in diameter.

Percutaneous transluminal angioplasty and stent placement for recurrent carotid artery stenosis

1999 ◽  
Vol 90 (4) ◽  
pp. 688-694 ◽  
Author(s):  
Giuseppe Lanzino ◽  
Robert A. Mericle ◽  
Demetrius K. Lopes ◽  
Ajay K. Wakhloo ◽  
Lee R. Guterman ◽  
...  
Keyword(s):  
Carotid Artery ◽  
Stent Placement ◽  
Carotid Artery Stenosis ◽  
Percutaneous Transluminal Angioplasty ◽  
Transluminal Angioplasty ◽  
Artery Stenosis ◽  
Content Type ◽  
Percutaneous Transluminal ◽  
Fine Print

Object. Treatment consisting of percutaneous transluminal angioplasty (PTA) and stent placement has recently been proposed as an alternative to surgical reexploration in patients with recurrent carotid artery stenosis following endarterectomy. The authors retrospectively reviewed their experience after performing 25 procedures in 21 patients to assess the safety and efficacy of PTA with or without stent placement for carotid artery restenosis.Methods. The mean interval between endarterectomy and the endovascular procedures was 57 months (range 8–220 months). Seven arteries in five patients were treated by PTA alone (including bilateral procedures in one patient and repeated angioplasty in the same vessel in another). Early suboptimum results and recurrent stenosis in some of these initial cases prompted the authors to combine PTA with stent placement in the treatment of 18 arteries over the past 3 years. No major periprocedural deficits (neurological or cardiac complications) or death occurred. There was one periprocedural transient neurological event, and in one patient a pseudoaneurysm of the femoral artery (at the access site) required surgical repair. In the 16 patients who each underwent at least 6 months of follow-up review, no neurological events ipsilateral to the treated artery had occurred after a mean follow-up period of 27 months (range 6–57 months). Three of five patients who underwent PTA alone developed significant (> 50%) asymptomatic restenoses that required repeated angioplasty in one and PTA with stent placement in two patients. Significant restenosis (55%) was observed in only one of the vessels treated by combined angioplasty and stent placement.Conclusions. Endovascular PTA and stenting of recurrent carotid artery stenosis is both technically feasible and safe and has a satisfactory midterm patency. This procedure can be considered a viable alternative to surgical reexploration in patients with recurrent carotid artery stenosis.

Cost of treating high-risk symptomatic carotid artery stenosis: stent insertion and angioplasty compared with endarterectomy

2004 ◽  
Vol 101 (6) ◽  
pp. 904-907 ◽  
Author(s):  
Robert D. Ecker ◽  
Robert D. Brown ◽  
Douglas A. Nichols ◽  
Robyn L. McClelland ◽  
Megan S. Reinalda ◽  
...  
Keyword(s):  
High Risk ◽  
Carotid Artery ◽  
Stent Placement ◽  
Stent Insertion ◽  
Key Factors ◽  
Content Type ◽  
Symptomatic Carotid ◽  
The North ◽  
Similar Risk ◽  
Fine Print

Object. Definitive data characterizing the safety and efficacy of carotid angioplasty with stent placement (CAS) for symptomatic, occlusive carotid artery (CA) disease require further refinements and standardization of techniques as well as large prospective studies on a par with the North American Symptomatic Carotid Endarterectomy Trial (NASCET). Despite the absence of such data, many surgeons have performed angioplasty and stent placement in patients with clinical or anatomical features known to add significant perioperative risk and capable of disqualifying the patients from participation in NASCET. There exists no cost analysis comparing high-risk endarterectomy with percutaneous angioplasty and stent insertion. Methods. Forty-five patients (29 men and 16 women) with high-risk, symptomatic CA stenosis have been treated with CAS at the authors' institution since 1996. Indications for this procedure included symptomatic recurrent stenosis following CA endarterectomy (CEA), active coronary disease, high CA bifurcation, and severe medical comorbidities. A long-standing CEA computer database was screened for control patients with similar risk factors; 391 patients (276 men and 115 women) were identified. Actual cost data, duration of hospital stay, and relevant clinical data from the time of treatment until hospital discharge were collected in each patient. The median total cost of CAS was $10,628, whereas that for CEA was $10,148 (p = 0.495). Conclusions. In patients with high-risk, NASCET-ineligible CA stenosis there was no overall statistically significant cost difference between CEA and CAS. Given that there may not be a cost advantage for either procedure, procedural risk, efficacy, and durability should be key factors in determining the optimal treatment strategy.

Stent placement for the treatment of occlusive atherosclerotic carotid artery disease in patients with concomitant coronary artery disease

2002 ◽  
Vol 96 (3) ◽  
pp. 490-496 ◽  
Author(s):  
Demetrius K. Lopes ◽  
Robert A. Mericle ◽  
Giuseppe Lanzino ◽  
Ajay K. Wakhloo ◽  
Lee R. Guterman ◽  
...  
Keyword(s):  
New York ◽  
Coronary Artery ◽  
Carotid Artery ◽  
Stent Placement ◽  
Carotid Artery Disease ◽  
Heart Association ◽  
Content Type ◽  
Carotid Artery Stent ◽  
Artery Disease ◽  
Fine Print

Object. The authors report their experience with carotid artery stent placement (CASP) in patients with concomitant carotid artery (CA) and coronary artery (CorA) diseases. Methods. In a review of 320 consecutive patients who underwent CASP, the authors identified 49 with severe CorA disease in addition to significant CA stenosis, who had undergone CASP before planned CorA bypass grafting (CorABG). The average age of these 49 patients was 68 years. In 39 patients (80%) the New York Heart Association functional classification grade was IV and in 10 the grade was III. In 26 patients 50% or greater stenosis of the left main CorA was found. Seventeen patients (35%) suffered from either significant hemodynamic contralateral CA stenosis (> 60% stenosis; eight patients) or contralateral CA occlusion (nine patients). Sixteen patients (33%) had symptomatic CA disease. No cerebrovascular events occurred during CorABG. Four patients (8%) died of cardiac arrest and one patient (2%) suffered a major stroke within 30 days after the CorABG procedure. No patient experienced clinically significant recurrent CA stenosis during the study period (average clinical follow-up period 27 months). Conclusions. Carotid artery stent placement should be considered as an alternative for the management of concomitant CA and CorA diseases. These preliminary results support the feasibility and durability of CASP in the population studied.

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